Posted on by Tammy Plotner

The Canis Major Constellation

Welcome back to Constellation Friday! Today, in honor of the late and great Tammy Plotner, we will be dealing with the “big dog” itself – the Canis Major constellation!

In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of all the then-known 48 constellations. This treatise, known as the Almagest, would be used by medieval European and Islamic scholars for over a thousand years to come, effectively becoming astrological and astronomical canon until the early Modern Age.

One of these constellations included in Ptolemy’s collection was Canis Major, an asterism located in the southern celestial hemisphere. As one of two constellations representing “the dogs” (which are associated with “the hunter” Orion) this constellation contains many notable stars and Deep Sky Objects. Today, it is one of the 88 constellations recognized by the IAU, and is bordered by Monoceros, Lepus, Columba and Puppis.

Name and Meaning:

The constellation of Canis Major literally translates to “large dog” in Latin. The first recorded mentions of any of the stars associated with this asterism are traced back to Ancient Mesopotamia, where the Babylonians recorded its existence in their Three Star Each tablets (ca. 1100 BCE). In this account, Sirus (KAK.SI.DI) was seen as the arrow aimed towards Orion, while Canis Major and part of Puppis were seen as a bow.

Artist's impression of a white dwarf star in orbit around Sirius (a white supergiant). Credit: NASA, ESA and G. Bacon (STScI)
Artist’s impression of a white dwarf star in orbit around Sirius (a white supergiant). Credit: NASA, ESA and G. Bacon (STScI)

To the ancient Greeks, Canis Major represented a dog following the great hunter Orion. Named Laelaps, or the hound of Prociris in some accounts, this dog was so swift that Zeus elevated it to the heavens. Its Alpha star, Sirius, is the brightest object in the sky (besides the Sun, the Moon and nearest planets). The star’s name means “glowing” or “scorching” in Greek, since the summer heat occurred just after Sirius’ helical rising.

The Ancient Greeks referred to such times in the summer as “dog days”, as only dogs would be mad enough to go out in the heat. This association is what led to Sirius coming to be known as the “Dog Star”. Depending on the faintness of stars considered, Canis Major resembles a dog facing either above or below the ecliptic. When facing below, since Sirius was considered a dog in its own right, early Greek mythology sometimes considered it to be two headed.

Together with the area of the sky that is deserted (now considered as the new and extremely faint constellations Camelopardalis and Lynx), and the other features of the area in the Zodiac sign of Gemini (i.e. the Milky Way, and the constellations Gemini, Orion, Auriga, and Canis Minor), this may be the origin of the myth of the cattle of Geryon, which forms one of The Twelve Lab ours of Heracles.

Sirius and the "Summer Triangle", . Credit: Greg Bacon/ STScI/ESA/NASA
Artist’s impression of Sirius and the “Summer Triangle”. Credit: G. Bacon (STScI)/ESA/NASA

Sirius has been an object of wonder and veneration to all ancient peoples throughout human history. In fact, the Arabic word Al Shi’ra resembles the Greek, Roman, and Egyptian names suggesting a common origin in Sanskrit, in which the name Surya (the Sun God) simply means the “shining one.” In the ancient Vedas this star was known as the Chieftain’s star; and in other Hindu writings, it is referred to as Sukra – the Rain God, or Rain Star.

Sirius was revered as the Nile Star, or Star of Isis, by the ancient Egyptians. Its annual appearance just before dawn at the Summer Solstice heralded the flooding of the Nile, upon which Egyptian agriculture depended. This helical rising is referred to in many temple inscriptions, where the star is known as the Divine Sepat, identified as the soul of Isis.

To the Chinese, the stars of Canis Major were associated with several different asterisms – including the Military Market, the Wild Cockerel, and the Bow and Arrow. All of these lay in the Vermilion Bird region of the zodiac, on of four symbols of the Chinese constellations, which is associated with the South and Summer.  In this tradition, Sirius was known Tianlang (which means “Celestial Wolf”) and denoted invasion and plunder.

This constellation and its most prominent stars were also featured in the astrological traditions of the Maori people of New Zealand, the Aborigines of Australia, and the Polynesians of the South Pacific.

Isis depicted with outstretched wings in an ancient wall painting (ca. 1360 BCE). Credit: Wikipedia Commons/Ägyptischer Maler
Isis depicted with outstretched wings in an ancient wall painting (ca. 1360 BCE). Credit: Wikipedia Commons/Ägyptischer Maler

History of Observation:

This constellation was one of the original 48 that Ptolemy included in his 2nd century BCE work the Amalgest. It would remain a part of the astrological traditions of Europe and the Near East for millennia. The Romans would later add Canis Minor, appearing as Orion’s second dog, using stars to the north-west of Canis Major.

In medieval Arab astronomy, the constellation became Al Kalb al Akbar, (“the Greater Dog”), which was transcribed as Alcheleb Alachbar by European astronomers by the 17th century. In 1862, Alvan Graham Clark, Jr. made an interesting discovery while testing an 18″ refractor telescope at the Dearborn Observatory at Northwestern University in Illinois.

In the course of observing Sirius, he discovered that the bright star had a faint companion – a white dwarf later named Sirius B (sometimes called “the Pup”). These observations confirmed what Friedrich Bessel proposed in 1844, based on measurements of Sirius A’s wobble. In 1922, the International Astronomical Union would include Canis Major as one of the 88 recognized constellations.

Canis Major as depicted in Urania's Mirror, a set of constellation cards published in London c.1825. Credit: Library of Congress
Canis Major as depicted in Urania’s Mirror, a set of constellation cards published in London c.1825. Credit: Library of Congress

Notable Features:

Canis Major has several notable stars, the brightest being Sirius A. It’s luminosity in the night sky is due to its proximity (8.6 light years from Earth), and the fact that it is a magnitude -1.6 star. Because of this, it produces so much light that it often appears to be flashing in vibrant colors, an effect caused by the interaction of its light with our atmosphere.

Then there’s Beta Canis Majoris, a variable magnitude blue-white giant star whose traditional name (Murzim) means the “The Heralder”. It is a Beta Cephei variable star and is currently in the final stages of using its hydrogen gas for fuel. It will eventually exhaust this supply and begin using helium for fuel instead. Beta Canis Majoris is located near the far end of the Local Bubble – a cavity in the local Interstellar medium though which the Sun is traveling.

Next up is Eta Canis Majoris, known by its traditional name as Aludra (in Arabic, “al-aora”, meaning “the virgin”). This star shines brightly in the skies in spite of its distance from Earth (approx. 2,000 light years from Earth) due to it being many times brighter (absolute magnitude) than the Sun. A blue supergiant, Aludra has only been around a fraction of the time of our Sun, yet is already in the last stages of its life.

Another “major” star in this constellation is VY Canis Majoris (VY CMa), a red hypergiant star located in the constellation Canis Major. In addition to being one of the largest known stars, it is also one of the most luminous ever observed. It is located about 3,900 light years (~1.2 kiloparsecs) away from Earth and is estimated to have 1,420 solar radii.

VY Canis Majoris. The biggest known star.
Size comparison between the Sun and VY Canis Majoris, which once held the title of the largest known star in the Universe. Credit: Wikipedia Commons/Oona Räisänen

Canis Major is also home to several Deep Sky Objects, the most notable being Messier 41 (NGC 2287). Containing about 100 stars, this impressive star cluster contains several red giant stars. The brightest of these is spectral type K3, and located near M41’s center. The cluster is estimated to be between 190 and 240 million years old, and its is believed to be 25 to 26 light years in diameter.

Then there’s the galactic star cluster NGC 2362. First seen by Giovanni Hodierna in 1654 and rediscovered William Herschel in 1783, this magnificent star cluster may be less than 5 million years old and show shows signs of nebulosity – the remains of the gas cloud from which it formed. What makes it even more special is the presence of Tau Canis Major.

Easily distinguished as the brightest star in the cluster, Tau is a luminous supergiant of spectral type O8. With a visual magnitude of 4.39, it is 280,000 times more luminous than Sol. Tau CMa is also brighter component of a spectroscopic binary and studies of NGC 2362 suggest that it will survive longer than the Pleiades cluster (which will break up before Tau does), but not as long as the Hyades cluster.

Then there’s NGC 2354, a magnitude 6.5 star cluster. While it will likely appear as a small, hazy patch to binoculars, NGC 2354 is actually a rich galactic cluster containing around 60 metal-poor members. As aperture and magnification increase, the cluster shows two delightful circle-like structures of stars.

The Canis Major Dwarf Galaxy - the Milky Way's current dinner. Image Credit: APOD
The Canis Major Dwarf Galaxy – currently recognized as being the closet neighbor to the Milky Way. Credit: APOD

For large telescopes and GoTo telescopes, there are several objects worth studying, like the Canis Major Dwarf Galaxy (RA 7 12 30 Dec -27 40 00). An irregular galaxy that is now thought to be the closest neighboring galaxy to our part of the Milky Way, it is located about 25,000 light-years away from our Solar System and 42,000 light-years from the Galactic Center.

It has a roughly elliptical shape and is thought to contain as many stars as the Sagittarius Dwarf Elliptical Galaxy, which was discovered in 2003 and thought to be the closest galaxy at the time. Although closer to the Earth than the center of the galaxy itself, it was difficult to detect because it is located behind the plane of the Milky Way, where concentrations of stars, gas and dust are densest.

Globular clusters thought to be associated with the Canis Major Dwarf galaxy include NGC 1851, NGC 1904, NGC 2298 and NGC 2808, all of which are likely to be a remnant of the galaxy’s globular cluster system before its accretion (or swallowing) into the Milky Way. NGC 1261 is another nearby cluster, but its velocity is different enough from that of the others to make its relation to the system unclear.

Finding Canis Major:

Finding Canis Major is quite easy, thanks to the presence of Sirius – the brightest star to grace the night sky. All you need to do is find Orion’s belt, discern the lower left edge of constellation (the star Kappa Orionis, or Saiph), and look south-west a few degrees. There, shining in all it glory, will be the “Dog Star”, with all the other stars stemming outwards from it.

The location of the Canis Major constellation in the southern sky. Credit: IAU
The location of the Canis Major constellation in the southern sky. Credit: IAU

Unfortunately, Sirius A’s luminosity means that the means that poor “Pup” hardly stands a chance of being seen. At magnitude 8.5 it could easily be caught in binoculars if it were on its own. To find it, you’ll need a mid-to-large telescope with a high power eyepiece and good viewing conditions – a stable evening (not night) when Sirius is as high in the sky as possible. It will still be quite faint, so spotting it will take time and patience.

Between Sirius at the northern tip, and Adhara at the south, you can also spot M41 residing almost about halfway. Using binoculars or telescopes, all one need do is aim about 4 degrees south of Sirius – about one standard field of view for binoculars, about one field of view for the average telescope finderscope, and about 6 fields of view for the average wide field, low power eyepiece.

Thousands of years later, Canis Major remains an important part of our astronomical heritage. Thanks largely to Sirius, for burning so brightly, it has always been seen as a significant cosmological marker. But as our understanding of the cosmos has improved (not to mention our instruments) we have come to find just how many impressive stars and stellar objects are located in this region of space.

We have written many interesting articles about the constellation here at Universe Today. Here is What Are The Constellations?, What Is The Zodiac?, and Zodiac Signs And Their Dates.

Be sure to check out The Messier Catalog while you’re at it!

For more information, check out the IAUs list of Constellations, and the Students for the Exploration and Development of Space page on Canes Venatici and Constellation Families.

Sources:

 

Posted on by Tammy Plotner

The Canes Venatici Constellation

The canes venatici constellation, located in the northern skies in proximity to Bootes, Ursa Major and Coma Berenices. Credit: maps.seds.org

Welcome back to Constellation Friday! Today, in honor of the late and great Tammy Plotner, we will be dealing with Canes Venatici constellation.

In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of the then-known 48 constellations. His treatise, known as the Almagest, would be used by medieval European and Islamic scholars for over a thousand years to come. Today, this list has been expanded to include the 88 constellations recognized by the IAU.

One of these is known as Canes Venatici, a small northern constellation that is bordered by Ursa Major to the north and west, Coma Berenices to the south, and Boötes to the east. Canes Venatici belongs to the Ursa Major family of constellations, along with Boötes, Camelopardalis, Coma Berenices, Corona Borealis, Draco, Leo Minor, Lynx, Ursa Major, and Ursa Minor.

Name and Meaning:

The small northern constellation of Canes Venatici represents the hunting dogs – Chara and Asterion – of Boötes. It is also one of three constellations that represent dogs, along with Canis Major and Canis Minor. Given its comparatively recent origin, there is no real mythology associated with this asterism. However, it does have an interesting history.

Canes Venatici depicted in Hevelius's star atlas. Note that, per the conventions of the time, the image is mirrored. Credit: Wikipedia Commons/Atlas Coelestis
Canes Venatici depicted in Hevelius’s star atlas. Note that, per the conventions of the time, the image is mirrored. Credit: Wikipedia Commons/Atlas Coelestis

History of Observation:

During Classic Antiquity, the stars of Canes Venatici did not appear very brightly in the night sky. As such, they were listed by Ptolemy as unfigured stars below the constellation Ursa Major in the Almagest, rather than as a distinct constellation. During the Middle Ages, the identification of these stars as being the dogs of Boötes arose due to a mistranslation.

Some of the component stars in the nearby constellation of Boötes (which was known as the “herdsman”) were traditionally described as representing his cudgel. When the Almagest was translated from Greek to Arabic, the translator – the Arab astronomer Hunayn ibn Ishaq – did not know the Arabic word for cudgel.

As such, he chose the closest translation in Arabic – “al-`asa dhat al-kullab” -which literally means “the spearshaft having a hook” (possibly in reference to a shepherd’s crook). When the Arabic text was later translated into Latin, the translator mistook the Arabic word “kullab” for “kilab” – which means “dogs” – and wrote the name as hastile habens canes (“spearshaft having dogs”).

This representation of Boötes having two dogs remained popular and became official when, in 1687, Johannes Hevelius decided to designate them as a separate constellation. The northern of the two hunting dogs was named Asterion (‘little star’) while the southern dog was named Chara – from the Greek word for ‘joy’,.

Canes Venatici can be seen in the orientation they appear to the eyes in this 1825 star chart from Urania's Mirror. Credit: Wikipedia Commons/Library of Congress
Canes Venatici can be seen in the orientation they appear to the eyes in this 1825 star chart from Urania’s Mirror. Credit: Wikipedia Commons/Library of Congress

Notable Features:

The constellation’s brightest star is Cor Caroli, which is perhaps one of the most splendid of all colorful double stars. The name literally means “Charles’ heart”, and was named by Sir Charles Scarborough in honor of Charles I – who was executed in the aftermath of the English Civil War. The star is also associated with Charles II of England, who was restored to the throne after the interregnum following his father’s death.

Cor Caroli is a binary star with a combined apparent magnitude of 2.81 which marks the northern vertex of the Diamond of Virgo asterism. The two stars are 19.6 arc seconds apart and are easily resolved in small telescopes and steady binoculars. The system lies approximately 110 light years from Earth. It’s main star, a² Canum Venaticorum, is the prototype of a class of Spectral Type A0 variable stars (the so-called a² Canum Venaticorum stars).

These stars have a strong stellar magnetic field, which is believed to produce starspots of enormous extent. Due to these starspots, the brightness of a² Canum Venaticorum stars varies considerably during their rotation. Their brightness also varies between magnitude +2.84 and +2.98 with a period of 5.47 days.  The companion, a¹ Canum Venaticorum (a spectral type F0 star), is considerably fainter at +5.5 magnitude.

Y CVn, and a simulation of what it would look like close-up, created using Celestia. Credit: Wikipedia Commons/Kirk39
Y CVn, “La Superba”, and a simulation of what it would look like close-up, created using Celestia. Credit: Wikipedia Commons/Kirk39

Next up is Y Canum Venaticorum (Y CVn), which was named “La Superba” by 19th century astronomer Angelo Secchi for its uncommonly beautiful red color. This name was certainly appropriate, since it is  one of the reddest stars in the sky, and one the brightest of the giant red “carbon stars”.

La Superba is the brightest J-star in the sky, a very rare category of carbon stars that contain large amounts of carbon-13. Its surface temperature is believed to be about 2800 K (~2526 °C; 4580 °F), making it one of the coldest  true stars known. Its appearance, temperature and composition are all indications that it is currently in the Red Giant phase of its life-cycle.

Y CVn is almost never visible to the naked eye since most of its output is outside the visible spectrum. Yet, when infrared radiation is considered, Y CVn has a luminosity 4400 times that of the Sun, and its radius is approximately 2 AU. If it were placed at the position of our sun, the star’s surface would extend beyond the orbit of Mars.

Canes Venatici is also home to several Deep Sky Objects. For starters, there’s the tremendous globular cluster known as Messier 3 (M3). Messier 3 has an apparent magnitude of 6.2, making it visible to the naked eye. It was first resolved into stars by William Herschel around 1784. This cluster is one of the largest and brightest, made up of around 500,000 stars, and is located about 33,900 light-years away from our solar system.

The 51st entry in Charles Messier's famous catalog is perhaps the original spiral nebula--a large galaxy with a well defined spiral structure also cataloged as NGC 5194. Over 60,000 light-years across, M51's spiral arms and dust lanes clearly sweep in front of its companion galaxy, NGC 5195. Image data from the Hubble's Advanced Camera for Surveys was reprocessed to produce this alternative portrait of the well-known interacting galaxy pair. The processing sharpened details and enhanced color and contrast in otherwise faint areas, bringing out dust lanes and extended streams that cross the small companion, along with features in the surroundings and core of M51 itself. The pair are about 31 million light-years distant. Not far on the sky from the handle of the Big Dipper, they officially lie within the boundaries of the small constellation Canes Venatici. Image Credit: NASA
Messier 51, aka. the Whirlpool Galaxy, is a spiral nebula – a large galaxy with a well defined spiral structure located over 60,000 light-years across. Credit: NASA

Then there’s the Whirlpool Galaxy, also known as Messier 51 or NGC 5194. This  interacting, grand-design spiral galaxy is located at a distance of approximately 23 million light-years from Earth. It is one of the most famous spiral galaxies in the night sky, for both its grace and beauty. The galaxy and its companion (NGC 5195) are easily observed by amateur telescopes, and the two galaxies may even be seen with larger binoculars.

Canes Venatici is also home of the Sunflower Galaxy (aka. Messier 63 and NGC 5055), an unbarred spiral galaxy consisting of a central galactic disc surrounded by many short spiral arm segments. It is part of the M51 galaxy group, which also includes the Whirlpool Galaxy (M51). In the mid-1800s, Lord Rosse identified the spiral structure within the galaxy, making this one of the first galaxies in which “spiral nebulae” were identified.

Now hop over to the barred spiral galaxy known as Messier 94 for some comparison. It was discovered by Pierre Méchain in 1781 and catalogued by Charles Messier two days later. Although some references describe M94 as a barred spiral galaxy, the “bar” structure appears to be more oval-shaped. The galaxy is also notable in that it has two ring structures, an inner ring with a diameter of 70″ and an outer ring with a diameter of 600″.

These rings appear to form at resonance locations within the disk of the galaxy. The inner ring is the site of strong star formation activity and is sometimes referred to as a starburst ring. This star formation is fueled by gas that is dynamically driven into the ring by the inner oval-shaped bar-like structure.

Messier 63, also known as the Sunflower Galaxy, seen here in a new image from the NASA/ESA Hubble Space Telescope. Credit: NASA/ESA/HST
Messier 63, also known as the Sunflower Galaxy, seen here in an image from the  Hubble Space Telescope. Credit: NASA/ESA/HST

For a completely different galaxy, try Messier 106 (NGC 4258). This spiral galaxy is about 22 to 25 million light-years away from Earth. It is also a Seyfert II galaxy, which means that due to x-rays and unusual emission lines detected, it is suspected that part of the galaxy is falling into a supermassive black hole in the center. Nearby NGC 4217 is a possible companion galaxy.

The constellation does not have any stars with known planets, and there is one meteor shower associated with the constellation – the Canes Venaticids.

Finding Canes Venatici:

While it basically consists of only two bright stars, the Canes Venatici constellation is still fairly easy to locate and is bordered by Ursa Major, Boötes and Coma Berenices. It can be spotted with the naked eye on a clear night where light conditions are favorable. However, for those using binoculars, finderscopes and small telescopes, the constellation has much to offer the amateur astronomer and stargazer.

The location of the Canes Venatici constellation. Credit: IAU and Sky&Telescope magazine
The location of the Canes Venatici constellation. Credit: IAU/Sky&Telescope magazine

It’s brightest star, Cor Calroli can be found at RA 12h 56m 01.6674s Dec +38° 19′ 06.167″, while beautiful Y Canum Venaticorum (aka. “La Superba”) can be seen at RA 12f 45m 07s Dec +45° 26′ 24″. And M51 is easy to find by following the easternmost star of the Big Dipper, Eta Ursae Majoris, and going 3.5° southeast. Its declination is +47°, so it is circumpolar for observers located above 43°N latitude.

We have written many interesting articles about the constellation here at Universe Today. Here is What Are The Constellations?, What Is The Zodiac?, and Zodiac Signs And Their Dates.

Be sure to check out The Messier Catalog while you’re at it!

For more information, check out the IAUs list of Constellations, and the Students for the Exploration and Development of Space page on Canes Venatici and Constellation Families.

Sources:

Posted on by Tammy Plotner

Messier 28 – The NGC 6626 Globular Cluster

Messier 28, Messier 22 and Kaus Borealis. Credit: Wikisky

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Globular Cluster known as Messier 28. Enjoy!

Back in the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of them so that others would not make the same mistake he did. In time, this list would come to include 100 of the most fabulous objects in the night sky.

One of these objects was the globular cluster now known as Messier 28. Located in the direction of the Sagittarius constellation, some 17,900 light-years from Earth, this “nebulous” cluster is easily detectable in the night sky. It is also the third largest known clustering of millisecond pulsars in the known Universe.

Description:

Compressed into a sphere measuring about 60 light years in diameter, globular star cluster Messier 28 happily orbits our galactic center about 19,000 light years away from Earth. In all of its thousands upon thousands of stars, M28 contains 18 known RR Lyrae variables and a W Virginis variable star. This very different variable is a Type II, or population II Cepheid that has a precise change rate which occurs every 17 days.

Image based on observations made with the NASA/ESA Hubble Space Telescope, and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre (CADC/NRC/CSA).
Image of Messier 28, based on observations made with the NASA/ESA Hubble Space Telescope, and obtained from the Hubble Legacy Archive. Credit: STScI/NASA/ST-EFC/ESA/CADC/NRC/CSA

There has also been a second long period variable discovered, which could very well be an RV Tauri type, too. However, one of M28’s biggest claims to fame happened in 1986, when it became the first globular cluster known to contain a millisecond pulsar. This was discovered by the Lovell Telescope at Jodrell Bank Observatory. The work on the pulsar was later picked up by Chandra researchers.

As Martin C. Weisskopf (et al) of the Space Sciences Department put it in a 2002 study of the object:

“We report here the results of the first Chandra X-Ray Observatory observations of the globular cluster M28 (NGC 6626). We detect 46 X-ray sources of which 12 lie within one core radius of the center. We measure the radial distribution of the X-ray sources and fit it to a King profile finding a core radius. We measure for the first time the unconfused phase-averaged X-ray spectrum of the 3.05-ms pulsar B1821–24 and find it is best described by a power law with photon index. We find marginal evidence of an emission line centered at 3.3 keV in the pulsar spectrum, which could be interpreted as cyclotron emission from a corona above the pulsar’s polar cap if the magnetic field is strongly different from a centered dipole. We present a spectral analyses of the brightest unidentified source and suggest that it is a transiently accreting neutron star in a low-mass X-ray binary, in quiescence. In addition to the resolved sources, we detect fainter, unresolved X-ray emission from the central core.”

And the search has far from ended as even more X-ray counterparts have been discovered inside this seemingly quiet globular cluster! As W. Becker and C.Y. Hui of the Max Planck Institute wrote in their 2007 study:

“A recent radio survey of globular clusters has increased the number of millisecond pulsars drastically. M28 is now the globular cluster with the third largest population of known pulsars, after Terzan 5 and 47 Tuc. This prompted us to revisit the archival Chandra data on M28 to evaluate whether the newly discovered millisecond pulsars find a counterpart among the various X-ray sources detected in M28 previously. The radio position of PSR J1824-2452H is found to be in agreement with the position of CXC 182431-245217 while some faint unresolved X-ray emission near to the center of M28 is found to be coincident with the millisecond pulsars PSR J1824-2452G, J1824-2452J, J1824-2452I and J1824-2452E.”

Messier 28. Credit: NASA/ESA/HST
The globular cluster Messier 28, image by the Hubble Space Telescope. Credit: NASA/ESA/HST

So is it possible that these can be seen? According to the 2001 study – “A search for the optical counterpart to PSR B1821-24 in M 28” – by Hubble researcher A Golden (et al.):

“We have analyzed archival HST/WFPC2 images in both the F555W & F814W bands of the core field of the globular cluster M 28 in an attempt to identify the optical counterpart of the magnetospherically active millisecond pulsar PSR B1821-24. Examination of the radio derived error circle yielded several potential candidates, down to a magnitude of V $\sim$ 24.5 (V0 $\sim$ 23.0). Each were further investigated, both in the context of the CMD of M 28, and also with regard to phenomenological models of pulsar magnetospheric emission. The latter was based on both luminosity-spindown correlations and known spectral flux density behaviour in this regime from the small population of optical pulsars observed to date. None of the potential candidates exhibited emission expected from a magnetospherically active pulsar. The fact that the magnetic field & spin coupling for PSR B1821-24 is of a similar magnitude to that of the Crab pulsar in the vicinity of the light cylinder has suggested that the millisecond pulsar may well be an efficient nonthermal emitter. ASCA’s detection of a strong synchrotron-dominated X-ray pulse fraction encourages such a viewpoint. We argue that only future dedicated 2-d high speed photometry observations of the radio error-circle can finally resolve this matter.”

History of Observation:

This globular cluster was an original discovery in July 1764 of Charles Messier who wrote in his notes:

“In the night of the 26th to the 27th of the same month, I have discovered a nebula in the upper part of the bow of Sagittarius, at about 1 degree from the star Lambda of that constellation, and little distant from the beautiful nebula which is between the head and the bow: that new one may be the third of the older one, and doesn’t contain any star, as far as I have been able to judge when examining it with a good Gregorian telescope which magnifies 104 times: it is round, its diameter is about 2 minutes of arc; one sees it with difficulty with an ordinary refractor of 3 feet and a half of length. I have compared the middle with the star Lambda Sagittarii, and I have concluded its right ascension of 272d 29′ 30″, and its declination of 37d 11′ 57″ south.”

As always, Sir William Herschel would often revisit with Messier’s objects for his own private observations and in his notes he states:

“It may be called insulated though situated in a part of the heavens that is very rich in stars. It may have a nucleus, for it is much compressed towards the centre, and the situation is too low for seeing it well. The stars of the cluster are pretty numerous.” It would be his son, John Herschel who would give M28 its New General Catalog Number and describe it as “Not very bright; but very rich, excessively compressed globular cluster; stars of 14th to 15th magnitude; much brighter toward the middle; a fine object.”

The location of Messier 28, in the direction of the Sagittarius Constellation. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)
The location of Messier 28, in the direction of the Sagittarius Constellation. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

Regardless of whether or not you use binoculars or a telescope on M28, part of the joy of this object is understand how very rich the stellar field is in which it appears. As John Herschel once said of M28 in his many observations, “Occurs in the milky way, of which the stars here are barely visible and immensely numerous.”

Locating Messier 28:

Finding M28 is another easy object once you’ve familiarized yourself with the “teapot” asterism of the constellation of Sagittarius. In binoculars, simply center Lambda in the field of view and you will see Messier 28 as a small, faded grey circular area in the 1:00 position away from the marker star.

In the finderscope of telescope, you can start by centering on Lambda and go to the eyepiece and simply shift the telescope to the northwest slowly and Messier 28 will pop into view. While this globular cluster is easily bright enough to be seen in the smallest of optics, it will require at least a 4″ telescope before it begins any resolution of individual stars and telescopes in the 10″ and larger range will fully appreciate all it has to offer.

And here are the quick facts to help you get started:

Object Name: Messier 28
Alternative Designations: M28, NGC 6626
Object Type: Class IV Globular Cluster
Constellation: Sagittarius
Right Ascension: 18 : 24.5 (h:m)
Declination: -24 : 52 (deg:m)
Distance: 18.3 (kly)
Visual Brightness: 6.8 (mag)
Apparent Dimension: 11.2 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.

Sources:

Posted on by Tammy Plotner

Messier 27 – The Dumbbell Nebula

Image of the Messier 27 planetary nebula, taken by NASA's Spitzer Space Telescope. Credit: NASA/JPL-Caltech/J. Hora (Harvard-Smithsonian CfA)

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the famous and easily-spotted Dumbbell Nebula. Enjoy!

Back in the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of them so that others would not make the same mistake he did. In time, this list would come to include 100 of the most fabulous objects in the night sky.

Known today as the Messier Catalog, this work has come to be viewed as one of the most important milestones in the study of Deep Space Objects. One of these is the famed Dumbbell Nebula – also known as Messier 27, the Apple Core Nebula, and NGC 6853. Because it of its brightness, it is easily viewed with binoculars and amateur telescopes, and was the first planetary Nebula to be discovered by Charles Messier.

Description:

This bright planetary nebula is located in the direction of the Vulpecula constellation, at a distance of about 1,360 light years from Earth. Located within the equatorial plane, this nebula is essentially a dying star that has been ejecting a shell of hot gas into space for roughly 48,000 years.

Picture of M27 processed and combined using IRAF and MaxIm DL by Mohamad Abbas. Credit: Mohamad Abbas
Picture of M27 processed and combined using IRAF and MaxIm DL. Credit: Wikipedia Commons/Mohamad Abbas

The star responsible is an extremely hot blueish subdwarf star, which emits primarily highly energetic radiation in the non-visible part of the electromagnetic spectrum. This energy is absorbed by exciting the nebula’s gas, and then re-emitted by the nebula. Messier 27 particular green glow (hence the nickname “Apple Core Nebula”) is due to the presence of doubly-ionized oxygen in its center, which emits green light at 5007 Angstroms.

For many years I quested to understand the distant and mysterious M27, but no one could answer my questions. I researched it, and learned that it was made up of doubly ionized oxygen. I had hoped that perhaps there was a spectral reason to what I viewed year after year – but still no answer.

Like all amateurs, I became the victim of “aperture fever” and I continued to study M27 with a 12″ telescope, never realizing the answer was right there – I just hadn’t powered up enough. Several years later while studying at the Observatory, I was viewing through a friend’s identical 12″ telescope and, as chance would have it, he was using about twice the magnification that I normally used on the “Dumbbell.”

Imagine my total astonishment as I realized for the very first time that the faint central star had an even fainter companion that made it seem to wink! At smaller apertures or low power, this was not revealed. Still, the eye could “see” a movement within the nebula – the central, radiating star and its companion.

Image from a ground-based telescope at Westview Observatory in Cridersville, OH. Credit: Wikipedia Commons/Charlemagne920
Image from a ground-based telescope at Westview Observatory in Cridersville, OH. Credit: Wikipedia Commons/Charlemagne920

As W.G. Mathews of the University of California put it in his study “Dynamical Evolution of a Model Planetary Nebula”:

“As the gas at the inner edge begins to ionize, the pressure throughout the nebula is equalized by a shock which moves outward through the neutral gas. Later, when about 1/10 of the nebular mass is ionized, a second shock is released from the ionized front, and this shock moves through the neutral shell reaching the outer edge. The density of the HI gas just behind the shock is quite large and the outward gas velocity increases within until it reaches a maximum of 40-80 km per second just behind the shock front. The projected appearance of the nebula during this stage has a double ring structure similar to many observed planetaries.”

R.E. Lupu of John Hopkins has also made studies of motion as well, which they published in a study titled “Discovery of Lyman-alpha Pumped Molecular Hydrogen Emission in the Planetary Nebulae NGC 6853 and NGC 3132“. As they indicated, and found them to “have low surface brightness signatures in the visible and near infrared.”

But, movement or no movement, Messier 27 is known as one of the top “polluters” of the interstellar medium. As Joseph L. Hora ( et al.) of the Harvard-Smithsonian Center for Astrophysics said in his 2008 study “Planetary Nebulae: Exposing the Top Polluters of the ISM“:

“The high mass loss rates of stars in their asymptotic giant branch (AGB) stage of evolution is one of the most important pathways for mass return from stars to the ISM. In the planetary nebulae (PNe) phase, the ejected material is illuminated and can be altered by the UV radiation from the central star. PNe therefore play a significant role in the ISM recycling process and in changing the environment around them…

“A key link in the recycling of material to the Interstellar Medium (ISM) is the phase of stellar evolution from Asymptotic Giant Branch (AGB) to white dwarf star. When stars are on the AGB, they begin to lose mass at a prodigious rate. The stars on the AGB are relatively cool, and their atmospheres are a fertile environment for the formation of dust and molecules. The material can include molecular hydrogen (H2), silicates, and carbon-rich dust. The star is fouling its immediate neighborhood with these noxious emissions. The star is burning clean hydrogen fuel, but unlike a “green” hydrogen vehicle that outputs nothing except water, the star produces ejecta of various types, some of which have properties similar to that of soot from a gas-burning automobile. A significant fraction of the material returned to the ISM goes through the AGB – PNe pathway, making these stars one of the major sources of pollution of the ISM.

“However, these stars are not done with their stellar ejecta yet. Before the slow, massive AGB wind can escape, the star begins a rapid evolution where it contracts and its surface temperature increases. The star starts ejecting a less massive but high velocity wind that crashes into the existing circumstellar material, which can create a shock and a higher density shell. As the stellar temperature increases, the UV flux increases and it ionizes the gas surrounding the central star, and can excite emission from molecules, heat the dust, and even begin to break apart the molecules and dust grains. The objects are then visible as planetary nebulae, exposing their long history of spewing material into the ISM, and further processing the ejecta. There are even reports that the central stars of some PNe may be engaging in nucleosynthesis for purposes of self-enrichment, which can be traced by monitoring the elemental abundances in the nebulae. Clearly, we must assess and understand the processes going on in these objects in order to understand their impact on the ISM, and their influence on future generations of stars.”

Messier 27 and the Summer Triangle. Credit: Wikisky
Messier 27 and the Summer Triangle. Credit: Wikisky

History of Observation:

So, chances are on July 12th, 1764, when Charles Messier discovered this new and fascinating class of objects, he didn’t really have a clue as to how important his observation would be. From his notes of that night, he reports:

“I have worked on the research of the nebulae, and I have discovered one in the constellation Vulpecula, between the two forepaws, and very near the star of fifth magnitude, the fourteenth of that constellation, according to the catalog of Flamsteed: One sees it well in an ordinary refractor of three feet and a half. I have examined it with a Gregorian telescope which magnified 104 times: it appears in an oval shape; it doesn’t contain any star; its diameter is about 4 minutes of arc. I have compared that nebula with the neighboring star which I have mentioned above [14 Vul]; its right ascension has been concluded at 297d 21′ 41″, and its declination 22d 4′ 0″ north.”

Of course, Sir William Herschel’s own curiosity would get the better of him and although he would never publish his own findings on an object previously cataloged by Messier, he did keep his own private notes. Here is an excerpt from just one of his many observations:

“1782, Sept. 30. My sister discovered this nebula this evening in sweeping for comets; on comparing its place with Messier’s nebulae we find it is his 27. It is very curious with a compound piece; the shape of it though oval as M. [Messier] calls it, is rather divided in two; it is situated among a number of small [faint] stars, but with this compound piece no star is visible in it. I can only make it bear 278. It vanishes with higher powers on account of its feeble light. With 278 the division between the two patches is stronger, because the intermediate faint light vanishes more.”

So where did Messier 27 get its famous moniker? From Sir John Herschel, who wrote: “A most extraordinary object; very bright; an unresolved nebula, shaped something like an hour-glass, filled into an oval outline with a much less dense nebulosity. The central mass may be compared to a vertebra or a dumb-bell. The southern head is denser than the northern. One or two stars seen in it.”

It would be several years, and several more historical astronomers, before the true nature of Messier 27 would even be hinted at. At one level, they understood it to be a nebula – but it wasn’t until 1864 when William Huggins came along and began to decode the mystery:

“It is obvious that the nebulae 37 H IV (NGC 3242), Struve 6 (NGC 6572), 73 H IV (NGC 6826), 1 H IV (NGC 7009), 57 M, 18 H. IV (NGC 7662) and 27 M. can no longer be regarded as aggregations of suns after the order to which our own sun and the fixed stars belong. We have with these objects to do no longer with a special modification only of our own type of suns, but find ourselves in the presence of objects possessing a distinct and peculiar plan of structure. In place of an incandescent solid or liquid body transmitting light of all refrangibilities through an atmosphere which intercepts by absorption a certain number of them, such as our sun appears to be, we must probably regard these objects, or at least their photo-surfaces, as enormous masses of luminous gas or vapour. For it is alone from matter in the gaseous state that light consisting of certain definite refrangibilities only, as is the case with the light of these nebulae, is known to be emitted.”

Whether or not you enjoy M27 as one of the most superb planetary nebula in the night sky (or as a science object) you will 100% agree with the words of of Burnham: “The observer who spends a few moments in quiet contemplation of this nebula will be made aware of direct contact with cosmic things; even the radiation reaching us from the celestial depths is of a type unknown on Earth…”

Locating Messier 27:

When you first begin, Messier 27 will seem like such an elusive target – but with a few simple sky “tricks”, it won’t be long until you’ll be finding this spectacular planetary nebula under just about any sky conditions. The hardest part is simply sorting out all the stars in the area to know the right ones to aim at!

The way I found easiest to teach others was to start BIG. The cruciform patterns of the Cygnus and Aquila constellations are easy to recognize and can be seen from even urban locations. Once you’ve identified these two constellations, you’re going smaller by locating Lyra and the tiny kite-shape of Delphinus.

Now you’ve circled the area and the hunt for Vulpecula the Fox begins! What’s that you say? You can’t distinguish Vulpecula’s primary stars from the rest of the field? You’re right. They don’t stand out like they should, and being tempted to simply aim halfway between Albeireo (Beta Cygni) and Alpha Delphini is too much of a span to be accurate. So what are we going to do? Here’s where some patience comes into play.

If you give yourself time, you’ll begin to notice the stars of Sagitta are ever so slightly brighter than the rest of the field stars around it, and it won’t be long until you pick out that arrow pattern. In your mind, measure the distance between Delta and Gamma (the 8 and Y shape on a starfinder map) and then just aim your binoculars or finderscope exactly that same distance due north of Gamma.

The location of M27 in the constellation Vulpecula. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)
The location of M27 in the constellation Vulpecula. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

You’ll find M27 every time! In average binoculars it will appear as a fuzzy, out of focus large star in a stellar field. In the finderscope, it may not appear at all… But in a telescope? Be prepared to be blown away! And here are the quick facts on the Dumbbell Nebula to help get you started:

Object Name: Messier 27
Alternative Designations: M27, NGC 6853, The Dumbbell Nebula
Object Type: Planetary Nebula
Constellation: Vulpecula
Right Ascension: 19 : 59.6 (h:m)
Declination: +22 : 43 (deg:m)
Distance: 1.25 (kly)
Visual Brightness: 7.4 (mag)
Apparent Dimension: 8.0×5.7 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.

Sources:

Posted on by Tammy Plotner

Messier 26 – The NGC 6694 Open Star Cluster

Messier 26 and Delta Scuti. Credit: WIkisky

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at Messier 26 open star cluster. Enjoy!

Back in the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of these objects so that others wouldn’t make the same mistake. Consisting of 100 objects, the Messier Catalog would come to be viewed by posterity as a major milestone in the study of Deep Space Objects.

One of these objects is Messier 26, an open star cluster located about 5,000 light years from the Earth in the direction of the Scutum Constellation. While somewhat faint compared to other objects that share its section of the sky, this star field remains a source of mystery to astronomers, thanks to what appears to be a low-density star field at its nucleus.

Description:

When this cloud of stars formed some 89 million years ago, it was probably far more compact than today’s size of a 22 light year span. At a happy distance of about 5,000 light years from our solar system, we can’t quite see into the nucleus to determine just how dense it may actually be because of an obscuring cloud of interstellar matter.

The Open Star Cluster, Messier 26. Credit: Wikisky
Image of the Messier 26 Open Star Cluster. Credit: Wikisky

However, we do know a little bit about the stars contained within it. As astronomer James Cuffey suggested in a paper titled “The Galactic Clusters NGC 6649 and NGC 6694“, which appeared in July 1940 issue of The Astrophysical Journal:

“The relations between color and apparent magnitude show that NGC 6694 contains a well-defined main sequence and a slight indication of a giant branch. A zone of low star density 3′ from the center of NGC 6694 is noted. The ratio between general and selective absorption is estimated from the available data on red color indices in obscured clusters. Although uncertain in many cases, the results tend to confirm the ratio predicted by the law of scattering.”

However boring a field of stars may look upon first encounter, studies are important to our understanding how our galaxy evolved and the timeline incurred. As Kayla Young of the Manhasset Science Research team said:

“Star Clusters are unique because all of the stars in the cluster essentially have the same age and are roughly the same distance from Earth. Therefore, the purpose was to determine if a correlation exists between mean absolute magnitude and age of a star cluster. The absolute magnitude for star cluster NGC 6694 was calculated to be about 1.34 + .9. Using the B-V (Photometric Analysis) data ages were also calculated. After a scatter plot was created, the line of best fit demonstrated an exponential relation between the age and absolute magnitude.”

The M26 Open Star Cluster. Credit: NOAO/AURA/NSF
The M26 Open Star Cluster. Credit: NOAO/AURA/NSF

History of Observation:

Messier 26 was first observed by Charles Messier himself on June 20th, 1764. As he wrote of the discovery at the time:

“I discovered another cluster of stars near Eta and Omicron in Antinous [now Alpha and Delta Scuti] among which there is one which is brighter than the others: with a refractor of three feet, it is not possible to distinguish them, it requires to employ a strong instrument: I saw them very well with a Gregorian telescope which magnified 104 times: among them one doesn’t see any nebulosity, but with a refractor of 3 feet and a half, these stars don’t appear individually, but in the form of a nebula; the diameter of that cluster may be 2 minutes of arc. I have determined its position with regard to the star o of Antinous, its right ascension is 278d 5′ 25″, and its declination 9d 38′ 14″ south.”

Later, Bode would report a few stars with nebulosity – a field that simply wouldn’t resolve to his telescope. William Herschel would spare it but only a brief glance, saying: “A cluster of scattered stars, not rich.” While John Herschel would later go on to class it with its NGC designation, it was Admiral Smyth who would most aptly describe M26 for the true galactic cluster we know it to be. As he wrote upon viewing it in April of 1835:

“A small and coarse, but bright, cluster of stars, preceding the left foot of Antinous, in a fine condensed part of the Milky Way; and it follows 2 Aquilae by only a half degree. The principle members of this group lie nearly in a vertical position with the equatorial line, and the place is that of a small pair in the south, or upper portion of the field [in telescope]. This neat double star is of the 9th and 10th magnitudes, with an angle [PA] = 48 deg, and is followed by an 8th [mag star], the largest [brightest] in the assemblage, by 4s. Altogether the object is pretty, and must, from all analogy, possess affinity among its various components; but the collocation and adjustment of these wondrous firmamental clusters, and their probable distances, almost stun our present faculties. There are many astral splashes in this crowded district of the Galaxy, among which fine specimens of what may be termed luminiferous ether, are met with.”

The location of Messier 26 within the Scutum Constellation. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)
The location of Messier 26 within the Scutum Constellation. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

Locating Messier 26:

Finding Messier 26 in binoculars is easy as far as location goes – but not so easy distinguishing it from the starfield. Begin with the constellation of Aquila and its brightest star – Alpha. As you move southwest, count the stars down the Eagle’s back. When you reach three you are at the boundary of the constellation of Scutum. While maps make Scutum’s stars appear easy to find, they really aren’t.

The next most easily distinguished star in the line in Alpha Scutii. Aim your binoculars or finderscope there and you’ll see northern Epsilon and southern Delta to the east. Messier 26 is slightly southeast of Delta and will appear as a slight compression in the starfield, and you will be able to resolve a few individual stars to larger ones. Using a finderscope, it will appear as a very vague brightening – perhaps not seen at all depending on your finder’s aperture.

In even a small telescope, however, you’ll be pleased with what you see! Medium magnification will light up this 8th magnitude galactic star cluster and mid-sized instruments will fully resolve it. Power up! See how many stars you can – and can’t – resolve in this dusty, curtained, distant beauty!

And here are the quick facts to help you on your way!

Object Name: Messier 26
Alternative Designations: M26, NGC 6694
Object Type: Open Galactic Star Cluster
Constellation: Scutum
Right Ascension: 18 : 45.2 (h:m)
Declination: -09 : 24 (deg:m)
Distance: 5.0 (kly)
Visual Brightness: 8.0 (mag)
Apparent Dimension: 15.0 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.

Sources:

Posted on by Tammy Plotner

The Cancer Constellation

The constellation Cancer as it can be seen by the naked eye. Credit: AlltheSky/Till Credner

Welcome back to Constellation Friday! Today, we will be dealing with one of the best-known constellations, that crabby asterism known as “Cancer”!

In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of the then-known 48 constellations. His treatise, known as the Almagest, would be used by medieval European and Islamic scholars for over a thousand years to come. One of these constellations is Cancer, which is represented by “the Crab”.

As one of the twelve constellations of the zodiac, this medium-sized constellation is located on the ecliptic plane, where it is bordered by Gemini to the west, Lynx to the north, Leo Minor to the northeast, Leo to the east, Hydra to the south, and Canis Minor to the southwest. Today, it is one of the 88 constellation that are recognized by the International Astronomical Union (IAU) today.

Name and Meaning:

In mythology, Cancer was part of the Twelve Labors of Hercules. While Hercules was busy fighting the multi-headed monster (Hydra), the goddess Hera – who did not like Hercules – sent the Crab to distract him. Cancer grabbed onto the hero’s toe with its claws, but was crushed by Hercule’s mighty foot. Hera, grateful for the little crustacean’s heroic sacrifice, gave it a place in the sky. Given that the crab did not win, the gods didn’t give it any bright stars.

The planets, including Earth, orbit within a relatively flat plane. As we watch them cycle through their orbits, two or more occasionally bunch close together in a conjunction. We see them projected against the
Illustration of the ecliptic of the Solar System, showing the position of the twelve constellations of the zodiac. Credit: Bob King

History of Observation:

The first recorded examples of the Cancer constellation come from the 2nd millennium BCE, where it was known to Akkadian astronomers as the “Sun of the South”. This was most likely due to its position at the summer solstice during ancient antiquity. By classical antiquity, Cancer came to be called the “Gate of Men”, based on the beleif that it was the portal through which souls came and went from the heavens.

Given its relative faintness in the night sky, Cancer was often described as the “Dark Sign” throughout history. For instance, the medieval Italian poet Dante alluded to its faintness and position of Cancer in heavens as follows (in the Paradiso section of The Divine Comedy):

“Then a light among them brightened,
So that, if Cancer one such crystal had,
Winter would have a month of one sole day.”

Cancer’s stature as a constellation of the Zodiac has remained steadfast over the millennia, thought its position has changed. Over two thousand years ago, the sun shone in front of the constellation during the Northern Hemisphere’s summer solstice. Today, the Sun resides in front of the constellation Taurus when the summer solstice sun reaches its northernmost point.

ancer’s stature as a constellation of the Zodiac has remained steadfast over the millennia. Over two thousand years ago, the sun shone in front of the constellation Cancer during the Northern Hemisphere’s summer solstice. That’s not the case today, however. Today, the sun resides in front of the constellation Taurus when the summer solstice sun reaches its northernmost point for the year on or near June 21. Nonetheless, Cancer still seems to symbolize the height and glory of the summer sun. To this day, we say the sun shines over the Tropic of Cancer – not the “tropic of Taurus” – on the June solstice. That’s in spite of the fact that the sun in our time passes in front of the constellation Cancer from about July 21 until August 10. Dates of sun’s entry into each constellation of the Zodiac Nowadays, the sun doesn’t enter the constellation Cancer until about a month after the Northern Hemisphere’s summer solstice. Credit: US Library of Congress
Cancer as depicted in Urania’s Mirror, a set of constellation cards published in London c.1825. Credit: US Library of Congress

Notable Features:

Though comparatively faint, the Cancer constellation contains several notable stars. For starters, there is Beta Cancri, which is also known by the Arabic name of Al Tarf (“the eye” or “the glance”). Beta Cancri is the brightest star in Cancer and is about 660 times brighter than our Sun.

This K-class orange giant star is about 290 light years away from Earth, and is part of a binary system that includes a 14th magnitude star. This second star is so far away – about 65 times the distance of Pluto from the Sun – that their orbital period is at least 76,000 years!

Then there is Delta Cancri – an orange giant star approximately 180 light-years away. This is the second-brightest star in the Cancer constellation, and also where the famous Beehive Cluster (Messier 44) can be found (see below). It is also known by its Latin name of Asellus Australis, which means “southern donkey colt” (or “southern ass” if you’re feeling comedic!).

A bit further north is Gamma Cancri, an A-type white subgiant located 158 light years from Earth. Its Latin name is Asellus Borealis, which means (you guessed it!) “northern ass”. Both this star and Delta Cancri are significant because of their mythological connection and proximity to Messier 44.

Next up is Alpha Cancri, the fourth brightest star in the constellation, which is also known as Acubens. The star also goes by the names of Al Zubanah or Sertans, which are derived from the Arabic az-zub?nah (which means “claws”), while Sertan is derived from sara??n, which means “the crab.” Located approximately 174 light years from Earth, Alpha Cancris is actually a multiple star system – Alpha Cancri A and B (a white A-type dwarf and an 11th magnitude star, respectively.

Messier 44, otherwise known as the Beehive Cluster. Credit & Copyright: Bob Franke
Messier 44, otherwise known as the Beehive Cluster. Credit & Copyright: Bob Franke

Cancer is also home to many Deep Sky Objects. For instance, there is the aforementioned Beehive Cluster (Messier 44). This open cluster is the nearest of its type relative to our Solar System, and contains a larger star population than most other nearby clusters. Under dark skies the Beehive Cluster looks like a nebulous object to the unaided eye; thus it has been known since ancient times.

The classical astronomer Ptolemy called it “the nebulous mass in the heart of Cancer,” and it was among the first objects that Galileo studied with his telescope. The cluster’s age and proper motion coincide with those of the Hyades stellar association, suggesting that both share a similar origin. Both clusters also contain red giants and white dwarfs, which represent later stages of stellar evolution, along with main sequence stars of spectral classes A, F, G, K, and M.

So far, eleven white dwarfs have been identified, representing the final evolutionary phase of the cluster’s most massive stars, which originally belonged to spectral type B. Brown dwarfs, however, are extremely rare in this cluster, probably because they have been lost by tidal stripping from the halo.

Then there’s M67, which can be viewed due west of Alpha Cancri. M67 is not the oldest known galactic cluster, but there are very few in the Milky Way known to be older. M67 is an important laboratory for studying stellar evolution, since all its stars are at the same distance and age, except for approximately 30 anomalous blue stragglers, whose origins are not fully understood.

The Messier 67 star cluster, one of the oldest known open star clusters. located in the constellation Cancer. Credit & Copyright: Noel Carboni/Greg Parker
The Messier 67 star cluster, one of the oldest known open star clusters. located in the constellation Cancer. Credit & Copyright: Noel Carboni/Greg Parker

M67 has more than 100 stars similar to the Sun and many red giants, though the total star count has been estimated at over 500. The cluster contains no main sequence stars bluer than spectral type F, since the brighter stars of that age have already left the main sequence. In fact, when the stars of the cluster are plotted on the Hertzsprung-Russell diagram, there is a distinct “turn-off” representing the stars which are just about to leave the main sequence and become red giants.

It appears that M67 does not contain an unbiased sample of stars. One cause of this is mass segregation, the process by which lighter stars (actually, systems) gain speed at the expense of more massive stars during close encounters, which causes the lighter stars to be at a greater average distance from the center of the cluster or to escape altogether.

Then there’s NGC 2775, which is positioned some 60 million light years away. NGC 2775 is a peculiar blend of spiral galaxy with a smooth bulge in the center. The star formation is confined to this ring of tightly wound arms, and the galaxy has been the location of 5 supernovae explosions in the past 30 years!

Next up is DX Cancri, a faint, magnitude 14, cool red dwarf star that has less than 9% the mass of our Sun. It is a flare star that has intermittent changes in brightness by up to a five-fold increase. This star is far too faint to be seen with the naked eye, even though it is the 18th closest star system to the Sun at a distance of 11.82 light years, and is the closest star in the constellation Cancer.

Artist’s impression of the super-Earth 55 Cancri e in front of its parent star. Credit: ESA/NASA
Artist’s impression of the super-Earth 55 Cancri e in front of its parent star. Credit: ESA/NASA

Now set your mark on 55 Cancri (located at RA 8 52 35 Dec +28 19 59). Also known as Rho1 Cancri, this binary star system is located approximately 41 light-years away from Earth and has a whole solar system of its own! The system consists of a yellow dwarf star and a smaller red dwarf star, separated by over 1,000 times the distance from the Earth to the Sun.

As of 2007, five extrasolar planets have been confirmed to be orbiting the primary – 55 Cancri A (the yellow dwarf). The innermost planet is thought to be a terrestrial “super-Earth” planet, with a mass similar to Neptune, while the outermost planets are thought to be Jovian planets with masses similar to Jupiter.

Finding Cancer:

As one of the 12 constellations along the ecliptic, Cancer is relatively easy to find with small telescopes and even binoculars. It lies in the second quadrant of the northern hemisphere (NQ2) and can be seen at latitudes between +90° and -60°. It occupies an area of 506 square degrees, making it the 31st largest constellation in the night sky.

There is only one meteor shower associated with the constellation of Cancer. The peak date for the Delta Cancrids is on or about January 16th. The radiant, or point of origin is just west of Beehive. It is a minor shower and the fall rate averages only about 4 per hour and the meteors are very swift.

The location of the Caner constellation. Credit: IAU
The location of the Caner constellation. Credit: IAU

Like all of the traditional constellations that belong to the Zodiac family, the significance of Cancer has not waned, despite the passage of several thousand years. Best of luck finding it, though you won’t need much!

We have written many interesting articles about the constellation here at Universe Today. Here is What Are The Constellations?, What Is The Zodiac?, and Zodiac Signs And Their Dates.

Be sure to check out The Messier Catalog while you’re at it!

For more information, check out the IAUs list of Constellations, and the Students for the Exploration and Development of Space page on Cancer and Constellation Families.

Sources:

Posted on by Tammy Plotner

Messier 25 – The IC 4725 Open Cluster

Messier 25, shown in proximity to the Sagittarius Constellation. Credit: Wikisky

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Messier 25 open star cluster. Enjoy!

Back in the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of these objects so that other astronomers wouldn’t make the same mistake. Consisting of 100 objects, the Messier Catalog has come to be viewed as a major milestone in the study of Deep Space Objects.

One of these objects is Messier 25, an open star cluster located in the direction of the Sagittarius Constellation. At  a distance of about 2000 light years from Earth, it is one of the few Messier Objects that is visible to the naked eye (on a clear night when light conditions are favorable).

Description:

This galactic star cluster was originally discovered by Philippe Loys de Cheseaux in 1745 and included in Charles Messier’s catalog in 1764. Oddly enough, it was one of those curious objects that didn’t get cataloged by Sir John Herschel – therefore it never received a New General Catalog (NGC) number.

This is odd, considering that it was part of the 1777 catalog of Johann Elert Bode, observed by William Herschel in 1783, written about by Admiral Smyth in 1836 and even commented on by the Reverend Thomas William Webb in 1859! It was until J.L.E. Dreyer in 1908 that poor little M25 ended up getting added to the second Index Catalog.

Messier 25. Atlas Image mosaic obtained as part of the Two Micron All Sky Survey (2MASS), a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.
Atlas Image mosaic of Messier 25,obtained as part of the Two Micron All Sky Survey (2MASS). Credit: Univ. of Mass./IPAC/Caltech/NASA/NSF

Cruising along peacefully about 2,000 light-years away from Earth, this little group of stars spans across about 19 light years of space. Caught inside of its influence are four giant stars – two of spectral type M and two of type G. As we know, it contains the variable star U Sagittarii, a Delta Cephei-type, which lets us know this group of 86 or so stars may have began life together as long ago as 90 million years.

But how many stars are really in there? If you’re using a large aperture telescope, you’re probably detecting the signature of several just beyond the threshold limits. And so has more recent scientific studies. According to a study by A.L. Tadross (et al.) of the National Research Institute of Astronomy and Geophysics:

“The young open star cluster M25 (IC 4725) is located in the direction of the galactic center in a crowded region, near much irregular absorption features on Sagittarius arm. This cluster has some difficult observing problems due to its southern location. The mass data available in the literature have been gathered to re investigate this cluster using most photometric tools to determine its main photometric parameters. More than 220 stars with mean reddening of 0.50 mag and absorption of 1.62 mag are found within the cluster.”

Core region of the Messier 25 open star cluster. Credit: Sergio Eguivar
Core region of the Messier 25 open star cluster. Credit: Sergio Eguivar

And how many of those stars are surprises? Let’s try a few blue straggler stars. According to a study titled “Blue Stragglers, Be stars and X-ray binaries in open clusters“, by A. Marco (et al):

“Combination of high-precision photometry and spectroscopy allows the detailed study of the upper main sequence in open clusters. We are carrying out a comprehensive study of a number of clusters containing Be stars in order to evaluate the likelihood that a significant number of Be stars form through mass exchange in a binary. Our first results show that most young open clusters contain blue stragglers. In spite of the small number of clusters so far analyzed, some trends are beginning to emerge.In younger open clusters, such as NGC 869 and NGC 663, there are many blue stragglers, most of which are not Be stars. In older clusters, such as IC 4725, the fraction of Be stars among blue stragglers is very high. Two Be blue stragglers are moderately strong X-ray sources, one of them being a confirmed X-ray binaries. Such objects must have formed through binary evolution. We discuss the contribution of mass transfer in a close binary to the formation of both blue stragglers and Be stars.”

History of Observation:

Perhaps we know more about it today than our historic antecedents, but our knowledge of its existence is owed to astronomers like Charles Messier, who took the time to catalog it. As he wrote in his notes:

“In the same night, June 20 to 21, 1764, I have determined the position of another star cluster in the vicinity of the two preceding, between the head and the extremity of the bow of Sagittarius, and almost on the same parallel as the two others: the closest known star is that of the sixth magnitude, the twenty-first of Sagittarius, in the catalog of Flamsteed: this cluster is composed of small stars which one sees with difficulty with an ordinary refractor of 3 feet: it doesn’t contain any nebulosity, and its extension may be 10 minutes of arc. I have determined its position by comparing with the star Mu Sagittarii; its right ascension has been found at 274d 25′, and its declination at 19d 5′ south.”

Finder Chart for M25 (also shown M8->M9, M16->M18, M20->M24 and M28). Credit: freestarcharts
Finder Chart for M25 (also shown M8->M9, M16->M18, M20->M24 and M28). Credit: freestarcharts

Perhaps William Herschel understood there was more there to be seen, for he commented in his unpublished notes; “Very large, bright, stars and some small, faint ones; I counted 70, and there are many more within no considerable extent.”

Yet, it was Admiral Smyth who really understood what lay beyond. From his observations, he wrote:

“A loose cluster of large and small stars in the Galaxy, between the Archer’s head and Sobieski’s shield; of which a pair og 8th magnitudes, the principle of a set something in the form of a jew’s harp, are above registered. The gathering portion of the group assumes an arched form, and is thickly strewn in the south, on the upper part, where a pretty knot of minute glimmers occupies the center, with much star-dust around. It was discovered in 1764 by Messier, and estimated by him at 10′ in extent: it is 5 deg to the north-east of Mu Sagittarii, and nearly on the parallel of Beta Scorpii, which glimmers far away in the west.”

Locating Messier 25:

Finding Messier 25 with binoculars is quite easy. Simply start at the teapot “lid” star, Lambda, and aim about a fist width almost due north. Here you will encounter a a Cepheid variable – U Sagittarii. This one is a quick change artist, going from magnitude 6.3 to 7.1 in less than seven days, so although it is a cluster member, it may fade on you from time to time as a marker star!

The location of Messier 25. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)
Location of Messier 25 and other Deep Sky Objects in proximity to the Sagittarius Constellation. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

M25 will appear a a loose, but bright association of stars in binoculars and as a faint hazy spot in binoculars – but behold incredible resolution in a telescope. You’ll love the different magnitudes, so stick to around low to medium magnifications to enjoy it most.

As always, here are the quick facts. Enjoy!

Object Name: Messier 25
Alternative Designations: M25, IC 4725
Object Type: Open Galactic Star Cluster
Constellation: Sagittarius
Right Ascension: 18 : 31.6 (h:m)
Declination: -19 : 15 (deg:m)
Distance: 2.0 (kly)
Visual Brightness: 4.6 (mag)
Apparent Dimension: 32.0 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.

Sources:

Posted on by Tammy Plotner

Messier 24 – the Sagittarius Star Cloud

M24 (the Small Sagittarius Cloud) and nearby Messier Objects. Credit: Wikisky

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Messier 24 star cloud. Enjoy!

Back in the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of these objects so that other astronomers wouldn’t make the same mistake. Consisting of 100 objects, the Messier Catalog has come to be viewed as a major milestone in the study of Deep Space Objects.

One such object is Messier 24, otherwise known as the Sagittarius Star Cloud (or Delle Caustiche). Located in the Sagittarius constellation, located approximately 100,000 light years from Earth, this cluster of the Milky Way is one of the densest concentration of individual stars in the night sky.

Description:

Messier 24 is one of the most curious of the catalog entries because it really isn’t a star cluster – simply an oddity. What we are looking at is thousands of stars that belong to the Sagittarius arm of the Milky Way galaxy seen through a chance hole in the gas and dust… a clear “window” in space.

Messier 24 (Sagittarius Star Cloud, Delle Caustiche), showing other objects like the dark nebula Barnard 92, the dark nebula Barnard 93, and the open cluster NGC 6603. Credit: Wikipedia Commons/Tomasmazon
Messier 24 (Sagittarius Star Cloud, Delle Caustiche), also showing like the dark nebula Barnard 92, the dark nebula Barnard 93, and the open cluster NGC 6603. Credit: Wikipedia Commons/Tomasmazon

And speaking of space, M24 fills a space of significant volume, to a depth of 10,000 to 16,000 light-years. This makes it the most dense concentration of individual stars visible using binoculars, with around 1,000 stars visible within a single field of view!

Still, it is sometimes referred to as the Small Sagittarius Star Cloud in order to differentiate it from the Great Sagittarius Star Cloud located north of Gamma Sagittarii and Delta Sagittarii. When viewing this awesome area, take into account how many different objects you can spot just within this region – like dim open cluster, NGC 6603.

E.E. Barnard has cataloged two dark nebulae in the northern region as objects 92 and 93. How about lesser known clusters like Collinder 469 and Markarian 38? Along the southern edge you’ll find emission nebula IC 1283-1284, with two adjacent reflection nebulae, NGC 6589 and NGC 6590.

Their fueling source is the notable little open cluster NGC 6595. Take a tour on the western edge of M24 and see if you can spot 12th-magnitude planetary nebula NGC 6567. Need more? Then how about Delta Cephei variable WZ Sagittarii in the southern area. Its a pulsating giant star that varies in brightness between magnitude 7.5 and 8.5 in slightly less than 22 days!

The Sagittarius constellation. Credit: iau.org
The Sagittarius constellation. Credit: iau.org

History of Observation:

As bright as the Sagittarius Star Cloud is, we know that Messier probably wasn’t the first to see it – but he was the first to catalog it. As he wrote about it in his notes:

“In the same night, June 20 to 21, 1764, I have discovered on the same parallel as the star cluster I have just been talking about and near the extremity of the bow of Sagittarius, in the milky way, a considerable nebulosity, of about one degree and a half extension: in that nebulosity there are several stars of different magnitudes; the light which is between these stars is divided in several parts. I have determined approximately the position of the middle of this cloud of light; its right ascension is 270d 26′, and its declination 18d 26′, south.”

While other historic astronomers would also look at Messier’s “discovery”, they realized they were looking at a portion of the Milky Way and were somewhat less than enthusiastic. The Sagittarius Star Cloud was named “Delle Caustiche” by Fr. Secchi, “from the peculiar arrangement of its stars in rays, arches, caustic curves, and intertwined spirals.”

As is often the case with Messier Objects, it was the late Admiral Smyth who described it with flowering prose. As he wrote of the large star cloud in July of 1835:

“A beautiful field of stars, below the sinister base of the Polish shield, and in a richly clustering portion of the Milky Way. This object was discovered by Messier in 1764, and described as a mass of stars — a great nebulosity of which the light is divided in several parts. This was probably owing to want of power in the instruments used, as the whole is fairly resolvable, though there is a gathering spot with much star dust [This is NGC 6603!].”

M22, located in the direction of the Sagittarius constellation, shares that region of the sky with many Deep Sky Objects. Credit: freestarcharts.com
M24, located in the direction of the Sagittarius constellation, shares that region of the sky with many Deep Sky Objects. Credit: freestarcharts.com

Locating Messier 24:

From a dark sky location, M24 is easily located with the unaided eye. It will appear as a large hazy patch in northern portion of the constellation of Sagittarius, about a handspan above the teapot-shaped Sagittarius asterism. For those observing under urban skies, even the slightest optical aid will easily reveal this massive cloud of stars.

Spanning a degree and a half of sky means this huge object is going to cover anywhere from about 1/3 to 1/2 the field of view in most binoculars. It can easily be seen in all optical finderscopes and requires minimum magnification in all telescopes. Even then, you’ll only be able to study portions of the Sagittarius Star Cloud at a time. given its sheer size.

So go forth, and gather ye some star dust of your own. There’s plenty for everyone!

And here are the quick facts on the Sagittarius Stat Cloud to help you get started:

Object Name: Messier 24
Alternative Designations: M24, IC 4715, Sagittarius Star Cloud, Delle Caustiche
Object Type: Star Cloud – contains Open Cluster NGC 6603 and NGC 6595, Barnard 92, Barnard 93, Collinder 469, IC 1283-1284, NGC 6589/90 and planetary nebula NGC 6567
Constellation: Sagittarius
Right Ascension: 18 : 16.9 (h:m)
Declination: -18 : 29 (deg:m)
Distance: 10.0 (kly)
Visual Brightness: 4.6 (mag)
Apparent Dimension: 90 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.

Sources:

Posted on by Tammy Plotner

Messier 23 – The NGC 6494 Open Star Cluster

Messier 23, Messier 21, Trifid Nebula (M20) and Omega Nebula (M17). Credit: Wikisky

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Messier 23 open star cluster. Enjoy!

Back in the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of these objects so that other astronomers wouldn’t make the same mistake. Consisting of 100 objects, the Messier Catalog has come to be viewed as a major milestone in the study of Deep Space Objects.

One of these objects is Messier 23 (aka. NGC 6494), a large open star cluster that is located in the constellation Sagittarius. Given its luminosity, it can be found quite easily in the rich star fields of the summer Milky Way using small telescopes and even binoculars.

Description:

Located some 2,150 light years (659 Parsecs) away from Earth, this vast cloud of 176 confirmed stars stretches across 15 to 20 light years of space. At an estimated 220 to 300 million years old, Messier 23 is on the “senior citizen” list of galactic open clusters in our galaxy. At this age, its hottest stars reach spectral type B9, and it even contains a few blue straggler candidates.

Messier 23. Atlas Image mosaic obtained as part of the Two Micron All Sky Survey (2MASS), a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.
Mosaic image obtained as part of the Two Micron All Sky Survey (2MASS). Credit: UofM/IPAC/Caltech/NASA/NSF

Given that M23 has spent many centuries sweeping through the interstellar medium, astronomers have wondered how this would affect its metal content. Using UBV photometry, astronauts examined the metallicity of M23, and determined that it had no discernible effect. As W.L. Sanders wrote of the cluster in 1990:

“UBV photometric observations of 176 stars in the galactic cluster NGC 6494 are presented and analyzed. The effect of a gas poor environment on the metal abundance of NGC 6494 is studied. It is determined that the metallicity of NGC 6494, which has a delta(U – B) value = + 0.02, is not affected by the interarm region in which it dwelled.”

At the same time, astronomers have discovered that some of M23’s older stars – the red giants – are suffering mass loss. As G. Barbaro (et al.) of the Istituto di Fisica dell’Universita put it in 1969:

“A statistical research on evolved stars beyond hydrogen exhaustion is performed by comparing the H-R diagrams of about 60 open clusters with a set of isochronous curves without mass loss derived from Iben’s evolutionary tracks and time scales for Population I stars. Interpreting the difference in magnitude between the theoretical positions thus calculated and the observed ones as due to mass loss, when negative, the results indicate that this loss may be conspicuous only for very massive and red stars. However, a comparison with an analogous work of Lindoff reveals that the uncertainties connected with the bolometric and color corrections may invalidate by a large amount the conclusions which might be drawn from such research.”

Close-up image of the core of M23, showing some of its brightest member stars. Credit: Sharp/NOAO/AURA/NSF
Close-up of the core of M23, showing some of its brightest member stars. Credit: Sharp/NOAO/AURA/NSF

However, the most recent studies show that we have to determine radial velocities before we can really associate red giants as being cluster members. J.C. Mermilliod of Laboratoire d’Astrophysique de l’Ecole wrote in his 2008 study, “Red giants in open clusters“:

“The present material, combined with recent absolute proper motions, will permit various investigation of the galactic distribution and space motions of a large sample of open clusters. However, the distance estimates still remain the weakest part of the necessary data. This paper is the last one in this series devoted to the study of red giants in open clusters based on radial velocities obtained with the CORAVEL instruments.”

History of Observation:

This neat and tidy galactic star cluster was one of the original discoveries of Charles Messier. As he recorded of the cluster when first viewing it, which occurred on June 20th, 1764:

“In the night of June 20 to 21, 1764, I determined the position of a cluster of small stars which is situated between the northern extremity of the bow of Sagittarius and the right foot of Ophiuchus, very close to the star of sixth magnitude, the sixty-fifth of the latter constellation [Oph], after the catalog of Flamsteed: These stars are very close to each other; there is none which one can see easily with an ordinary refractor of 3 feet and a half, and which was taken for these small stars. The diameter of all is about 15 minutes of arc. I have determined its position by comparing the middle with the star Mu Sagittarii: I have found its right ascension of 265d 42′ 50″, and its declination of 18d 45′ 55″, south.”

The M23 open star cluster, as it appears in the night sky, flanked by M8 (Lagoon), M16 (Eagle), M17 (Omega), M20 (Trifid) and other deep sky objects. Credit & Copyright: Fernando Cabrerizo/NASA
The M23 open star cluster, as it appears in the night sky (a patch of red), flanked by M8 (Lagoon), M16 (Eagle), M17 (Omega), M20 (Trifid) and other deep sky objects. Credit & Copyright: Fernando Cabrerizo/NASA

While William Herschel did not publish his observations of Messier’s objects, he was still an avid observer. So of course, he had to look at this cluster, and wrote the following observations in his personal notes:

“A cluster of beautiful scattered, large stars, nearly of equal magnitudes (visible in my finder), it extends much farther than the field of the telescope will take in, and in the finder seems to be a nebula of a lengthened form extending to about half a degree.”

In July of 1835, Admiral Smyth would make an observation of Messier 23 and once again add his colorful remarks to the timeline:

“A loose cluster in the space between Ophiuchus’s left leg and the bow of Sagittarius. This is an elegant sprinkling of telescopic stars over the whole field, under a moderate magnifying power; the most clustering portion is oblique, in the direction sp to nf [south preceding to north following, SW to NE], with a 7th-magnitude star in the latter portion. The place registered it that of a neat pair, of the 9th and 10th magnitudes, of a lilac hue, and about 12″ apart. This object was discovered by Messier 1764, and it precedes a rich out-cropping of the Milky Way. The place is gained by differentiating the cluster with Mu Sagittarii, from which it bears north-west, distant about 5 deg, the spot being directed to by a line from Sigma on the shoulder, through Mu at the tip of the bow.”

Remember when observing Messier 23 that it won’t slap you in the face like many objects. Basically, it looks like a stellar scattering of freckles across the face of the sky when fully-resolved. It’s actually one of those objects that’s better to view with binoculars and low power telescopes.

messier-23-location

Locating Messier 23:

M23 can be easily found with binoculars about a finger’s width north and two finger widths west of Mu Sagittarii. Or, simply draw a mental line between the top star in the teapot lid (Lambda) and Xi Serpentis. You’ll find a slight compression in the star field about halfway between these two stars that shows up as an open cluster with binoculars.

Using a finderscope, the object will appear nicely as a hazy spot. And for those using telescopes of any size, you’ll need to use fairly low magnification to help set this cluster apart from the surrounding star field, and it will resolve well to almost all instruments.

And here are the quick facts on this object to help you get started:

Object Name: Messier 23
Alternative Designations: M23, NGC 6494
Object Type: Open Star Cluster
Constellation: Sagittarius
Right Ascension: 17 : 56.8 (h:m)
Declination: -19 : 01 (deg:m)
Distance: 2.15 (kly)
Visual Brightness: 6.9 (mag)
Apparent Dimension: 27.0 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.

Sources:

Posted on by Tammy Plotner

Messier 22 – The Sagittarius Nebula

The Sagittarius Cluster, aka. Messier 22. Credit: Wikipedia Commons/Hewholooks

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Sagittarius Cluster (aka. Messier 22). Enjoy!

Back in the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of these objects so that others wouldn’t make the same mistake. Consisting of 100 objects, this “Messier Catalog” would come to be viewed by posterity as a major milestone in the study of Deep Space Objects.

One of these objects is the Sagittarius Cluster, otherwise known as Messier 22 (and NGC 6656). This elliptical globular cluster, is located in the constellation Sagittarius, near the Galactic bulge region. It is one of the brightest globulars visible in the night sky, and was therefore one of the first of its kind to be discovered and later studied.

Description:

Located around 10,400 light years from our Solar System, in the direction of Sagittarius, M22 occupied a volume of space that is 200 light years in diameter and is receding away from us at 149 kilometers per second. M22 has a lot in common with many other clusters of its type, which includes being a gravitationally bound sphere of stars, and that most of its stars are all about the same age – about 12 billion years old.

Messier 22, showing its proximity to Messier 28 and Kaus Borealis. Credit: Wikisky
Messier 22, showing its proximity to Messier 28 and Kaus Borealis. Credit: Wikisky

It is part of our galactic halo, and may once have been part of a galaxy that our Milky Way cannibalized. But it’s there that the similarities end. For example, it consists of at least 70,000 individual stars, only 32 of which are variable stars. It also spans an incredible 32 arc minutes in the sky and ranks as the fourth brightness of all the known globular clusters in our galaxy.

And four must be its lucky number, because it is also one of only four globular clusters known to contain a planetary nebula. Recent Hubble Space Telescope investigations of Messier 22 have led to the discovery of an astonishing discovery. For starters, in 1999, astronomers discovered six planet-sized objects floating around inside the cluster that were about 80 times the mass of Earth!

Using a technique known as microlensing, which measures the way gravity bends the light of the background stars, the Hubble Space Telescope was able to determine the existence of the gas giant. Even though the Hubble can’t resolve them because the angle at which the light bends is about 100 times smaller than the telescope’s angular resolution, scientist know they are there because the gravity “powers up” the starlight, making it brighter each time a body passes in front of it.

Because a microlensing event is very rare and totally unpredictable, the Hubble team needed to monitor 83,000 stars every three days for nearly four months. Luckily, a sharp peak in brightness was all the proof they needed that they were on the right track.

The center of the globular cluster Messier 22, also known as M22, as observed by the NASA/ESA Hubble Space Telescope. Credit: ESA/HST/NASA
The center of the globular cluster Messier 22, also known as M22, as observed by the NASA/ESA Hubble Space Telescope. Credit: ESA/HST/NASA

Said Kailash Sahu, of the Space Telescope Science Institute, Baltimore, MD, of the discovery in 2007: “Hubble’s excellent sharpness allowed us to make this remarkable new type of observation, successfully demonstrating our ability to see very small objects. This holds tremendous potential for further searches for dark, low-mass objects.”

During their study time, the Hubble team caught six microlensing events that lasted less than 20 hours and one which endured for 18 days. By calculating the times of the eclipses and the spikes in brightness, astronomers could then estimate the mass of the object passing in front of the star. These wandering rogues might be planets torn away from their parent stars by the huge amounts of gravitational influence from so many closely packed suns – or (in the case of the long event) simply a smaller mass star passing in front of another.

They could be brown dwarfs, or even a totally new type of object. As co-investigator Nino Panagia of the European Space Agency and Space Telescope Science Institute said: “Since we know that globular clusters like M22 are very old, this result opens new and exciting opportunities for the discovery and study of planet-like objects that formed in the early universe,”

Two black holes were also discovered in M22 and confirmed by the Chandra X-ray telescope in 2012. The objects have between 10 and 20 solar masses, and their discovery suggests that there may be 5 to 100 black holes within the cluster (and maybe some multiple black holes as well). The presence of black holes and their interaction with the stars of M22 could explain the cluster’s unusually large central region.

These are optical images of M22 and the candidate companion stars to the radio sources M22-VLA1 and M22-VLA2: the globular cluster M22, on the left, and the location of the radio sources on archival Hubble images. Credit: Doug Matthews/Adam Block/NOA/AURA/NSF/HST/NASA/ESA
Optical images of M22 and the candidate companion stars to the radio sources M22-VLA1 and M22-VLA2. Credit: Doug Matthews/Adam Block/NOA/AURA/NSF/HST/NASA/ESA

Other objects of interesting include two black holes – M22-VLA1 and M22-VLA2 – both of which are part of binary star systems. Each has a companion star and is pulling matter from it. This gas and dust, in turn, forms an accretion disk around each black hole, creating emissions that scientists used to confirm their existence.

Messier 22 is one of only four known globular clusters that contain a planetary nebula. This nebula – catalogued as GJJC1 or IRAS 18333-2357 – is rather small and young, being only 3 arcseconds in diameter and 6,000 years old. It was discovered in 1986 using the infrared satellite IRAS, and identified as a planetary nebula in 1989.

History of Observation:

Chances are, magnificent Messier 22 was probably the first globular cluster to ever be recorded in the history of astronomy, most likely by Abraham Ihle in 1665. Over the years it has been included in many historic observations, including Edmund Halley’s list of 6 objects published 1715, and observed by De Chéseaux (his Number 17) and Le Gentil, as well as by Abbe Nicholas Louis de la Caille, who included it in his catalog of southern objects (as Lacaille I.12).

Atlas image mosaic of Messier 22 obtained as part of the Two Micron All Sky Survey (2MASS), a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology. Credit: NASA/NSF
Atlas image mosaic of Messier 22 obtained as part of the Two Micron All Sky Survey (2MASS). Credit:UoM/IPAC/Caltech/NASA/NSF

However, it was Charles Messier who made it famous when he cataloged it as M22 on June 5th, 1764. As he said of the object at the time:

“I have observed a nebula situated a bit below the ecliptic, between the head and the bow of Sagittarius, near the star of seventh magnitude, the twenty-fifth of that constellation, according to the catalog of Flamsteed. That nebula didn’t appear to me to contain any star, although I have examined it with a good Gregorian telescope which magnified 104 times: it is round, and one sees it very well with an ordinary refractor of 3 feet and a half; its diameter is about 6 minutes of arc. I have determined its position by comparing with the star Lambda Sagittarii: its right ascension has been concluded as 275d 28′ 39″, and its declination as 24d 6′ 11”. It was Abraham Ihle, a German, who discovered this nebula in 1665, when observing Saturn. M. le Gentil has examined it also, and he has made an engraving of the configuration in the volume of the Memoirs of the Academy, for the year 1759, page 470. He observed it on August 29, 1747, under good weather, with a refractor of 18 feet length: He also observed it on July 17, and on other days. “It always appeared to me,” he says, “very irregular in its figure, hair and distributing in space of rays of light all over its diameter.”

While Messier’s description is a wonder, let us remember that he was a comet hunter by profession. Once more, it was the observer Admiral Smythto whom we are indebted for the most detailed and vivid description of the cluster:

“A fine globular cluster, outlying that astral stream, the Via Lactea [Milky Way], in the space between the Archer’s head and bow, not far from the point of the winter solstice, and midway between Mu and Sigma Sagittarii. It consists of very minute and thickly condensed particles of light, with a group of small stars preceding by 3m, somewhat in a crucial form. Halley ascribes the discovery of this in 1665, to Abraham Ihle, the German; but it has been thought this name should have been Abraham Hill, who was one of the first council of the Royal Society, and was wont to dabble with astronomy. Hevelius, however, appears to have noticed it previous to 1665, so that neither Ihle nor Hill can be supported.

“In August, 1747, it was carefully drawn by Le Gentil, as seen with an 18-foot telescope, which drawing appears in the Mémoires de l’Académie for 1759. In this figure three stars accompany the cluster, and he remarks that two years afterwards he did not see the preceding and central one: I, however, saw it very plainly in 1835. In the description he says, “Elle m’a toujours parue tres-irrégulière dans sa figure, chevelue, et rependant des espèces de rayons de lumière tout autout de son diamètre.” This passage, I quote, “as in duty bound;” but from familiarity with the object itself, I cannot say that I clearly understand how or why his telescope exhibited these “espèces de rayons.” Messier, who registered it in 1764, says nothing about them, merely observing that it is a nebula without a star, of a round form; and Sir William Herschel, who first resolved it, merely describes it as a circular cluster, with an estimated profundity of the 344th order. Sir John Herschel recommends it as a capital test for trying the space-penetrating power of a telescope.

“This object is a fine specimen of the compression on which the nebula-theory is built. The globular systems of stars appear thicker in the middle than they would do if these stars were all at equal distances from each other; they must, therefore, be condensed toward the centre. That the stars should be accidentally disposed is too improbable a supposition to be admitted; whence Sir William Herschel supposes that they are thus brought together by their mutual attractions, and that the gradual condensation towards the centre must be received as proof of a central power of such kind.”

Messier 22 location. Image: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)
The location of Messier 22 in the night sky. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

Locating Messier 22:

From its position almost on the ecliptic plane, bright globular cluster M22 is easy to find in optics of all sizes. The most important clue is simply identifying the Sagittarius “teapot” shape. Once you’ve located it, just choose the “lid” star, Lambda (Kaus Borealis) and look about a fingerwidth (2 degrees) due northeast. In binoculars, if you center on Lambda, M22 will appear in the 10:00 region of your field of view.

In a finderscope, you will need to hop from Lambda northeast to 24 Sagittari and you’ll see it as a faint fuzzy nearby also to the northeast. From a dark sky location, Messier Object 22 can also sometimes be spotted with the unaided eye! No matter what size optics you use, this large, very luminous ball of stars is quite appealing. A joy to binocular users and an exercise in resolution to telescopes.

And here are the quick facts to help you get started:

Object Name: Messier 22
Alternative Designations: M22, NGC 6656
Object Type: Class VII Globular Star Cluster
Constellation: Sagittarius
Right Ascension: 18 : 36.4 (h:m)
Declination: -23 : 54 (deg:m)
Distance: 10.4 (kly)
Visual Brightness: 5.1 (mag)
Apparent Dimension: 32.0 (arc min)

Go on… Magnificent Messier 22 is waiting for you to appreciate it!

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.

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