Posted on by Tammy Plotner

The Corona Borealis Constellation

Alphecca is the brightest star in a C-shaped pattern of stars: the constellation Corona Borealis. It’s near the bright star Arcturus on the sky’s dome. Credit: EarthSky

Welcome to another edition of Constellation Friday! Today, in honor of the late and great Tammy Plotner, we take a look at the “Northern Crown” – the Corona Borealis constellation. Enjoy!

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 was Corona Borealis, otherwise known as the “Northern Crown”. This small, faint constellation is the counterpart to Corona Australis – aka. the “Southern Crown”. It is bordered by the constellations of Hercules, Boötes and Serpens Caput, and has gone on to become one of the 88 modern constellations recognized by the International Astronomical Union.

Name and Meaning:

In mythology, Corona Borealis was supposed to represent the crown worn by Ariadne – a present from Dionysus. In Celtic lore, it was known as Caer Arianrhod, or the “Castle of the Silver Circle”, home to the Lady Arianrhod. Oddly enough, it was also known to the Native Americans as well, who referred to it as the “Camp Circle” – a heavenly rendition of their celestial ancestors.

Hercules and Corona Borealis, as depicted in Urania’s Mirror (c.?1825). Credit: Library of Congress

History of Observation:

Corona Borealis was one of the original 48 constellations mentioned in the Almagest by Ptolemy. To the medieval Arab astronomers, the constellation was known as al-Fakkah,  which means “separated” or “broken up”– a reference to the resemblance of the constellation’s stars to a loose string of jewels (sometimes portrayed as a broken dish). The name was later Latinized as Alphecca, which was later given to Alpha Coronae Borealis. In 1920, it was adopted by the International Astronomical Union (IAU) as one of the 88 modern constellations.

Notable Objects:

Corona Borealis has no bright stars, 6 main stars and 24 stellar members with Bayer/Flamsteed designations. It’s brightest star – Alpha Coronae Borealis (Alphecca) – is an eclipsing binary located about 75 light years away. The primary components is a white main sequence star that is believed to have a large disc around it (as evidenced by the amount of infrared radiation it emits), and may even have a planetary or proto-planetary system.

The second brightest star, Beta Coronae Borealis (Nusakan), is a spectroscopic binary that is located 114 light years away. It is an Alpha-2 Canum Venaticorum (ACV) type star, a class of variable (named after a star in the constellation Canes Venatici) that are main sequence stars that are chemically peculiar and have strong magnetic fields. Its traditional name, Nusakan, comes from the Arabic an-nasaqan which means “the (two) series.”

Corona Borealis Galaxy Cluster – Abell 2065. Credit: NASA (Wikisky)

Corona Borealis contains few Deep Sky Objects that would be visible to amateur astronomers. The most notable is the Corona Borealis Galaxy Cluster (aka. Abell 2065), a densely-populated cluster located between 1 and 1.5 billion years from Earth. It lies about one degree southwest of Beta Coronae Borealis, in the southwest corner of the constellation. The cluster contains more than 400 galaxies in an area spanning about one degree in the sky.

Corona Borealis also has five stars that have confirmed exoplanets orbiting them, most of which were detected using the radial velocity method. These include the the orange giant Epsilon Coronae Borealis, which has a Super-Jupiter (6.7 Jupiter masses) that orbits it at a distance of 1.3 AU and with a period of 418 days.

There’s also Kappa Coronae Borealis, an orange subgiant that is orbited by both a debris disk and a gas giant. This planet is 2.5 times as massive as Jupiter and orbits the star with a period of 3.4 years. Omicron Coronae Borealis is a clump giant (a type of red giant) with one confirmed exoplanet – a gas giant with 0.83 Jupiter masses that orbits its star every 187 days.

HD 145457 is an orange giant that has one confirmed planet of 2.9 Jupiter masses that takes 176 days to complete an orbit. XO-1 is a yellow main-sequence star located approximately 560 light-years away with a hot Jupiter (roughly the same size as Jupiter) exoplanet. This planet was discovered using the transit method and completes an orbit around its star every three days.

Artist’s concept of “hot Jupiter” orbiting a distant star. Credit: NASA/JPL-Caltech

Finding Corona Borealis:

Corona Borealis is visible at latitudes between +90° and -50° and is best seen at culmination during the month of July. Using binoculars, let’s start with Alpha Coronae Borealis. It’s name is Gemma, or on some star charts – Alphecca. At 75 light years away, we have a nice binary star system whose companion star produces a very faint eclipse every 17.3599 days. Even though Gemma is quite some distance in relative sky terms from Ursa Major, you might be surprised to know that it’s actually part of the Ursa Major moving star group!

Shift your attention to Beta Coronae Borealis. It’s traditional name Nusakan. Again, it looks like one star, but it’s actually two. Nusakan is a double star that’s about 114 light-years and the primary is a variable star that changes every so slightly about every 41 days. The two components are separated by about 0.25 arc seconds – way too close for amateur telescopes – but that’s not all. In 1944 F.J. Neubauer found a small variation in the radial velocity of Nusakan which may lead to a third orbiting body about 10 times the size of Jupiter.

Now have a look at Gamma. Again, we have a binary star that’s just too darn close to split with anything but a large telescope. Struve 1967 is a close binary with an orbit of 91 years. The position angle is 265º and separation about 0.2″. Instead, try focusing your attention on Zeta 1 and Zeta 2. Known as Struve 1965, this pair is a pretty blue white and they are well spaced at 7.03″ and about one stellar magnitude in difference. Nu1 and Nu2 are also very pretty in binoculars. Here we have an optic double star. Although they aren’t physically related, this widely seperated pair of orange giant stars is a pleasing sight in binoculars!

The location of the Corona Borealis Constellation. Credit: IAU/Sky&Telescope magazine

Out of all the singular stars here, you definitely have to take a look at R Coronae Borelis – known as R Cor Bor. Discovered nearly 200 years ago by English amateur, Edward Pigot, R Coronae Borealis is the prototype star of the R Coronae Borealis (RCB) type variables. They are very unusual type of variable star – one where the variability is caused by the formation of a cloud of carbon dust in the line of sight. Near the stellar photosphere, a cloud is formed – dimming the star’s visual brightness by several magnitudes.

Then the cloud dissipates as it moves away from the star. All RCB types are hydrogen-poor, carbon- and helium-rich, and high-luminosity. They are simultaneously eruptive and pulsating. They could fade anywhere from 1 to 9 magnitudes in a month… Or in a hundred days. It’s normally magnitude 6… But it could be magnitude 14. No wonder it has the nickname “Fade-Out star,” or “Reverse Nova”!

Unfortunately, Corona Borealis contains no bright deep sky objects, but it does have one claim to fame – the highly concentrated galaxy cluster, Abell 2065. For observers with larger telescope, many members of this fascinating 1-1.5 billion light years distant group are visible. This rich cluster of galaxies is located slightly more than a degree southwest of Beta Cor Bor and covers about a full degree of sky! Not for the faint of heart… Some of these galaxies list at magnitude 18….

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 46 – the NGC 2437 Open Star Cluster

The open star clusters of Messier 46 and Messier 47, located in the southern skies in the Puppis constellation. Credit: Wikisky

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at Orion’s Nebula’s “little brother”, the De Marian’s Nebula!

During 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 (known as the Messier Catalog) would come to include 100 of the most fabulous objects in the night sky.

One of these objects is the open star cluster known as Messier 46, which is located about 5,500 light years away in the southern Puppis constellation. Located in close proximity to another open cluster (Messier 47), this bright, rich cluster is about 300 million years old and is home to many stars – an estimated 500 – and some impressive nebulae too.

Description:

Crammed into about 30 light years of space, around 150 resolvable stars and up to 500 possible stellar members all took off together on a journey through space some 300 million years ago. At this point in time, they are about 5,400 light years away from our solar system, but they aren’t standing still. They’re pulling away from us at a speed of 41.4 kilometers per second.

The Messier 46 open star cluster. Credit: Jose Luis Martinez

If you notice something just a bit different about one of the stars along the northern edge – then you’ve caught on to one of the most famous features of Messier 46 – its resident planetary nebula. While radial velocities show it probably isn’t a true member of the cluster, it’s still a cool feature!

But, is there more to this cluster than that? You bet. Messier 46 has also been highly studied for its core properties. As Saurabh Sharma (et al) indicated in a 2006 study:

“The study of Galactic open clusters is of great interest in several astrophysical aspects. Young open clusters provide information about current star formation processes and are key objects for clarifying questions of Galactic structure, while observations of old and intermediate-age open clusters play an important role in studying the theories of stellar and Galactic evolution. A detailed analysis of the structure of coronae of open clusters is needed to understand the effects of external environments, like the Galactic tidal field and impulsive encounters with interstellar clouds, etc., on dynamical evolution of open clusters. Extensive studies of the coronal regions of clusters have not been carried out so far mainly because of unavailability of photometry in a large field around open star clusters. The ability to obtain improved photometry of thousands of stars means that large-scale studies of open clusters can be conducted to study the spatial structure and stability of Galactic open clusters. With the addition of photometry of a nearby field region it is possible to construct luminosity functions (LFs) and MFs, which are useful for understanding cluster-formation processes and the theory of star formation in open clusters.”

History of Observation:

Messier 46 is an original discovery of Charles Messier, caught on February 19, 1771, just after he released his first catalog of entries. In his journal, he wrote:

“A cluster of very small stars, between the head of the Great Dog and the two hind feet of the Unicorn, [its position] determined by comparing this cluster with the star 2 Navis, of 6th-magnitude, according to Flamsteed; one cannot see these stars but with a good refractor; the cluster contains a bit of nebulosity.”

Messier 46 and NGC 2437. Credit: NASA

At the time of its discovery, Messier had not published his findings quite as immediately as we do today, so another astronomer also independently discovered this cluster as well… Caroline Herschel. “March 4th, [17]83. 1 deg S following the nebula near the 2nd Navis… a Nebula the figure is done by memory. My Brother observed it with 227 and found it to be, an astonishing number of stars. it is not in Mess. catalogue.”

It would be John Herschel in 1833 who would discover the planetary nebula while cataloging it: “The brightest part of a very fine rich cluster; stars of 10th magnitude; which fills the field. Within the cluster at its northern edge is a fine planetary nebula.”

But, as always, Admiral Symth has a way with words and observations. As he wrote of the object:

“A very delicate double star in a fine cluster, outlying the Galaxy, over Argo’s poop. A 8 1/2 [mag], and B 11, both pale white.A noble though rather loose assemblage of stars from the 8th to the 13th magnitude, more than filling the field, especially in length, with power 93; the most compressed part trending sf [south following, SE] and np [north preceding, NW]. Among the larger [brighter] stars on the northern verge is an extremely faint planetary nebula, which is 39 H. IV. [NGC 2438], and 464 of his son’s Catalogue. This was discovered by Messier in 1769, who considered it as being “rather enveloped in nebulous matter;” this opinion, however, must have arisen from the splendid glow of mass, for judging from his own remark, it is not likely that he perceived the planetary nebula on the north. WH [William Herschel], who observed it in 1786, expressly says, “no connexion with the cluster, which is free from nebulosity.” Such is my own view of attentively gazing; but the impression left on the senses, is that of awful vastness and bewildering distance, – yet including the opinion, that those bodies bespangled the vastness of space, may differ in magnitude and other attributes.”

Pretty amazing considering these gentlemen did all of their observations visually and knew nothing about today’s parallaxes, radial velocities or any other type of thing. May your own observations be as talented…

Locating Messier 46:

There is no simple way of finding Messier 46 in the finderscope of a telescope, but it’s not too hard with binoculars. Begin your hunt a little more than a fistwidth east/northeast of bright Sirius (Alpha Canis Majoris)… or about 5 degrees (3 finger widths) south of Alpha Monoceros. There you will find two open clusters that will usually appear in the same average binocular field of view. M46 is the easternmost of the pair.

Messier 46 location. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

It will appear slightly dimmer and the stars will be more concentrated. In the finderscope it will appear as a slightly foggy patch, while neighboring western M47 will try to begin resolution. Because M46’s stars are fainter, it is better suited to darker sky conditions, showing as a compression in binoculars and will resolve fairly well with even a small telescope. However, you will need at least a 6″ telescope to perceive the planetary nebula.

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

Object Name: Messier 46
Alternative Designations: M46, NGC 2437
Object Type: Open Galactic Star Cluster
Constellation: Puppis
Right Ascension: 07 : 41.8 (h:m)
Declination: -14 : 49 (deg:m)
Distance: 5.4 (kly)
Visual Brightness: 6.0 (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

The Corona Australis Constellation

The southern constellation of Corona Astralis (aka. the "Southern Crown"). Credit: Torsten Bronger.

Welcome back to Constellation Friday! Today, in honor of the late and great Tammy Plotner, we will be dealing with the “Southern Crown” – the Corona Australis 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 was the Coronoa Australis constellation, otherwise known as the “Southern Crown”.  This small, southern constellation is one of the faintest in the night sky, where it is bordered by the constellations of Sagittarius, Scorpius, Ara and Telescopium. Today, it is one of the 88 modern constellations recognized by the International Astronomical Union.

Name and Meaning:

Corona Australis – the “Southern Crown” – is the counterpart to Corona Borealis – the “Northern Crown”. To the ancient Greeks, this constellation wasn’t seen as a crown, but a laurel wreath. According to some myths, Dionysus was supposed to have placed a wreath of myrtle as a gift to his dead mother into the underworld as well. Either way, this small circlet of dim stars definitely has the appearance of a wreath – or crown – and belongs to legend!

False-colour image from the ESO’s Very Large Telescope of the star-forming region NGC 6729. Credit: ESO

History of Observation:

Like many of the Greek constellations, it is believed that Corona Australis was recorded by the ancient Mesopotamian in the MUL.APIN – where it may have been called MA.GUR (“The Bark”). While recorded by the Greeks as early as the 3rd century BCE, it was not until Ptolemy’s time (2nd century CE) that it was recorded as the “Southern Wreath”, a name that has stuck ever since.

In Chinese astronomy, the stars of Corona Australis are located within the Black Tortoise of the North and were known as ti’en pieh (“Heavenly Turtle”). During the Western Zhou period, the constellation marked the beginning of winter. To medieval Islamic astronomers, Corona Australis was known alternately as Al Kubbah (“the Tortoise”), Al Hiba (“the Tent”) or Al Udha al Na’am (“the Ostrich Nest”).

In 1920, the constellation was included in the list of 88 constellations formally recognized by the IAU.

Notable Objects:

Corona Australis is a small, faint constellation that has no bright stars, consists of 6 primary stars and contains 14 stellar members with Bayer/Flamsteed designations. There is one meteor shower associated with Corona Australis – the Corona-Australids which peak on or about March 16 each year and are active between March 14th through the 18th. The fall rate is minimal, with an average of about 5 to 7 per hour.

It’s brightest star, Alpha Coronae Australis (Alphekka Meridiana), is a class A2V star located about 130 light years from Earth. It is also the only properly-named star in the constellation. It’s second brightest star, Beta Coronae Australis, is a K-type bright giant located approximately 510 light years distant.

And then there’s R Coronae Australis, a well-known variable star that is located approximately 26.8 light years from Earth. This relatively young star is still in the process of formation – accreting material onto its surface from a circumstellar disk – and is located within a star forming region of dust and gas known as NGC 6726/27/29.

Corona Australis is also home to several Deep Sky Objects, such as the Corona Australis Nebula. This bright reflection nebula, which is located about 420 light years away, was formed when several bright stars became entangled with a dark cloud of dust. The cloud is a star-forming region, with clusters of young stars embedded inside, and consists of three nebulous regions – NGC 6726, NGC 6727, and NGC 6729.

Other reflection nebulas include NGC 6726/6727 and the fan-shaped NGC 6729. Corona Australis also boasts many star clusters, such as the large, bright globular cluster known as NGC 6541. There’s also the Coronet cluster, a small open star cluster that is located approximately 420 light years from Earth. The cluster lies at the heart of the constellation and is one of the nearest known regions that experiences ongoing star formation.

Color image of the Coronet Australis Nebula, taken by NASA’s WISE (Wide-field Infrared Survey Explorer). Credit: NASA/Caltech

Finding Corona Australis:

Corona Australis is visible at latitudes between +40° and -90° and is best seen at culmination during the month of August. It can be explored using both binoculars and small telescopes. Let’s start with binoculars and a look at Alpha Coronae Australis – the only star in the constellation to have a proper name.

Called Alfecca Meridiana – or “the sixth star in the river Turtle” – Alpha is a spectral class A2V star which is located about 160 light years from Earth. Alfecca Meridiana is a fast rotator, spinning at least at 180 kilometers per second at its equator, 90 times faster than our Sun and making a full rotation in about 18 hours.

Even more interesting is the fact that Alpha is a Vega-like star, pouring out excess infrared radiation that appears to be coming from a surrounding disk of cool dust. Just what does that mean? It means that Alfecca Meridiana could possibly have a planetary system!

Now have a look at Beta. Although this orange class K (K0) giant star is rather ordinary, where it’s at is not. It’s sitting on the edge of the Corona Australis Molecular Cloud, a dusty, dark star-forming region which contains huge amounts of nebulae. While Beta does seem pretty plain, it is almost 5 times larger than our Sun and 730 times brighter. Not bad for a star that’s about a hundred million years old!

Image of the globular cluster NGC 6541 in Corona Australis, based on observations made with the NASA/ESA Hubble Space Telescope. Credit: STScI/NASA/ST-ECF/ESA/CADC/NRC/CSA.

Now, take a look at a really bizarre star – Epsilon Coronae Australis. At a distance of 98 light years, there doesn’t seem to be much going on with this fifth magnitude, faint stellar point, but there is. That’s because Epsilon isn’t one star – but two. Epsilon is an eclipsing binary with two very similar eclipses that take place within an orbital period of 0.5914264 days, as first a faint star passes in front of the bright one that gives us 95 or so percent of the light, and then the bright one passes in front of the fainter.

So what does that mean? It means that if you sit right there at watch, you can see the changes in less than 7 hours. While watching for hours for a half magnitude drop might not seem like your cup of tea, think about what you’re watching…. These two stars are actually contacting each other as they go by! Can you imagine stars spinning so fast that they produce huge amounts of magnetic activity and dark starspots that also add to the variation as they swing in and out of view? Sharing mass and pulling at each other in just a matter of hours? Now that’s a show worth watching…

Now try variable star R Coronae Borealis (RA 19 53 65 Dec -36 57 97). Here we have another unusual one – a “Herbig Ae/Be” pre-main sequence star. The star is an irregular variable with more frequent outbursts during times of greater average brightness, but it also has a long-term periodic variation of about 1,500 days and about 1/2 magnitude that may be linked to changes in its circumstellar shell, rather than to stellar pulsations. Although R Coronae Australis is 40 times brighter than Sol, and about 2 to 10 times larger, most of its stellar luminosity is obscured because the star is still accreting matter. Protoplanetary bodies? Maybe!

Keep your binoculars handy and get out the telescope as we start deep sky first with NGC 6541. Also known as Caldwell 78 and Bennett 104, this beautiful 6th magnitude globular cluster was first discovered by N. Cacciatore on March 19, 1826. It belongs in our Milky Way galaxy’s inner halo structure and it is rather metal poor in structure – but beautifully resolved in a telescope. In binoculars, this splendid southern sky study will appear as a large faint globular with a bright star to the northeast.

The location of the southern constellation of Corona Astralis. Credit: IAU/ Sky&Telescope magazine

Now head for the telescope and NGC 6496 (RA 17 59 0 Dec -44 16). At right around magnitude 9, this globular cluster also has a bonus nebula attached to it. Collectively known as Bennett 100, Dreyer described it as a “nebula plus cluster” but it will take dark skies to make out both. Look for 5th magnitude star SAO 228562 that accompanies it. In a small telescope, only a hazy, faint patch can be seen, but larger aperture does get some resolution.

Try emission/reflection nebula NGC 6729 (RA 19 01 55 Dec -36 57 30) next. In a wide field, you can place NGC 6726, NGC 6727, NGC 6729 and the double star BSO 14 in the same eyepiece. The three nebulae NGC 6726-27, and NGC 6729 were discovered by Johann Friedrich Julius Schmidt, during his observations at Athen Observatory in 1861. The nebula are very faint and almost comet-like in appearance and the double star is easily split. Don’t forget to mark your notes as having captured Caldwell 68!

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 45 – The Pleiades Cluster

Pleiades stars. Image: NASA, ESA, AURA/Caltech, Palomar Observatory. Credit: D. Soderblom and E. Nelan (STScI), F. Benedict and B. Arthur (U. Texas), and B. Jones (Lick Obs.)

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the universally-renowned cluster known for its seven major points of light – The Pleiades Cluster!

During 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 (known as the Messier Catalog) would come to include 100 of the most fabulous objects in the night sky.

One of these is the famous Pleiades Cluster, also known as the Seven Sisters (and countless other names). An open star cluster located approximately 390 to 456 light years from Earth in the constellation of Taurus, this cluster is dominated by very bright, hot blue stars. Being both bright and of one of the nearest star clusters to Earth, this cluster is easily visible to the naked eye in the night sky.

Description:

The nine brightest stars of the Pleiades are named for the Seven Sisters of Greek mythology: Sterope, Merope, Electra, Maia, Taygete, Celaeno, and Alcyone, along with their parents Atlas and Pleione. To the X-ray telescopes on board the orbiting ROSAT observatory, the cluster also presents an impressive, but slightly altered, appearance.

An optical image of the Pleiades. Credit: NOAO/AURA/NSF

This false color image was produced from ROSAT observations by translating different X-ray energy bands to visual colors – the lowest energies are shown in red, medium in green, and highest energies in blue. (The green boxes mark the position of the seven brightest visual stars.)

The Pleiades stars seen in X-rays have extremely hot, tenuous outer atmospheres called coronas and the range of colors corresponds to different coronal temperatures. This helps to determine mass and the presence of brown dwarf stars within Messier 45. As Greg Ushomirsky (et al) said in a 1998 study:

“We present an analytic calculation of the thermonuclear depletion of the light elements lithium, beryllium, and boron in fully convective, low-mass stars. Under the presumption that the pre-main-sequence star is always fully mixed during contraction, we find that the burning of these rare light elements can be computed analytically, even when the star is degenerate. Using the effective temperature as a free parameter, we constrain the properties of low-mass stars from observational data, independently of the uncertainties associated with modeling their atmospheres and convection. Our analytic solution explains the dependence of the age at a given level of elemental depletion on the stellar effective temperature, nuclear cross sections, and chemical composition. These results are also useful as benchmarks to those constructing full stellar models. Most importantly, our results allow observers to translate lithium nondetections in young cluster members into a model-independent minimum age for that cluster. Using this procedure, we have found lower limits to the ages of the Pleiades (100 Myr) and Alpha Persei (60 Myr) clusters. Dating an open cluster using low-mass stars is also independent of techniques that fit upper main-sequence evolution. Comparison of these methods provides crucial information on the amount of convective overshooting (or rotationally induced mixing) that occurs during core hydrogen burning in the 5-10 Mo stars typically at the main-sequence turnoff for these clusters.”

As one of the closest of star clusters to our solar system, M45 is dominated by hot blue stars that have only formed within the last 100 million years. Alongside Maia is a reflection nebula discovered by Tempel faint nebula which accompanies Merope was discovered by master observer E.E. Barnard. These were first believed to be left over from the formation of the cluster.

Messier 45. Credit: Boris Stromar

However, it didn’t take many years of observation of proper motion for astronomers to realize the Pleiades were actually moving through a cloud of interstellar dust. While this pleasing blue group is still only 440 light years away, it only has about another 250 million years left before tidal interactions will tear it apart. By then, its relative motion will have carried it from the constellation of Taurus to the southern portion of Orion!

Of course, many observers aren’t quite sure if they are seeing the nebulosity in M45 or not. Chances are, if you’re seeing what appears to be a “fog” around the bright stars – you’re on it. Only large aperture or photography reveals the full extent of the reflection nebula… and there’s a whole lot of scientific reasons for it. Said Steven Gibson (et al) in a 2003 study:

“The scattering geometry analysis is complicated by the blending of light from many stars and the likely presence of more than one scattering layer. Despite these complications, we conclude that most of the scattered light comes from dust in front of the stars in at least two scattering layers, one far in front and extensive, the other nearer the stars and confined to areas of heavy nebulosity. The first layer can be approximated as an optically thin, foreground slab whose line-of-sight separation from the stars averages ~0.7 pc. The second layer is also optically thin in most locations and may lie at less than half the separation of the first layer, perhaps with some material among or behind the stars. The association of nebulosity peripheral to the main condensation around the brightest stars is not clear. Models with standard grain properties cannot account for the faintness of the scattered UV light relative to the optical. Some combination of significant changes in grain model albedo and phase function asymmetry values is required. Our best-performing model has a UV albedo of 0.22+/-0.07 and a scattering asymmetry of 0.74+/-0.06. Hypothetical optically thick dust clumps missed by interstellar sight line measurements have little effect on the nebular colors but might shift the interpretation of our derived scattering properties from individual grains to the bulk medium.”

Since the Pleaides really is close to our solar system, have astronomers been able to detect anything within its boundaries that has surprised them? The answer is yes. according to a 1998 study by E.L. Martin:

“We present the discovery of an object in the Pleiades open cluster, named Teide 2, with optical and infrared photometry that places it on the cluster sequence slightly below the expected substellar mass limit. We have obtained low- and high-resolution spectra that allow us to determine its spectral type (M6), radial velocity, and rotational broadening and to detect H? in emission and Li I in absorption. All the observed properties strongly support the membership of Teide 2 in the Pleiades. This object has an important role in defining the reappearance of lithium below the substellar limit in the Pleiades.”

The M45 cluster. Credit: Wikipedia Commons/Did23

And what star is that? One cataloged as known as HD 23514, which has a mass and luminosity a bit greater than our Sun. But it’s a star surrounded by an extraordinary number of hot dust particles.  “Unusually massive amounts of dust, as seen at the Pleiades and Aries stars, cannot be primordial but rather must be the second-generation debris generated by collisions of large objects,” said Song, “”Collisions between comets or asteroids wouldn’t produce anywhere near the amount of dust we are seeing.”

The astronomers analyzed emissions from countless microscopic dust particles and concluded that the most likely explanation is that the particles are debris from the violent collision of planets or planetary embryos. Song calls the dust particles the “building blocks of planets,” which can accumulate into comets and small asteroid-size bodies and then clump together to form planetary embryos, eventually becoming full-fledged planets.

“In the process of creating rocky, terrestrial planets, some objects collide and grow into planets, while others shatter into dust,” Song said. “We are seeing that dust.”

History of Observation:

The recognition of the Pleiades dates back to antiquity, and its stars are known by many names in many cultures. The Greeks and Romans referred to them as the “Starry Seven,” the “Net of Stars,” “The Seven Virgins,” “The Daughters of Pleione,” and even “The Children of Atlas.” The Egyptians referred to them as “The Stars of Athyr;” the Germans as “Siebengestiren” (the Seven Stars); the Russians as “Baba” after Baba Yaga – the witch who flew through the skies on her fiery broom.

The Pleiades by Elihu Vedder (1885). Credit: Metropolitan Museum of Art, New York City.

The Japanese call them “Subaru;” Norsemen saw them as packs of dogs; and the Tongans as “Matarii” (the Little Eyes). American Indians viewed the Pleiades as seven maidens placed high upon a tower to protect them from the claws of giant bears, and even Tolkien immortalized the star group in The Hobbit as “Remmirath.” The Pleiades were even mentioned in the Bible! So, you see, no matter where we look in our “starry” history, this cluster of seven bright stars has been part of it.

Charles Messier would log it on March 4, 1769 where his only comment would be: “Cluster of stars known by the name Pleiades: the position reported is that of the star Alcyone.” Even though historic astronomers did little more than comment on M45’s presence, we’re still glad the Charles logged it – for it never received another “official” catalog designation!

Locating Messier 45:

Most normally the Pleiades are easily found with the unaided eye as a very visible cluster of stars about a hand span northwest of Orion. However, if sky conditions are bright, M45 might be a little more difficult to spot. If so, look for bright, red star Aldebaran and set your sights about 10 degrees (an average fist width) northwest.

It will show very easily in any size optics and under virtually any conditions – except for clouds and daylight! Messier 45’s large size makes it an ideal candidate for binoculars, where it will cover about half the average field of view. When using a telescope, chose the least amount of magnification possible to see the entire cluster and use higher magnification to study individual stars.

The location of the Centaurus constellation in the southern sky. Credit: IAU/Sky & Telescope magazine/Roger Sinnott & Rick Fienberg

And as always, here are the quick facts on this Messier Object to help you get started:

Object Name: Messier 45
Alternative Designations: M45, the Pleiades, Seven Sisters, Subaru
Object Type: Open Galactic Star Cluster, Reflection Nebula
Constellation: Taurus
Right Ascension: 03 : 47.0 (h:m)
Declination: +24 : 07 (deg:m)
Distance: 0.44 (kly)
Visual Brightness: 1.6 (mag)
Apparent Dimension: 110.0 (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 44 – The Beehive Cluster (Praesepe)

Messier 44 location. Credit: Wikisky

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at that buzzing nest of stars – the Beehive Cluster!

During 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 (known as the Messier Catalog) would come to include 100 of the most fabulous objects in the night sky.

One of these is the Beehive Cluster (aka. Messier 44, or Praesepe), an open star cluster located in the the Cancer constellation. In addition to containing a larger population of stars than most clusters in its vicinity, it is also one of the nearest open clusters to the Solar System – at a distance of 577 light years (177 parsecs). As such, astronomers have been aware of it since Classical Antiquity.

Description:

According to ancient lore, this group of stars (often called the Praesepe) foretold a coming storm if it was not visible in otherwise clear skies. Of course, this came from a time when combating light pollution meant asking your neighbors to dim their candles. But, once you learn where it’s at, it can be spotted unaided even from suburban settings. Hipparchus called it the “Little Cloud,” but not until the early 1600s was its stellar nature revealed.

Close up of the Praesepe (Messier 44) open star cluster. Credit: Wikisky

Believed to be about 550 light-years away, this awesome cluster consists of hundreds of members – with at least four orange giants and five white dwarfs. M44’s age is similar to that of the Pleiades, and it is believed that both clusters have a common origin. Although you won’t see any nebulosity in the Beehive, even the very smallest of binoculars will reveal a swarm of bright stars and large telescopes can resolve down to 350 faint stars.

Messier 44 is the nearest open cluster of its type to our Solar System, and it 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.

Messier 44 is home to 5 red giant stars and a handful of white dwarf stars. But, M44 also contains one peculiar blue star. Among its members, there is the eclipsing binary TX Cancri, the metal line star Epsilon Cancri, and several Delta Scuti variables of magnitudes 7-8, in an early post-main-sequence state. And in all those stars, there’s a lot of other peculiarities to be found!

Atlas Image of the Beehive Cluster obtained as part of the Two Micron All Sky Survey (2MASS). Credit: UMass/IPAC (Caltech)/NASA/NSF

As Sergei M. Andrievsky indicated in a 1998 study:

“We present the results of a spectroscopic study of four blue stragglers from old galactic open cluster NGC 2632 (Praesepe). The LTE analysis based on Kurucz’s atmosphere models and synthetic spectra technique has shown that three stars, including the hottest star of the cluster HD73666, possess an uniform chemical composition: they show a solar-like abundance (or slight overabundance) of iron and an apparent deficiency of oxygen and silicon. Two stars exhibit a remarkable barium overabundance. The chemical composition of their atmospheres is typical for Am stars. One star of our sample does not share such uniform elemental distribution, being generally deficient in metals.”

But is there more hiding in there? Perhaps the kind of stuff that could eventually make planets? According to a 2009 study done by A. Gaspar (et al), this was certainly thought to the be the case:

“Mid-IR excesses indicating debris disks are found for one early-type and for three solar-type stars. The incidence of excesses is in agreement with the decay trend of debris disks as a function of age observed for other cluster and field stars. We show that solar-type stars lose their debris disk 24 um excesses on a shorter timescale than early-type stars. Simplistic Monte Carlo models suggest that, during the first Gyr of their evolution, up to 15%-30% of solar-type stars might undergo an orbital realignment of giant planets such as the one thought to have led to the Late Heavy Bombardment, if the length of the bombardment episode is similar to the one thought to have happened in our solar system.”

In September of 2012, two planets were confirmed to be orbiting around two separate stars in the Beehive Cluster. The finding was significant since the stars were similar to Earth’s Sun, and this was the first instance where exoplanets were found orbiting a Sun-like star within a stellar cluster. These planets were designated as Pr0201b and Pr0211b, both of which are “Hot Jupiters” (i.e. gas giants that orbit close to their stars). In 2016, additional observations showed that the Pr0211 system actually has two planets, the second one being Pr0211-c.

History of Observation:

This beautiful, nearby star cluster has been known since ancient times and played wonderful roles in mythology. Aratos mentioned this object as “Little Mist” as far back as 260 BC, and Hipparchus included this object in his star catalog and called it “Little Cloud” or “Cloudy Star” in 130 BC. Ptolemy mentions it as one of seven “nebulae” he noted in his Almagest, and describes it as “The Nebulous Mass in the Breast (of Cancer)”.

According to Burnham, it appeared on Johann Bayer’s chart (about 1600 AD) as “Nubilum” (“Cloudy” Object). It was even resolved by Galileo in 1609 who said: “The nebula called Praesepe contains not one star only but a mass of more than 40 small stars. We have noted 36 besides the Aselli (Gamma and Delta Cancri).”

Messier 44 was partly resolved by Orion nebula’s discoverer, Peiresc, in 1611, who said, “Nebula was seen in the vicinity of Jupiter to the east. in which more than 15 stars have been counted.” and added to Hevelius’ catalog as number 291. De Cheseaux charted it as his number 11 and Bode as his number 20. Small wonder Messier felt the need to add his own numbers to it as well when he recorded:

“At simple view [with the naked eye], one sees in Cancer a considerable nebulosity: this is nothing but a cluster of many stars which one distinguishes very well with the help of telescopes, and these stars are mixed up at simple view [to the unaided eye] because of their great proximity. The position in right ascension of one of the stars, which Flamsteed has designated with the letter c, reduced to March 4, 1769, should be 126d 50′ 30″, for its right ascension, and 20d 31′ 38″ for its northern declination. This position is deduced from that which Flamsteed has given in his catalog.”

Image of M44 Beehive cluster taken by the author, Miguel Garcia. Credit: Intihuatana (Miguel Garcia)

While Sir William Herschel would ignore it and Caroline Herschel would only write that she “observed it”, John Herschel would go on to give it an NGC designation and Admiral Smyth would sing its poetic praises. Is it possible that watching this star cluster could help fortell the weather? If you believe the words of Aratos, it just might.

“Watch, too, the Manger. Like a faint mist in the North it plays the guide beneath Cancer. Around it are borne two faintly gleaming stars, not far apart nor very near but distant to the view a cubit.s length, one on the North, while the other looks towards the South. They are called the Asses [in the constellation Cancer], and between them is the Manger. On a sudden, when all the sky is clear, the Manger wholly disappears, while the stars that go on either side seem nearer drawn to one another: not slight then is the storm with which the fields are deluged. If the Manger darken and both stars remain unaltered, they herald rain. But if the Ass to the North of the Manger shine feebly through a faint mist, while the Southern Ass is gleaming bright, expect wind from the South: but if in turn the Southern Ass is cloudy and the Northern bright, watch for the North wind.”

And watch for a swarm of incredible starlight!

Locating Messier 44:

Messier 44 is so bright that it easily shows to the unaided eye as a nebulous patch just above the conjunction of the faint, upside down “Y” asterism of the Cancer constellation. However, not everyone lives where dark skies are a rule – so try using both Pollux and Procyon to form the base of an imaginary triangle. Now aim your binoculars or finderscope near the point of the apex to discover M44 – the Beehive.

The location of Messier 44 in the Cancer constellation. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

Since Messier 44 is about a degree and a half in diameter, it will require that you use your lowest magnification eyepiece in a telescope, and it is very well suited to binoculars of all sizes. Because its major stars are also quite bright, it stands up to urban sky and moonlight conditions, but many more stars are revealed with higher magnification and darker skies. Because M44 is very near the ecliptic plane, you’ll often find a planet or the Moon mixing it up with the stars!

Object Name: Messier 44
Alternative Designations: M44, NGC 2632, Beehive Cluster, The Praesepe, The Manger
Object Type: Open Galactic Star Cluster
Constellation: Cancer
Right Ascension: 08 : 40.1 (h:m)
Declination: +19 : 59 (deg:m)
Distance: .577 (kly)
Visual Brightness: 3.7 (mag)
Apparent Dimension: 95.0 (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 Columba Constellation

The southern constellation Columba. Credit: Torsten Bronger

Welcome back to Constellation Friday! Today, in honor of the late and great Tammy Plotner, we will be dealing with the dove – the Columba 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.

Since then, thanks to the efforts of astronomer and explorers, many more constellations have come to be recognized. One of these is the constellation Columba (also known as “the dove”), which was discovered in the 16th century. Located in the southern hemisphere, this small constellation is bordered by the constellations of Caelum, Canis Major, Lepus, Pictor, and Puppis.

Name and Meaning:

Since Columba was unknown to the ancient Greeks and Romans, no mythlogy is associated with it, but its original name was Columba Noachi, which refers to the Torah’s and Bible’s Dove of Noah that was the first bird to find land after the Deluge.

It could also belong to the story of Argo, where a dove was sent out to lead the Argonauts to safety between the clashing rocks. The legend of the dove is supported by the brightest star in the constellation – Alpha – whose name is Phact, Arabic for “ring dove”.

The constellation seen as “Columba Noachi” in Urania’s Mirror (1825). Credit: US Library of Congress/Wikipedia Commons

History of Observation:

Columba first appeared on the constellation charts of Petrus Plancius – a sixteen century Dutch astronomer and cartographer. In 1589, he created a celestial globe using what little information he could gather from the times explorers to help “fill in” the blank area around the south celestial pole.

Columba was then introduced into a large wall map of the constellations in 1592 and later included in Johann Bayer’s Uranometria sky atlas. In 1920, it was included among the 88 constellations recognized by the IAU, where it has remained to this day.

Notable Objects:

Columba has several major stars associated with it. The brightest is Alpha Columbae (aka. Phact), which is located approximately 270 light years from Earth. Phact is a double star that belongs to the spectral class B7IVe, and is omposed of a Be-type subgiant and a faint companion star. Its name is derived from the Arabic world Al-Fakhita, which means “the dove”.

Beta Columbae (aka. Wezn) is the second brightest star in the constellation, a giant K1-type star located 86 light years from Earth. It’s name is derived from the Arabic word Al-Wazen, which means “the weight”. Third is Delta Columbae (aka. Ghusn al Zaitun), a spectroscopic binary that is located approximately 237 light years away. Its name is derived from the Arabic phrase al-ghasn alzzaytun, which means “olive branch.”

The barred spiral galaxy NGC 1808. Credit: Jim Flood (Amateur Astronomers Inc., Sperry Observatory), Max Mutchler (STScI)

Columba is also home to several Deep Sky Objects. There’s NGC 1808, a barred spiral galaxy that is located approximately 40 million light years from Earth. Similar in many ways to the Milky Way, this galaxy has an unusual nuclear which is shaped like a warped disk and is believed to have a lot of star-forming activity within it.

There’s also NGC 1851 (aka. Caldwell 73), is a globular cluster located approximately 39,500 light years away, and NGC 1792, a starburst spiral galaxy that also goes by the name Bulliens Columbae (or the “bubbling galaxy”). This is due to its appearance, which is characterized by the patchy distribution of dust throughout the galaxy and the way this dust is heated by young stars.

Last, there’s ESO 306-17, a fossil group giant elliptical galaxy that is located at a distance of about 493 million light years from Earth. The galaxy spans about 1 million light years in diameter and is believed to have cannibalized smaller galaxies in its neighbourhood. Hence why it is designated as a fossil group, which refers to the fact that it is believed to be the end-result of a galaxy colliding and merging with a regular galaxy group.

Finding Columba:

Columba consists of 1 bright star and 5 primary stars, with 18 Bayer/Flamsteed designated stellar members. It is bordered by the constellations of Lepus, Caelum, Pictor, Puppis and Canis Major. Columba is easily visible to viewers at latitudes between +45° and -90° and is best seen at culmination during the month of February.

The globular cluster NGC 1851. Credit: NASA, JPL-Caltech, SSC

Get out your telescope and take a look at Alpha Columbae – the A symbol on the map. Here we have a a subgiant star – a star that has just stopped fusing hydrogen to helium – with an an apparent magnitude of approximately 2.6. Located about 268 light years from Earth, Phact is spinning rapidly… at a speed of at least 180 kilometers per second at its equator.

That’s over 90 times faster than our Sun! This rapid rotation causes Phact to flatten at its poles and to spin off a low density envelope about twice its radius. Now, look closely you’ll see that Phact is actually a binary star system. Its faint companion has an apparent magnitude of 12.3 and is 13.5″ distant from the main star.

Now aim binoculars at Beta Columbae – the B symbol on the map. Its proper name is Wazn the “Weight”. If you don’t think there is anything particularly interesting about this 86 light-year distant, spectral class K1IIICN+1, 3.12 magnitude star, then you better think again. This calm looking, core helium fusing giant star might be a little on the small side as giant stars go, but it is about 12 times the size of our own Sun and shines 53 times brighter.

Of course, that’s not all that unusual either. Nor is the fact that Wazn is about 2 billion years old. What is really strange is that Beta Columbae is scooting along through space at a speed of 103 kilometers per second. That’s about six to seven time faster than what’s considered “normal”! Why? It’s a runaway star, just like Mu Columbae.

Turn your binoculars toward the U symbol on the map and have a look. At 1,300 light years from our solar system, Mu is one of the few O-class stars that is visible to the unaided eye. Like Phact, Mu is a relatively fast rotating star that completes a full revolution approximately every 1.5 days.

Colour composite image of the starburst spiral galaxy NGC 1792. Credit: ESO

But Mu is also like Wazn – speeding along at relative velocity of over 200 km/s. Just where did these these two “runaways” come from? Chances are Wazn came from the other side of the Milky Way, while Mu may have originated from a binary star collision in Orion. Catch them while they’re still there!

Now aim your binoculars or telescopes at 7th magnitude globular cluster, NGC 1851 (RA 5 14 6.7 Dec -40 2 48). This Class II beauty was discovered by James Dunlop on May 29, 1826 and cataloged as Dunlop 508. What you’ll find is a very rich, almost impenetrable core surrounded by a nice halo of resolvable stars in a delightful field.

NGC 1851 has two distinct stellar populations with very different initial metal mixtures: a normal alpha-enhanced component, and one characterized by strong anti correlations among the CNONa abundances. Known in the Caldwell Catalog as Object 73, this fine object does well in all aperture sizes – even to Dunlop who almost 200 years ago wrote:

“An exceedingly bright, round, well-defined nebula, about 1.5′ diameter, exceedingly condensed, almost to the very margin. This is the brightest small nebula that I have seen. I tried several magnifying powers on this beautiful globe; a considerable portion round the margin is resolvable, but the compression to the centre is so great that I cannot reasonably expect to separate the stars. I compared this with the 68 Conn. des Temps, and this nebula greatly exceeds the 68 in condensation and brightness.”

Image of ESO 306-17, taken by the Advanced Camera for Surveys aboard the NASA/ESA Hubble Space Telescope. Credit: NASA/ESA/Michael West (ESO)

For a telescope challenge, try NGC 1792 (RA 05 05.2 Dec -37 59). Despite being billed at slightly fainter than magnitude 10, you’ll find the surface brightness of this spiral galaxy a little more in need of larger aperture. Noted as a starburst galaxy, NGC 1792 has a patchy distribution of dust throughout the galactic disc. The galaxy itself is abundant in neutral hydrogen gas and is in the star formation process.

The galaxy is characterized by unusually luminous far-infrared radiation from the young stars heating the dust with their intense activity. This activity could be caused by gravitational interaction with galaxy NGC 1808 (RA 5 7 42.3 Dec -37 30 47) – also a Seyfert galaxy. Easily seen in larger telescopes as an elongated glow, with a bright, round central core. There’s a reason for that…

The barred spiral galaxy NGC 1808 is undergoing an episode of intense star formation near its very center, perhaps triggered by rotation of the bar or by material transported inward along the bar. This new star formation is somehow being organized into clusters of between 10 and 100 light years in diameter, and filaments of dark, obscuring dust are mixed in with the gas and stars.

Thanks to studies done with the XMM-Newton and Chandra observatories, they have directly proved the co-existence of thermal diffuse plasma and non-nuclear unresolved point-like sources associated with the starburst activity, along with a Low Luminosity Active Galactic Nucleus (LLAGN) or an Ultra Luminous X-ray source (ULX). What a show!

Now try your luck with galactic star cluster NGC 1963 (RA 05 32.2 Dec -36 23). While it is not a very rich and populous star cluster, it is an interesting stellar association of perhaps two dozen stars arranged in chains over a wide field with a size of 10.0′. Look for an asterism that appears like the number 3!

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.

Source:

Posted on by Tammy Plotner

Messier 43 – the De Marian’s Nebula

The De Mairan's Nebula (aka. Messier 43) and the Orion Nebula. Credit: Wikisky

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at Orion’s Nebula’s “little brother”, the De Marian’s Nebula!

During 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 (known as the Messier Catalog) would come to include 100 of the most fabulous objects in the night sky.

One of these if the diffuse nebula known as the De Marian’s Nebula (aka. Messier 43). Located in the direction of the Orion constellation (in close proximity to the Orion Nebula), this nebula lies at a distance of 1,600 light years from Earth. Together with the Orion Nebula, it is part of one of the most active star-forming regions visible in the night sky.

Description:

The diffuse nebula M43 surrounds the variable star N U Orionis (HD 37061) – a rather cool, young star cooking in a rich HII region. But is the light that’s reaching us actually coming through a tunnel in this dusty cloud? As Karl Wurm and Mario Perinotto explained in a 1970 study:

“Most of the areas with identical monochromatic features show a high deficiency of cluster stars correlated with a low surface brightnesss and a reduced gas density. This is explained by an opaqueness of the emission strata in the direction in the line of sight and a position of the same nearer to the observer than the extension of the cluster. There appear surface structures at large distances from the Trapezium which show a correlation between the intensity of scattered star light and the intensity of the emission of the higher ions ([Oiii], [Neiii]). This observation is considered as a proof that canals through the nebular cloud complex allow in some directions the exciting radiation to reach large distances from the star without having suffered an appreciable absorption or scattering.”

De Mairan’s Nebula, M43, NGC 1982. Image: NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team
De Mairan’s Nebula, M43, NGC 1982. Credit: NASA/ESA/M. Robberto (Space Telescope Science Institute/ESA)/Hubble Space Telescope Orion Treasury Project Team

However, N U is far from being alone…. The whole complex is littered with stars being born! As Bo Reipurth (et al), stated in a 1999 study:

“The OMC-2/3 molecular clouds contain one of the highest concentrations of protostars known in nearby molecular clouds. We have observed an area of about 6 × 15 (0.8 pc × 2 pc) covering the OMC-2/3 region with the Very Large Array in the D configuration at 3.6 cm, matching well the area of a recent 1300 m survey. We detected 14 sources, of which it is highly probable that 11 sources are either protostars or very young stars. This testifies to the star-forming activity and extreme youth of the OMC-2/3 region. The 3.6 cm flux is free-free emission probably due to shocks in outflowing material. Three of the sources are extended even with the relatively low resolution of the present observations, and two of these may be collimated radio jets. The large fraction of submillimeter continuum sources that have a radio continuum counterpart is evidence that outflow is already common at the very earliest evolutionary stages. No relation is found between the radio continuum flux and the 1300 m flux of the associated submillimeter dust clumps.”

History of Observation:

In 1731, Jean-Jacques Dortous de Mairan was the first to notice this independent portion of the Orion nebula, stating:

“Finally I will add that close to the luminous space in Orion [M42], one sees the star d of Huygens [NU Orionis] currently (1731) surrounded by a brilliance very similar to that which produces, as I believe, the atmosphere of our Sun, if it were dense enough and extensive enough to be visible in Telescopes at a similar distance. See it in the form and the situation [given by] D, according to what was determined with the Reticule.”

On March 4, 1771, Charles Messier would also come to the same conclusion as he states in his observing notes:

“The star which is above, and has little distance from that nebula, and of which is spoken in the Traite de l’Aurore boreale [Treat of the Northern Light] by M. de Mairan is surrounded, and equally by a very thin light; the star doesn’t have the same brilliance as the four of the great nebula: its light is pale, and it appears covered by fog. I determined its position; its right ascension was 81d 3′ 0″, and its declination 5d 26′ 37″ south.”

Close-up view of the Orion Nebula’s little brother, Messier 43, taken by NASA/ESA Hubble Space Telescope. Credit: ESA/Hubble & NASA

While Sir William Herschel was very careful not to assign his own catalog numbers to Messier Objects, he, too, was fascinated by the M43 region. In his personal notes he writes:

“In the year 1774, the 4th of March, I observed the nebulous star, which is the 43d of the Connoissance des Temps and is not many minutes north of the great nebula; but at the same time I also took notice of two similar, but much smaller nebulous stars; one on each side of the large one, and at nearly equal distance from it. Fig. 37 is a copy of the drawing which was made at the time of observation.

“In 1783, I reexamined the nebulous star, and found it to be faintly surrounded with a circular glory of whitish nebulosity, faintly joined to the great nebula. About the latter end of the same year I remarked that it was not equally surrounded, but most nebulous toward the south.

“In 1784, I began to entertain an opinion that the star was not connected with the nebulosity of the great nebula in Orion, but was one of those which are scattered over that part of the heavens.

“In 1801, 1806, and 1810 this opinion was fully confirmed, by the gradual change which happened in the great nebula, to which the nebulosity surrounding this star belongs. For the intensity of the light about the nebulous star had by this time been considerably reduced, by attenuation or dissipation of nebulous matter; and it seemed now to be pretty evident that the star is far behind the nebulous matter, and that consequently its light in passing through it is scattered and deflected, so as to produce the appearance of a nebulous star. A similar phenomenon may be seen whenever a planet or a star of the 1st or 2nd magnitude happens to be involved in haziness; for a diffused circular light will then be seen, to which, but in a much inferior degree, that which surrounds this nebulous star bears a great resemblance.

“When I reviewed this interesting object in December 1810, I directed my attention particularly to the two small nebulous stars, by sides of the large one, and found that they were perfectly free from every nebulous appearance; which confirmed not only my former surmise of the great attenuation of the nebulosity, but also proved that their former nebulous appearance had been entirely the effect of the passage of their feeble light through the nebulous matter spread out before them.

The 19th of January 1811, I had another critical examination of the same object in a very clear view through the 40-feet telescope; but notwithstanding the superior light of this instrument, I could not perceive any remains of nebulosity about the two small stars, which were perfectly clear, and in the same situation, where about thirty-seven years before I had seen them involved in nebulosity.”

May this wonderful region entertain your brain for as many years as it did Bill Herschel!

The location of Messier 43 in the constellation of Orion. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

Locating Messier 43:

Locating M43 is as easy as locating… well… M42! This small star cluster accompanied by an emission/reflection nebula just to the north of the Orion Nebula’s “Trapezium” region is often mistake for part of the great nebula itself. However, if you look closely, you’ll see the two are separated by a dark dust lane.

Begin by locating the asterism of three stars known as Orion’s Belt. If you cover it with your fist held at arm’s length in a “thumb’s down” gesture with your left hand, the tip of your thumb will just about mark the correct spot in the sky. From a dark location when no Moon is present, you can easily see the haze of the Orion nebula surrounding the stars in the “sword” asterism. While it is easily seen in binoculars on a dark night, it will fade significantly under light pollution or moonlight.

And here are the quick facts on Messier 43 to help you get started:

Object Name: Messier 43
Alternative Designations: M43, NGC 1982, De Mairan’s Nebula, Companion of the Orion Nebula
Object Type: Emission/Reflection Nebula and Open Cluster
Constellation: Orion
Right Ascension: 05 : 35.6 (h:m)
Declination: -05 : 16 (deg:m)
Distance: 1.3 (kly)
Visual Brightness: 9.0 (mag)
Apparent Dimension: 20×15 (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 Circinus Constellation

Celestial map of the constellation Circinus, the Pair of Compasses. Credit: Torsten Bronger

Welcome back to Constellation Friday! Today, in honor of the late and great Tammy Plotner, we will be dealing with the compass – the Circinus 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.

Over time, the number of recognized constellations has grown as astronomers and explorers became aware of other stars visible from other location around the world. By the 20th century, the IAU adopted a modern catalog of 88 Constellations. One of these is the Circinus constellation, a small, faint constellation located in the southern skies. It is bordered by the constellations Apus, Centaurus, Lupus, Musca, Norma, Triangulum Australe.

Name and Meaning:

Because Circinus was unknown to the ancient Greeks and Romans, it has no mythology associated with it. The three brightest stars form a narrow triangle. The shape is reminiscent of a drawing (or drafting) compass of the sort used to plot sea and sky charts. Nicolas Louis de Lacaille had a fascination with secular science and the thought of naming a constellation after a science tool fascinated him.

Lacaille’s table, showing his representations of the constellations. Credit: gallica.bnf.fr

In this case, Circinus represents a drafting tool used in navigation, mathematics, technical drawing, engineering drawing, in cartography (drawing maps) – and which many elementary school age children use to learn to draw circles and in geometry to bi-sect lines, draw arcs and so forth. In this case, the device should not be confused with Pyxis, a constellation associated with a ship’s compass… despite the similarity in names with the Latin language!

History of Observation:

The small, faint southern constellation Circinus was created by Nicholas de Lacaille during his stay at the Cape of Good Hope in the mid-18th century. Circinus was given its current name in 1763, when Lacaille published an updated sky map with Latin names for the constellations he introduced.

On the map he created, Lacaille portrayed the constellations of Norma, Circinus, and Triangulum Australe as a set of draughtsman’s instruments – as a ruler, compass, and a surveyor’s level, respectively. This constellation has endured and became one of the 88 modern constellation recognized by the IAU in 1920.

Notable Features:

Circinus has no bright stars and consists of only 3 main stars and 9 Bayer/Flamsteed designated stars. However, the constellation does have several Deep Sky Objects associated with it. For instance, there’s the Circinus Galaxy, a spiral galaxy located approximately 13 million light years distant that was discovered in 1975. The galaxy is notable for the gas rings inside it, one of which is a massive star-forming region, and its black hole-powered core.

Composite image of the central regions of the nearby Circinus galaxy, located about 12 million light years away. Credit: NASA/Chandra/HST

Then there’s the X-ray double star known as Circinus X-1, which is located approximately 30,700 light years away and was discovered in 1969. This system is composed of a neutron star orbiting a main sequence star. Circinus is also home to the bright planetary nebula known as NGC 5315, which was created when a star went supernova and cast off its outer layers into space.

Then there’s NGC 5823 (aka. Caldwell 88), an open cluster located on the border between Circinus and Lupus. Located about 3,500 light years away, this cluster is about 800 million years old and spans about 12 light years.

Finding Circinus:

Circinus is visible at latitudes between +10° and -90° and is best seen at culmination during the month of June. Start by taking out your binoculars for a look at Alpha Circini – a great visual double star. Located about 53.5 light years from Earth, this stellar pair isn’t physically related but does make a unique target. The brighter of the two, Alpha, is a F1 Bright Yellow Dwarf that is a slight variable star. This contrasts very nicely with the fainter, red companion.

For the telescope, take a look at Gamma Circini – a faint star a little over five hundred light years from the Solar System. In the sky, it lies in the Milky Way, between bright Alpha Centauri and the Southern Triangle. Gamma Circini is a binary system, containing a blue giant star with a yellow, F-type, companion. Gamma is unique because it possess a stellar magnetic buoyancy!

Location of the Circinus constellation. Credit: IAU

For larger binoculars and telescopes, have a look at galactic star cluster NGC 5823 (RA 15 : 05.7 Dec -55 : 36). This dim cluster will appear to have several brighter members which are actually foreground stars, but does include Mira-type variable Y Circini. While it will be hard to distinguish from the rich, Milky Way star fields, you will notice an elliptical shaped compression of stars with an asterism which resembles and open umbrella.

For large telescopes, check out ESO 97-G13 – the “Circinus Galaxy”. Located only 4 degrees below the Galactic plane, and 13 million light-years away (RA 14h 13m 9.9s Dec 65° 20? 21?), this Seyfert Galaxy is undergoing tumultuous changes, as rings of gas are being ejected from the galactic core. While it can be spotted in a small telescope, science didn’t notice it until 25 years ago!

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 42 – The Orion Nebula

The stunning, shaped clouds of gas in the Orion Nebula make it beautiful, but also make it difficult to see inside of. This image of the Orion Nebula was captured by the Hubble Telescope. Image: NASA, ESA, M. Robberto (STScI/ESA) and The Hubble Space Telescope Orion Treasury Project Team
The stunning, shaped clouds of gas in the Orion Nebula make it beautiful, but also make it difficult to see inside of. This image of the Orion Nebula was captured by the Hubble Telescope. Image: NASA, ESA, M. Robberto (STScI/ESA) and The Hubble Space Telescope Orion Treasury Project Team

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at that Great and most brightest of nebulae – the Orion Nebula!

During 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 (known as the Messier Catalog) would come to include 100 of the most fabulous objects in the night sky.

One of these objects is the Orion Nebula, a diffuse nebula situated just south of Orion’s Belt in the Orion constellation. Located between 1,324 and 1,364 light years distant, it is the closest massive star forming region to Earth. Little wonder then why it  is the brightest nebula in the night sky and can be seen on a clear evening with the naked eye.

Description:

Known as “The Great Orion Nebula,” let’s learn what makes it glow. M42 is a great cloud of gas spanning more than 20,000 times the size of our own solar system and its light is mainly florescent. For most observers, it appears to have a slight greenish color – caused by oxygen being stripped of electrons by radiation from nearby stars.

A pair of binoculars will make the “Curlicue” pop in Orion’s Belt. Although the stars aren’t related, they form a delightfully curvy line-of-sight pattern. Credit: Bob King

At the heart of this immense region is an area known as the “Trapezium” – its four brightest stars form perhaps the most celebrated multiple star system in the night sky. The Trapezium itself belongs to a faint cluster of stars now approaching main sequence and resides in an area of the nebula known as the “Huygenian Region” (named after 17th century astronomer and optician Christian Huygens who first observed it in detail).

Buried amidst the bright ribbons and curls of this cloud of predominately hydrogen gas are many star forming regions. Appearing like “knots,” these Herbig-Haro objects are thought to be stars in the earliest stages of condensation. Associated with these objects are a great number of faint red stars and erratically luminous variables – young stars, possibly of the T Tauri type.

There are also “flare stars,” whose rapid variations in brightness mean an ever changing view. “Orion may seem very peaceful on a cold winter night, but in reality it holds very massive, luminous stars that are destroying the dusty gas cloud from which they formed,” said Tom Megeath, an astronomer at the Harvard-Smithsonian Center for Astrophysics.

While studying M42, you’ll note the apparent turbulence of the area – and with good reason. The “Great Nebula’s” many different regions move at varying speeds. The rate of expansion at the outer edges may be caused by radiation from the very youngest stars present. Said Massimo Roberto, an astronomer at the Space Science Telescope Institute in Baltimore:

“In this bowl of stars we see the entire formation history of Orion printed into the features of the nebula: arcs, blobs, pillars and rings of dust that resemble cigar smoke. Each one tells a story of stellar winds from young stars that impact the environment and the material ejected from other stars.”

The star Alnitak and Flame Nebula in Orion. Credit and copyright: César Cantú.

Although M42 may have been luminous for as long as 23,000 years, it is possible that new stars are still forming, while others were ejected by gravitation – known as “runaway” stars. A tremendous X-ray source (2U0525-06) is quite near the Trapezium and hints at the possibility of a black hole present within M42. The Trapezium’s stellar winds also are responsible for the formation of stars inside the nebula – their shock waves compressing the medium and igniting starbirth.

“When you look closely, you see that the nebula is filled with hundreds of visible shock waves,” said Bob O’Dell, an astronomer from Vanderbilt University. O’Dell was fortunate enough to use Hubble to map Orion’s stellar winds and create a map of two of Orion’s three star-forming regions… Regions where the winds have been blowing continuously for nearly 1,500 years!

What else have we learned about the Great Orion nebula in recent years? Try the discovery of 13 drifting gas planets. These rare, “free-floating” objects were confirmed by Patrick Roche of the University of Oxford and Philip Lucas of the University of Hertfordshire just before the turn of the century. They were found with the Hubble Space Telescope while looking for faint stars and brown dwarfs. As he explained:

“The objects are likely to be large gas planets similar in size to Jupiter and consisting primarily of hydrogen and helium. From the measured brightness and the known distance to the Orion nebula, we knew they did not have enough material for any nuclear processing in their interiors.”

Orion's Horsehead Nebula Credit & Copyright Ryan Steinberg & Family, Adam Block, NOAO, AURA, NSF
Orion’s Horsehead Nebula Credit & Copyright Ryan Steinberg & Family, Adam Block, NOAO, AURA, NSF

Chances are very good these planets may be failed stars – much like our own Jupiter. But these planets don’t orbit a star the same way our solar system’s planets orbit the Sun… they simply roam around. Dr. Roche said that the 13 objects “probably formed in a different way from the planets in our solar system” in that they were not made “out of the residue of material left over from the birth of the sun.”

Instead, they formed “like stars via the collapse of a cloud of cold gas,” explained Lucas. “But they possess most of the physical properties and structure of gas giant planets,” added Lucas.

History of Observation:

Messier 42 was possibly discovered 1610 by Nicholas-Claude Fabri de Peiresc and was recorded by by Johann Baptist Cysatus, Jesuit astronomer, in 1611. For fans of the great Galileo, he was the first to mention the Trapezium cluster in 1617, but did not see the nebula. (However, do not despair! For it is my belief that he was simply using too much magnification and therefore could not see the extent of what he was looking at.)

The first known drawing of the Orion nebula was created by Giovanni Batista Hodierna, and after all of these documents were lost, the Orion nebula was once again credited to Christian Huygens 1656, documented by Edmund Halley in 1716. It then went on to Jean-Jacques d’Ortous de Mairan in his nebulae descriptions, to be added by Philippe Loys de Chéseaux to his list, expounded by Guillaume Legentil in his review.

Horsehead Nebula at the Orion Credit & Copyright Adam Block, Mt. Lemmon SkyCenter, U. Arizona
Horsehead Nebula at the Orion. Credit & Copyright Adam Block, Mt. Lemmon SkyCenter, U. Arizona

At last, Charles Messier added the nebula to his catalog on March 4, 1769. As he wrote of the stunning objectL

“The drawing of the nebula in Orion, which I present at the Academy, has been traced with the greatest care which is possible for me. The nebula is represented there as I have seen it several times with an excellent achromatic refractor of three and a half feet focal length, with a triple lens, of 40 lignes [3.5 inches] aperture, and which magnifies 68 times. This telescope made in London by Dollond, belongs to M. President de Saron. I have examined that nebula with the greatest attention, in an entirely serene sky, as follows: February 25 & 26, 1773. Orion in the Meridian. March 19, between 8 & 9 o’clock in the evening. [March] 23, between 7 & 8 o’clock. The 25th & 26th of the same month, at the same time. These combined observations and the drawings brought together, have enabled me to represent with care and precision its shape and its appearances.

“This drawing will serve to recognize, in following times, if this nebula is subject to any changes. There may be already cause to presume this; for, if one compares this drawing with those given by MM. Huygens, Picard, Mairan and by le Gentil, one finds there such a change that one would have difficulty to figure out that this was the same. I will make these observations in the following with the same telescope and the same magnification. In the figure which I give, the circle represents the field of the telescope in its true aperture; it contains the Nebula and thirty Stars of different magnitudes. The figure is inverted, as it is shown in the instrument; one recognizes there also the extension and the limits of this nebula, the sensible difference between its clearest or most apparent light with that which merges gradually with the background of the sky. The jet of light, directed from the star no. 8 to the star no. 9, passing by a small star of the 10th magnitude, which is extremely rare, as well as the light directed to the star no. 10, and that which is opposite, where there are the eight stars contained in the nebula; among these stars, there is one of the eighth magnitude, six of the tenth, and the eighth of the eleventh magnitude. M. de Mairan, in his Traite de l’Aurore Boreale, speaks of the star no. 7. I report it in my drawing below such as it is at present, and as I have seen; so to speak surrounded by a thin nebulosity. In the night of October 14 to 15, 1764, in a serene sky, I determined with regard to Theta in the nebula, the positions of the more apparent stars in right ascension and declination, by the means of a micrometer adapted to a Newtonian telescope of 4 1/2 feet length. These stars are numbered up to ten; I have reported them in the drawing containing the field of the telescope; and an eleventh of them is beyond the circle. The positions of the stars which are not marked with numbers have been fixed by estimating their relative alignments. One will know easily also the magnitude of the Stars by the model which I have reported on the figure. Those of the tenth and the eleventh magnitude are absolutely telescopic and very difficult to find.”

However, it would be Sir William Herschel who would devote much love, time, and attention to the Great Orion Nebula – even though his findings would never be made public. As a true master observer, he had quite a talent for sensing what truly might lay beyond the boundary:

“In 1783, I reexamined the nebulous star, and found it to be faintly surrounded with a circular glory of whitish nebulosity, faintly joined to the great nebula. About the latter end of the same year I remarked that it was not equally surrounded, but most nebulous toward the south. In 1784 I began to entertain an opinion that the star was not connected with the nebulosity of the great nebula in Orion, but was one of those which are scattered over that part of the heavens. In 1801, 1806, and 1810 this opinion was fully confirmed, by the gradual change which happened in the great nebula, to which the nebulosity surrounding this star belongs. For the intensity of the light about the nebulous star had by this time been considerably reduced, by attenuation or dissipation of nebulous matter; and it seemed now to be pretty evident that the star is far behind the nebulous matter, and that consequently its light in passing through it is scattered and deflected, so as to produce the appearance of a nebulous star. A similar phenomenon may be seen whenever a planet or a star of the 1st or 2nd magnitude happens to be involved in haziness; for a diffused circular light will then be seen, to which, but in a much inferior degree, that which surrounds this nebulous star bears a great resemblance.”

But of course, the great Sir William Herschel also had nights from his many notes on M42 where he simply said: “The nebula in Orion which I saw by the front-view was so glaring and beautiful that I could not think of taking any place of its extent.”

Locating Messier 42:

Finding Messier 42 is very easy from a dark sky location by centering on the glowing region in the center of Orion’s “sword”. However, from urban locations, these stars might not be visible, so aim your binoculars or telescope about a fist width south of the three prominent stars that make the asterism known as Orion’s Belt. It’s a very bright and large object well suited to all sky conditions and instruments!

This chart shows the location of Messier 78 in the famous constellation of Orion (The Hunter). Credit: ESO, IAU and Sky & Telescope

Remember to use low power to get the full majesty of M42 and to increase magnification to study various regions. And trust us when we tell you, you are in for some pretty awesome viewing!

And of course, here are the quick facts on Messier 42 to help you get started:

Object Name: Messier 42
Alternative Designations: M42, NGC 1976, The Great Orion Nebula, Home of the Trapezium
Object Type: Emission and Reflection Nebula with Open Galactic Star Cluster
Constellation: Orion
Right Ascension: 05 : 35.4 (h:m)
Declination: -05 : 27 (deg:m)
Distance: 1.3 (kly)
Visual Brightness: 4.0 (mag)
Apparent Dimension: 85×60 (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 Chamaeleon Constellation

The Constellation Chamaeleon. Credit: Till Credner/AlltheSky.com

Welcome back to Constellation Friday! Today, in honor of the late and great Tammy Plotner, we will be dealing with that famous lizard that specializes at blending in – the Chamaeleon 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.

In time, this list would come to be expanded as astronomers became aware of more asterisms in the night sky. One of these is Chamaeleon, a small constellation located in the southern sky that was first defined in the 16th century. This constellation was appropriately named, given its ability to blend into the background! Today, it is one of the 88 constellations recognized by the IAU.

Name and Meaning:

Since Chamaeleon was unknown to the ancient Greeks and Romans, it has no mythology associated with it, but it’s not hard to understand how it came about its fanciful name. As exploration of the southern hemisphere began, what biological wonders were discovered! Can you imagine how odd a creature that could change its skin color to match its surroundings would be to someone who wasn’t familiar with lizards?

Map of the dark molecular clouds associated with the Chamaeleon constellation. Credit: Roberto Mura

Small wonder that a constellation that blended right in with the background stars could be considered a “chamaeleon” or that it might be pictured sticking its long tongue out to capture its insectile constellation neighbor – Musca the “fly”!

History of Observation:

Chamaeleon was one of twelve constellations created by Pieter Dirkszoon Keyser and Frederick de Houtman between 1595 and 1597. Both were Dutch navigators and early astronomical explorers who made attempts to chart southern hemisphere skies. Their work was added to Johann Bayer’s “Uranometeria” catalog in 1603, where Chamaeleon was first introduced as one of the 12 new southern constellations and its stars given Bayer designations.

To this day, Chamaeleon remain as one of the 88 modern constellations recognized by the IAU and it is bordered by Musca, Carina, Volans, Mensa, Octans and Apus. It contains only 3 main stars, the brightest of which is 4th magnitude Alpha – but it also has 16 Bayer/Flamsteed designated stars within its boundaries.

Notable Features:

The Chamaeleon constellation is home to several notable stars. These include Alpha Chamaeleontis, a spectral type F5III star located approximately 63.5 light years from Earth. Beta Chamaeleontis is a main sequence star that is approximately 270 light years distant. This star is the third brightest in the constellation, after Alpha and Gamma Chamaeleontis.

Artist’s concept of “hot Jupiter”, a Jupiter-sized planet orbiting closely to its star. Credit: NASA/JPL-Caltech

And then there’s HD 63454, a K-type main sequence star located approximately 116.7 light years away. It lies near the south celestial pole and is slightly cooler and less luminous than the Sun. In February of 2005, a hot Jupiter-like planet (HD 63454 b) was discovered orbiting the star.

The “Chamaeleon” also disguises itself with a huge number of dark molecular clouds that are often referred to as the “Chamaeleon Cloud Complex”. Situation about 15 degrees below the galactic plane, it is accepted is one of the closest low mass star forming regions to the Sun with a distance of about 400 to 600 light years.

Within these clouds are pre-main sequence star candidates, and low-mass T Tauri stars. The southern region of the Chamaeleon Cloud is a complex pattern of dark knots connected by elongated, dark, wavy filaments, with a serpentine-like shape. Bright rims with finger-like extensions are apparent, and a web of very faint, extremely thin but very long and straight shining filaments.

These feeble structures, reflecting stellar light, extend over the entire Chamaeleon complex and are considered very young – not yet capable of the type of collapse needed to introduce major star formation. Thanks to Gemini Near Infrared Spectrograph (GNIRS) on Gemini South Telescope, a very faint infrared object confirmed – a very low-mass, newborn brown dwarf star and the lowest mass brown dwarf star found to date in the Chamaeleon I cloud complex.

A newly formed star lights up the surrounding cosmic clouds in this image from ESO’s La Silla Observatory in Chile. Credit: ESO

Chamaeleon is also home to the Eta Chamaeleontis Cluster (aka. Mamajek 1). This open star cluster, which is centered on the star Eta Chamaeleontis, is approximately 316 light years distant and believed to be around eight million years old. The cluster was discovered in 1999 and consists of 12 or so relatively young stars. It was also the first open cluster discovered because of its X-ray emissions its member stars emit.

Finding Chamaeleon:

Chamaeleon is visible at latitudes between +0° and -90° and is best seen at culmination during the month of April. Now take out your telescope and aim it towards Eta for a look at newly discovered galactic star cluster – the Eta Chamaeleontis cluster – Mamajek 1. In 1999, a cluster of young, X-ray-emitting stars was found in the vicinity of eta Chamaeleontis from a deep ROSAT high-resolution imager observation.

They are believed to be pre-main-sequence weak-lined T Tauri stars, with an age of up to 12 million years old. The cluster itself is far from any significant molecular cloud and thus it has mysterious origins – not sharing proper motions with other young stars in the Chamaeleon region. There’s every possibility it could be a moving star cluster that’s a part of the Scorpius/Centaurus OB star association!

For binoculars, take a look at fourth magnitude Alpha Chamaeleontis. It is a rare class F white giant star that is about 63.5 light years from Earth. It is estimated to be about 1.5 billion years old. Its spectrum shows it to be a older giant with a dead helium core, yet its luminosity and temperature show it to be a younger dwarf.

The location of the Chamaeleon Constellation. Credit: IAU /Sky&Telescope magazine

Now point your binoculars or telescope towards Delta Chamaeleontis. While these two stars aren’t physically connect to one another, the visual double star is exceptionally pleasing with one orange component and one blue.

Last, but not least, take a look at Gamma Chamaeleontis. Although the south celestial pole currently lacks a bright star like Polaris to mark its position, the precession of the equinoxes will change that. One day – in the next 7500 years – the south celestial pole will pass close to the stars Gamma Chamaeleontis. But don’t wait up…

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: