Posted on by Tammy Plotner

Messier 54 – the NGC 6715 Globular Cluster

Hubble image of Messier 54, a globular cluster located in the Sagittarius Dwarf Galaxy. Credit: ESA/Hubble & NASA

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at globular cluster known as Messier 54!

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 these objects so 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 globular cluster known as Messier 54. Located in the direction of the Sagittarius constellation, this cluster was once thought to be part of the Milky Way, located about 50,000 light years from Earth, In recent decades, astronomers have come to realize that it is actually part of the Sagittarius Dwarf Galaxy, located some 87,000 light-years away.

What You Are Looking At:

Running away from us at a speed of 142 kilometers per second, this compact globe of stars could be as wide as 150 light years in diameter and as far away as 87,400 light years. Wait… Hold the press… Almost 90 thousand light years? Yeah. Messier 54 isn’t part of our own Milky Way Galaxy!

In 1994 astronomers made a rather shocking discovery… this tough to resolve globular was actually part of the Sagittarius Dwarf Elliptical Galaxy. As Michael H. Siegal (et al) said in their study:

“As part of the ACS Survey of Galactic Globular Clusters, we present new Hubble Space Telescope photometry of the massive globular cluster M54 (NGC 6715) and the superposed core of the tidally disrupted Sagittarius (Sgr) dSph galaxy. Our deep (F606W ~ 26.5), high-precision photometry yields an unprecedentedly detailed color-magnitude diagram showing the extended blue horizontal branch and multiple main sequences of the M54+Sgr system. Multiple turnoffs indicate the presence of at least two intermediate-aged star formation epochs with 4 and 6 Gyr ages and [Fe/H]=-0.4 to -0.6. We also clearly show, for the first time, a prominent, ~2.3 Gyr old Sgr population of near-solar abundance. A trace population of even younger (~0.1-0.8 Gyr old), more metal-rich ([Fe/H]~0.6) stars is also indicated. The Sgr age-metallicity relation is consistent with a closed-box model and multiple (4-5) star formation bursts over the entire life of the satellite, including the time since Sgr began disrupting.”

Inside its compact depths lurk at least 82 known variable stars – 55 of which are the RR Lyrae type. But astronomers using the Hubble Space telescope have have also discovered there are two semi-regular red variables with periods of 77 and 101 days. Kevin Charles Schlaufman and Kenneth John Mighell of the National Optical Astronomy Observatory explained in their study:

“Most of our candidate variable stars are found on the PC1 images of the cluster center – a region where no variables have been reported by previous ground-based studies of variables in M54. These observations cannot be done from the ground, even with AO as there are far too many stars per resolution element in ground-based observations.”

The globular cluster Messier 54. Credit: NASA

But what other kinds of unusual stars could be discovered inside such distant cosmic stellar evolutionary laboratory? Try a phenomena known as blue hook stars! As Alfred Rosenberg (et al) said in their study:

“We present BV photometry centered on the globular cluster M54 (NGC 6715). The color-magnitude diagram clearly shows a blue horizontal branch extending anomalously beyond the zero-age horizontal-branch theoretical models. These kinds of horizontal-branch stars (also called “blue hook” stars), which go beyond the lower limit of the envelope mass of canonical horizontal-branch hot stars, have so far been known to exist in only a few globular clusters: NGC 2808, Omega Centauri (NGC 5139), NGC 6273, and NGC 6388. Those clusters, like M54, are among the most luminous in our Galaxy, indicating a possible correlation between the existence of these types of horizontal-branch stars and the total mass of the cluster. A gap in the observed horizontal branch of M54 around Teff = 27,000 K could be interpreted within the late helium flash theoretical scenario, which is a possible explanation for the origin of blue hook stars.”

But with the stars packaged together so tightly, even more has been bound to occur inside of Messier 54. As Tim Adams (et al) indicated in their study:

“We investigate a means of explaining the apparent paucity of red giant stars within post-core-collapse globular clusters. We propose that collisions between the red giants and binary systems can lead to the destruction of some proportion of the red giant population, by either knocking out the core of the red giant or by forming a common envelope system which will lead to the dissipation of the red giant envelope. Treating the red giant as two point masses, one for the core and another for the envelope (with an appropriate force law to take account of the distribution of mass), and the components of the binary system also treated as point masses, we utilize a four-body code to calculate the time-scales on which the collisions will occur. We then perform a series of smooth particle hydrodynamics runs to examine the details of mass transfer within the system. In addition, we show that collisions between single stars and red giants lead to the formation of a common envelope system which will destroy the red giant star. We find that low-velocity collision between binary systems and red giants can lead to the destruction of up to 13 per cent of the red giant population. This could help to explain the colour gradients observed in PCC globular clusters. We also find that there is the possibility that binary systems formed through both sorts of collision could eventually come into contact perhaps producing a population of cataclysmic variables.”

Messier 54, as imaged by the VLT Survey Telescope at ESO’s Paranal Observatory in northern Chile. Credit: ESO

But the discoveries haven’t ended yet…. Because 2009 studies have revealed evidence for an intermediate mass black hole inside Messier 54 – the first known to have ever been discovered in a globular cluster.

“We report the detection of a stellar density cusp and a velocity dispersion increase in the center of the globular cluster M54, located at the center of the Sagittarius dwarf galaxy (Sgr). The central line-of-sight velocity dispersion is 20.2 ± 0.7 km s-1, decreasing to 16.4 ± 0.4 km s-1 at 2farcs5 (0.3 pc). Modeling the kinematics and surface density profiles as the sum of a King model and a point-mass yields a black hole mass of ~9400 M sun.” says R. Ibata (et al), “However, the observations can alternatively be explained if the cusp stars possess moderate radial anisotropy. A Jeans analysis of the Sgr nucleus reveals a strong tangential anisotropy, probably a relic from the formation of the system.”

History of Observation:

On July 24, 1778 when Charles Messier first laid eyes on this faint fuzzy, he had no clue that he was about to discover the very first extra-galactic globular cluster. In his notes he writes: “Very faint nebula, discovered in Sagittarius; its center is brilliant and it contains no star, seen with an achromatic telescope of 3.5 feet. Its position has been determined from Zeta Sagittarii, of 3rd magnitude.”

Years later Sir William Herschel would also study M54, and in his private notes he writes: “A round, resolvable nebula. Very bright in the middle and the brightness diminishing gradually, about 2 1/2′ or 3′ in diameter. 240 shews too pretty large stars in the faint part of the nebulosity, but I rather suppose them to have no connection with the nebula. I believe it to be no other than a miniature cluster of very compressed stars.”

Countless other observations would follow as the M54 became cataloged by other astronomers and each would in turn describe it only as having a much brighter core and some resolution around the edges. Have fun trying to crack this one!

Locating Messier 54:

M54 isn’t hard to find… Just skip down to Zeta Sagittarii, the southwestern-most star of Sagittarius “teapot” and hop a half degree south and a finger width (1.5 degrees) west. The problem is seeing it! In small optics, such as binoculars or a finder scope, it will appear almost stellar because of its small size. However, if you just look for what appears like a larger, dim star that won’t quite come into perfect focus, then you’ve found it.

In smaller telescopes, you’ll get no resolution on this class III globular cluster because it is so dense. Large aperture doesn’t fare much better either, with only some individual stars making their appearance at the outer perimeters. Because of magnitude and size, Messier 54 is better suited to dark sky conditions.

The location of Messier 54 in the Sagittarius constellation. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

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

Object Name: Messier 54
Alternative Designations: M54, NGC 6715
Object Type: Class III Extragalactic Globular Cluster
Constellation: Sagittarius
Right Ascension: 18 : 55.1 (h:m)
Declination: -30 : 29 (deg:m)
Distance: 87.4 (kly)
Visual Brightness: 7.6 (mag)
Apparent Dimension: 12.0 (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 53 – the NGC 5024 Globular Cluster

Messier 53, as imaged by the Hubble Space Telescope. Credit: ESA/Hubble & NASA

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at globular cluster known as Messier 53!

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 these objects so 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 Messier 53, a globular cluster located in the northern Coma Berenices constellation. Located about 58,000 light years from the Solar System, it is almost equidistant from Galactic Center (about 60,000 light years). As Messier Objects go, it is relatively easy to find since it lies in the same area of the sky as Arcturus, the fourth brightest star in the night sky.

Description:

Heading towards us at a speed of 112 kilometers per second, globular cluster M53 is one of the furthest distant globular clusters in our Milky Way halo and lay almost equally distant between our solar system and the galactic center. This 220 light year diameter ball of stars in tightly compacted towards its core – where low metal is the name of the game and RR Lyra type variable stars once ruled. But recent studies have found that there are some new kids on the block. The blue stragglers…

Messier 53, as imaged by the Hubble Space Telescope. Credit: ESA/Hubble & NASA

According to G. Beccari (et al) the population of these definitely appears to violate standard theories of stellar evolution. And there not just a few blues… There’s a whole host of them. As Beccari noted in a 2008 study:

“We used a proper combination of high-resolution and wide-field multiwavelength observations collected at three different telescopes (HST, LBT, and CFHT) to probe the blue straggler star (BSS) population in the globular cluster M53. Almost 200 BSSs have been identified over the entire cluster extension. We have also used this database to construct the radial star density profile of the cluster; this is the most extended and accurate radial profile ever published for this cluster, including detailed star counts in the very inner region. A deviation from the model is noted in the most external region of the cluster. This feature needs to be further investigated in order to address the possible presence of a tidal tail in this cluster.”

Is this possible? Then take a closer look into this research. One where a millisecond pulsar was discovered inside. As S.R. Kulkarni (et al) indicated in a 1991 study:

“Millisecond pulsars are conventionally assumed to be spun up through the action of binary companions, although some subsequently lose their companions and appear as isolated pulsars. Such objects should therefore be more numerous in dense stellar systems. We report here the surprising discovery of two pulsars in low-density globular clusters: one is a single 10-ms pulsar (1639+36) in M13 (NGC 6205), the other a 33-ms pulsar (1310+18) in a 256-d binary in M53 (NGC 5024). Their ages, inferred from their luminosities and constraints on their period derivatives, seem to be 10 9 years, significantly greater than previously reported ages ( ! 10 8 years) of cluster pulsars. The implied birth rate is inconsistent with the conventional two-body tidal capture model, suggesting that an alternative mechanism such as tidal capture between primordial binaries and a reservoir of (hundreds of) primordial neutron stars may dominate the production of tidal binaries in such clusters. The period derivative of PSR1639+36 is surprisingly small, and may be corrupted by acceleration due to the mean gravitational potential of the cluster.”

The Messier 53 globular star cluster. Credit: Ole Nielsen

History of Observation:

This globular cluster was first discovered on February 3, 1775 by Johann Elert Bode, but independently recovered on February 26, 1777 by Charles Messier who writes:

“Nebula without stars discovered below & near Coma Berenices, a little distant from the star 42 in that constellation, according to Flamsteed. This nebula is round and conspicuous. The Comet of 1779 was compared directly with this nebula, & M. Messier has reported it on the chart of that comet, which will be included in the volume of the Academy for 1779. Observed again April 13, 1781: It resembles the nebula which is below Lepus [M79].”

Sir William Herschel would revisit M53, but he did not publish his findings when studying Messier objects. Very seldom did Herschel wax poetic in his writings, but of this particular object he said: “A cluster of very close stars; one of the most beautiful objects I remember to have seen in the heavens. The cluster appears under the form of a solid ball, consisting of small stars, quite compressed into one blaze of light, with a great number of loose ones surrounding it, and distinctly visible in the general mass.”

He would return again in later years to include in his notes: “From what has been said it is obvious that here the exertion of a clustering power has brought the accumulation and artificial construction of these wonderful celestial objects to the highest degree of mysterious perfection.”

The Messier 53 globular cluster. Credit: NASA/ESA/Hubble

Although it did not touch Sir John Herschel quite so much, M53 also engaged Admiral Smyth who wrote:

“A globular cluster, between Berenice’s tresses and the Virgin’s left hand, with a coarse pair of telescopic stars in the sf [south following, SE] quadrant, and a single one in the sp [south preceding, SW]. This is a brilliant mass of minute stars, from the 11th to the 15th magnitude, and from thence to gleams of star-dust, with stragglers to the np [north preceding, NW], and pretty diffused edges. From the blaze at the centre, it is evidently a highly compressed ball of stars, whose law of aggregation into so dense and compact a mass, is utterly hidden from our imperfect senses. It was enrolled by Messier in 1774 as No. 53, and resolved into stars by Sir W. Herschel. The contemplation of so beautiful an object, cannot but set imagination to work, though the mind may be soon lost in astonishment at the stellar dispositions of the great Creator and Maintainer. Thus, in reasoning by analogy, these compressed globes of stars confound conjecture as to the models in which the mutual attractions are prevented from causing the universal destruction of their system. Sir John Herschel thinks, that no pressure can be propagated through a cluster of discrete stars; whence it would follow, that the permanence of its form must be maintained in a way totally different from that which our reasoning suggest. Before quitting this interesting ball of innumerable worlds, I may mention that it was examined by Sir John Herschel, with Mr. Baily, in the 20-foot reflector; and that powerful instrument showed the cluster with curved appendages of stars, like the short claws of a crab running out from the main body. A line through Delta and Epsilon Virginis, northward, meeting another drawn from Arcturus to Eta Bootis, unite upon this wonderful assemblage; or it is also easily found by its being about 1 deg northeast of 42 Comae Berenices, the alignment of which is already given.”

Locating Messier 53:

M53 can be easily found just about a degree northeast of 42 Alpha Comae Berenices, a visual binary star. To located Alpha, draw a mental line from Arcturus via Eta Bootis where you’ll see it about a fist width west. Alternately you can starhop from Gamma Viginis to Delta and on to Epsilon where you can locate M53 approximately 4 fingerwidths to the north/northeast.

To see this small globular cluster in binoculars will require dark skies and it will appear very small, like a large, out of focus star. In small telescopes it will appear almost cometary – and thus why Messier cataloged these objects! However, with telescopes approaching the 6″ range, resolution will begin and larger telescopes will shatter this gorgeous globular cluster. Requires dark skies.

The location of Messier 53 in the northern Coma Berenices constellation. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

A ball of worlds… What a unique description! May you enjoy your observations as well!

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

Object Name: Messier 53
Alternative Designations: M53, NGC 5024
Object Type: Class V Globular Cluster
Constellation: Coma Berenices
Right Ascension: 13 : 12.9 (h:m)
Declination: +18 : 10 (deg:m)
Distance: 58.0 (kly)
Visual Brightness: 7.6 (mag)
Apparent Dimension: 13.0 (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 52 – the NGC 7654 Open Star Cluster

The location of the Messier 52 open star cluster, located in the direction of the southern constellation Cassiopeia. Credit: Wikisky

Welcome back to Messier Monday! We continue our tribute to our dear friend, Tammy Plotner, by looking at the open star cluster of Messier 52. Enjoy!

In the 18th century, while searching the night sky for comets, French astronomer Charles Messier kept noting the presence of fixed, diffuse objects in the night sky. In time, he would come to compile a list of approximately 100 of these objects, with the purpose of making sure that astronomers did not mistake them for comets. However, this list – known as the Messier Catalog – would go on to serve a more important function.

One of these objects is Messier 52, an open star cluster that can seen in proximity to the northern constellation Cassiopeia. Located about 5000 light years from Earth, this star cluster is easily spotted in the night sky because of its association with Cassiopeia’s familiar W-shape. It can viewed with binocular and telescopes, and will appears as a hazy, nebulous patch of light.

Description:

Located roughly 5000 light years away, this 35 million year old cluster of stars has around 200 members – one of which is a very peculiar Of star. According to A.K. Pandy (et al), M52 is an interesting cluster in which to study star formation history. As they stated in their 2001 study:

“The colour magnitude diagrams show a large age spread in the ages. Star formation was biased towards relatively higher masses during the early phase of star formation whereas most of the low mass stars of the cluster were formed during the later phase. The star formation seems to have been a gradual process that proceeded sequentially in mass and terminated with the formation of most massive stars.”

The Messier 52 open star cluster. Credit: Wikisky

Indeed, M52 has been very studied for its star structure, including a search for variables. As S.L. Kim (et al), wrote in a 2000 study:

“We have performed a long-term project of CCD photometry of open clusters. Its primary goal is to search for variable stars, in particular short-period (less than a few days) pulsating stars such as Delta Sct, Gamma Dor, and slowly pulsating B-type stars (SPBs). These pulsating stars are recognized as important objects in studying stellar structure and testing evolution theory of intermediate-mass main sequence stars. Thus these clusters are ideal targets to investigate whether Gamma Dor type variability occurs in old open clusters or not.”

And it’s not just the structure they’re looking at – but the time frame in which they formed. As Anil K. Pandey wrote in her 2001 study:

“The distribution of stars in NGC 7654 indicates that the star formation within the cluster is not coeval and has an age spread -50 Myr. We found that star formation took place sequentially in the sense that low mass stars formed first. The star formation history in NGC 7654 supports the conventional picture of star formation in cluster where ‘low mass stars’ form first and star formation continues over a long period of time. The star formation within the cluster terminates with the formation of most massive stars in the cluster.”

History of Observation:

M52 was an original discovery of Charles Messier, captured on the night of September 7th, 1774. As he wrote in his notes at the time:

“Cluster of very small stars, mingled with nebulosity, which can be seen only with an achromatic telescope. It was when he observed the Comet which appeared in this year that M. Messier saw this cluster, which was close to the comet on the 7th of September 1774; it is below the star d Cassiopeiae: that star was used to determine both the cluster of stars and the comet.”

Atlas Image mosaic of Messier 52, as part of the Two Micron All Sky Survey (2MASS). Credit: UMass/UPAC/Caltech/NASA/NSF

Sir William Herschel would also observe M52, but he would keep his notes private. As he wrote on August 29th, 1873:

“All resolved into innumerable small stars without any suspicion of nebulosity. 7 ft., 57. In the sweeper, 30, shews nebulosity, the stars being too obscure to be distinguished with its light tho’ considerable.” and again on December 23, 1805: “Review. Large 10 feet. This is a cluster of pretty condensed stars of different sizes. It is situated in a very rich part of the heavens and can hardly be called insulated, it may only be a very condensed part of the Milky Way which is here much divided and scattered. It is however so far drawn together with some accumulation that it may be called a cluster of the third order.”

Herschel’s son John would also add it to the General Catalog a few years later with less descriptive narrative, but it was Admiral Smyth who described M52’s beauty best when he said:

“An irregular cluster of stars between the head of Cepheus and his daughter’s throne; it lies north-west-by-west of Beta Cassiopeiae, and one third of the way towards Alpha Cephei. This object assumes somewhat of a triangular form, with an orange-tinted 8th-mag star at its vertex, giving it the resemblance of a bird with outspread wings. It is preceded by two stars of 7th and 8th magnitudes, and followed by another of similar brightness; and the field is one of singular beauty under a moderate magnifying power. While these were under examination, one of those bodies called falling stars passed through the outliers. This phenomenon was so unexpected and sudden as to preclude attention to it; but it appeared to be followed by a train of glittering and very minute spangles.”

May it glitter and spangle for you!

The location of Messier 52 in proximity to the constellation Cassiopeia. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

Locating Messier 52:

In the rich star cluster fields of Cassiopeia, M52 is distinctive for its size and brightness. It’s not hard to find! Begin by identifying the W-shape of Cassiopeia and focus on its two brightest stars – Alpha and Beta. Because this constellation is circumpolar, remembering to look at the side that has the brightest stars or the steepest angle, will help you remember how to find this great open cluster. Now, just draw a mental line between Alpha, the lower star, and Beta, the upper.

Extend that line into space about the same distance and aim your binoculars or finderscope there. In binoculars M52 will show clearly as a beginning to resolve star cloud and a hazy patch in a telescope finderscope. Even the smallest of telescopes can expect resolution from this multi-magnitude beauty and the more aperture you apply, the more stars you will see. M52 is well suited to urban or light polluted skies and stands up well to fairly moonlit conditions and hazy skies.

Object Name: Messier 52
Alternative Designations: M52, NGC 7654
Object Type: Open Galactic Star Cluster
Constellation: Cassiopeia
Right Ascension: 23 : 24.2 (h:m)
Declination: +61 : 35 (deg:m)
Distance: 5.0 (kly)
Visual Brightness: 7.3 (mag)
Apparent Dimension: 13.0 (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, 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 Crater Constellation

The Crater Constellation relative to others in the south sky. Credit: go-astronomy.com

Welcome to another edition of Constellation Friday! Today, in honor of the late and great Tammy Plotner, we take a look at “The Cup” – the Crater 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 constellation is Crater (aka. “The Cup”), an asterism located in the Southern Hemisphere. This small constellation is located south of the ecliptic plane, with no bright marker stars. As part of the Hercules family, it is bordered by the constellations of Leo, Sextans, Hydra, Corvus and Virgo. Today, it is one of the 88 modern constellations recognized by the International Astronomical Union.

Name and Meaning:

In Greek mythology, Crater represents the Cup of Apollo – the god of the skies – which is due to its chalice-like configuration. The cup is being held up by the Raven – Corvus – another figure in Greek mythology. The tale, much like many mythological stories, is a sad one, and begins with the Raven being sent to fetch water for his master, Apollo.

Unfortunately, Corvus (the Raven) was distracted as he became tempted by a fig, and then waited too long for it to ripen. When he realized his mistake, he returned sorrowfully to Apollo with his cup (Crater) and brought along the serpent Hydra in his claws as well. Angry, Apollo tossed all three into the sky for all eternity, where they became part of the starry firmament.

Corvus, Crater and other constellations seen around Hydra. From Urania's Mirror (1825). Credit: US Library of Congress
Corvus, Crater and other constellations seen around Hydra. From Urania’s Mirror (1825). Credit: US Library of Congress

History of Observation:

The Crater constellation comes to us from Classical Antiquity and was recorded by Ptolemy in his 2nd-century CE tract the Almagest. However, it was also recognized by Chinese astronomers, where the stars associated with it were viewed as being part the Vermillion Bird of the South (Nan Fang Zhu Que). Along with the some of the stars from Hydra, they depict the Red Bird’s wings.

Notable Objects:

Crater has only a few bright stars associated with it and no Messier Objects. The brightest, Delta Crateris, is an orange giant located approximately 196 light yeas from Earth. The star is also known as Labrum (Latin for “the lip”), due to the fact that it was sometimes associated with the story of the Holy Grail.

Next is Alpha Crateris, an orange giant located approximately 174 light-years from Earth which is 80 times more luminous than our Sun. It is also known as Alkes, derived from the Arabic word alkas, which means “the cup”. Then there’s Beta Crateris, a white sub-giant that is located approximately 266 light years from Earth. This star is also known by the name Al Sharasif, which means “the ribs” in Arabic.

In terms of Deep Sky Objects, Crater has no associated Messier Objects, but a few galaxies can be found in its region of the night sky. These include the Crater 2 dwarf galaxy, a satellite galaxy of the Milky Way that is located approximately 380,000 light years from Earth. There’s also the spiral galaxy known as NGC 3511, which has a slight bar and is seen from Earth nearly edge-on.

The spiral galaxy NGC 3887, located in the constellation Crater. Credit: NASA (Wikisky)

There’s also the NGC 3887 and NGC 2981 spiral galaxies, and the RX J1131 quasar, which is located 6 billion light years away from Earth. Interestingly, the black hole at the center of this quasar was the first to have its spin directly measured by astronomers.

Finding Crater:

Crater is visible at latitudes between +65° and -90° and is best seen at culmination during the month of April. It is comprised of only 4 main stars, and 12 stars with Bayer/Flamsteed designations. In order to spot these stars, observers should begin by looking for the Alpha star (the “a” shape on their star map) with binoculars.

Situated some 174 light-years from Earth, Alpha Crateris (the star’s official designation) is a spectral class K1 star – an orange giant that’s a little different from the rest. This is because Alkes is a “high velocity” star, which means it moves far faster than the stars around it. Another thing that sets it apart is its high metal content, which according to some researchers, it may have picked up when it came from the inner, metal-rich part of the Galaxy.

Artist's impression of Alkes - aka. Alpha Crateris. Credit: constellation-guide.com
Artist’s impression of Alkes – aka. Alpha Crateris – a K1 orange giant star in the Crater constellation. Credit: constellation-guide.com

Next, observers should look to Beta Crateris (the “B” shape on the map) which also goes by the name of Al Sharasif. This star is not an ordinary one either. For starters, Al Sharasif is about 265 light-years from our solar system, and it’s a white sub-giant star. To boot, it also has a low mass, white dwarf companion – which is why astronomers classify it as a Sirius-like system.

Next up is Delta Crateris – the “8” symbol on the map – which is an orange giant, spectral class K0III star with an apparent magnitude of 3.56. In time, this star will become an even larger giant, eventually turning into a Mira-type variable star before ending its life as a white dwarf. Oddly enough, Labrum has a very low metal content compared to its Crater-neighbors, containing about 40% as much iron as our own Sun.

At this point, observers with telescopes and have a look at Gamma Crateris – the “Y” shape on the map. Gamma Crateris is a fixed binary white dwarf star with an easy separation of 5.2″. Gamma itself is 89 light-years for Earth, which is rather hard to believe when you try to seek out the 9.5 stellar magnitude companion that accompanies it.

Although this is a disparate double star, it is still quite fun and easy to spot with a small telescope. For a challenge, try Iota Crateris – a close binary star with an 11th magnitude companion that’s only separated by 1.4″. Psi Crateris is an even closer binary. Both stars are within a half magnitude of each other, but the separation is only 0.2″.

Artist's impression of white dwarf binary pair CSS 41177. Image: Andrew Taylor.
Artist’s impression of white dwarf binary pair, a type of star system that describes Gamma Crateris. Credit: Andrew Taylor.

Next up is R Crateris, a variable star that can be observed with binoculars, and which is located at RA 10 56 Dec -17 47. You will notice it by its lovely red color and its nice change of magnitude, which goes from 8 to 9.5 in a period of about 160 days. And then there’s SZ Crateris, a magnitude 8.1 variable star. It is a nearby star system located about 44 light years from the Sun and is known as Gliese 425 – which in the past was known as Abt’s Star.

While there’s no brighter deep sky objects for binoculars or small telescopes, there are a couple of challenging galaxies in the Crater constellation that are well suited to a large aperture. Let’s start with the brightest – elliptical galaxy NGC 3962 – which is easy to spot (like all elliptical galaxies), though there’s not much detail to be seen. Even if it is not terribly exciting to behold, it is on the Herschel 400 observing list.

And then there’s NGC 3887 (11h47.1 -16 51), a nice spiral galaxy that’s only slightly fainter. It has two faint stars which accompany it and a stellar nucleus which occasionally makes an appearance and provides an opportunity for some very interesting viewing. Both of these galaxies are in the slightly fainter range, both being just under magnitude 11.

Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)
List of the stars that appear in the Crater constellation. Credit: IAU and Sky & Telescope magazine/Roger Sinnott & Rick Fienberg

Observers who are skilled with telescopes should also keep and eye out for NGC 3511 (11h03.4 -23 05), a spiral galaxy of magnitude 11.5. It is joined in the same field of view by NGC 3513, a barred spiral galaxy that is a full magnitude dimmer. People with larger telescopes should also take a crack at spotting NGC 3672 (11h25.0 -09 48), a faint spiral galaxy that nevertheless has nice halo and a bright, apparent nucleus.

And last, but not least, there is NGC 3981 (11h56.1 -19 54), a beautifully inclined, magnitude 12 spiral galaxy that has a bright nucleus, and which sometimes shows some spiral galaxy structure when observing conditions are right.

Drink up… the “Cup” is waiting!

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 51 – the Whirlpool Galaxy

Visible light (left) and infrared image (right) of the Whirlpool Galaxy, taken by NASA’s Hubble Space Telescope. Credit: NASA/ESA/M. Regan & B. Whitmore (STScI), & R. Chandar (U. Toledo)/S. Beckwith (STScI), & the Hubble Heritage Team (STScI/AURA

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at that swirling, starry customer, the Whirlpool Galaxy!

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 spiral galaxy located in the constellation Canes Venatici known as the Whirlpool Galaxy (aka. Messier 51). Located between 19 and 27 million light-years from the Milky Way, this deep sky object was the very first to be classified as a spiral galaxy. It is also one of the best known galaxies among amateur astronomers, and is easily observable using binoculars and small telescopes.

Description:

Located some 37 million light years away, M51 is the largest member of a small group of galaxies, which also houses M63 and a number of fainter galaxies. To this time, the exact distance of this group isn’t properly known… Even when a 2005 supernova event should have helped astronomers to correctly calculate! As K. Takats stated in a study:

“The distance to the Whirlpool galaxy (M51, NGC 5194) is estimated using published photometry and spectroscopy of the Type II-P supernova SN 2005cs. Both the expanding photosphere method (EPM) and the standard candle method (SCM), suitable for SNe II-P, were applied. The average distance (7.1 +/- 1.2 Mpc) is in good agreement with earlier surface brightness fluctuation and planetary nebulae luminosity function based distances, but slightly longer than the distance obtained by Baron et al. for SN 1994I via the spectral fitting expanding atmosphere method. Since SN 2005cs exhibited low expansion velocity during the plateau phase, similarly to SN 1999br, the constants of SCM were recalibrated including the data of SN 2005cs as well. The new relation is better constrained in the low-velocity regime, that may result in better distance estimates for such SNe.”

Visible light (left) and infrared image (right) of M51, taken by the Kitt Peak National Observatory and NASA’s Spitzer Space Telescope, respectively. Credit: NASA/JPL-Caltech/R. Kennicutt (Univ. of Arizona)/DSS

Of course, one of the most outstanding features of the Whirlpool Galaxy is its beautiful spiral structure – perhaps result of the close interaction between it and its companion galaxy NGC 5195? As S. Beckwith,

“This sharpest-ever image of the Whirlpool Galaxy, taken in January 2005 with the Advanced Camera for Surveys aboard NASA’s Hubble Space Telescope, illustrates a spiral galaxy’s grand design, from its curving spiral arms, where young stars reside, to its yellowish central core, a home of older stars. At first glance, the compact galaxy appears to be tugging on the arm. Hubble’s clear view, however, shows that NGC 5195 is passing behind the Whirlpool. The small galaxy has been gliding past the Whirlpool for hundreds of millions of years. As NGC 5195 drifts by, its gravitational muscle pumps up waves within the Whirlpool’s pancake-shaped disk. The waves are like ripples in a pond generated when a rock is thrown in the water. When the waves pass through orbiting gas clouds within the disk, they squeeze the gaseous material along each arm’s inner edge. The dark dusty material looks like gathering storm clouds. These dense clouds collapse, creating a wake of star birth, as seen in the bright pink star-forming regions. The largest stars eventually sweep away the dusty cocoons with a torrent of radiation, hurricane-like stellar winds, and shock waves from supernova blasts. Bright blue star clusters emerge from the mayhem, illuminating the Whirlpool’s arms like city streetlights.”

But there were more surprises just waiting to be found – like a black hole, surrounded by a ring of dust. What makes it even more odd is a secondary ring crosses the primary ring on a different axis, a phenomenon that is contrary to expectations and a pair of ionization cones extend from the axis of the main dust ring. As H. Ford,

“This image of the core of the nearby spiral galaxy M51, taken with the Wide Field Planetary camera (in PC mode) on NASA’s Hubble Space Telescope, shows a striking , dark “X” silhouetted across the galaxy’s nucleus. The “X” is due to absorption by dust and marks the exact position of a black hole which may have a mass equivalent to one-million stars like the sun. The darkest bar may be an edge-on dust ring which is 100 light-years in diameter. The edge-on torus not only hides the black hole and accretion disk from being viewed directly from earth, but also determines the axis of a jet of high-speed plasma and confines radiation from the accretion disk to a pair of oppositely directed cones of light, which ionize gas caught in their beam. The second bar of the “X” could be a second disk seen edge on, or possibly rotating gas and dust in MS1 intersecting with the jets and ionization cones.”

History of Observation:

The Whirlpool Galaxy was first discovered by Charles Messier on October 13th, 1773 and re-observed again for his records on January 11th, 1774. As he wrote of his discovery in his notes:

“Very faint nebula, without stars, near the eye of the Northern Greyhound [hunting dog], below the star Eta of 2nd magnitude of the tail of Ursa Major: M. Messier discovered this nebula on October 13, 1773, while he was watching the comet visible at that time. One cannot see this nebula without difficulties with an ordinary telescope of 3.5 foot: Near it is a star of 8th magnitude. M. Messier reported its position on the Chart of the Comet observed in 1773 & 1774. It is double, each has a bright center, which are separated 4’35”. The two “atmospheres” touch each other, the one is even fainter than the other.”

It would be his faithful friend and assistant, Pierre Mechain who would discover NGC 5195 on March 21st, 1781. Even though it would be many, many years before it was proven that galaxies were indeed independent systems, historic astronomers were much, much sharper than we gave them credit for. Sir William Herschel would observe M51 many times, but it would be his son John who would be the very first to comment on M51’s scheme:

“This very singular object is thus described by Messier: – “Nebuleuse sans etoiles.” “On ne peut la voir que difficilement avec une lunette ordinaire de 3 1/2 pieds.” “Elle est double, ayant chacune un centre brillant eloigne l’un de l’autre de 4′ 35″. Les deux atmospheres se touchent.” By this description it is evident that the peculiar phenomena of the nebulous ring which encircles the central nucleus had escaped his observation, as might have been expected from the inferior light of his telescopes. My Father describes it in his observations of Messier’s nebulae as a bright round nebula, surrounded by a halo or glory at a distance from it, and accompanied by a companion; but I do not find that the partial subdivision of the ring into two branches throughout its south following limb was noticed by him. This is, however, one of its most remarkable and interesting features. Supposing it to consist of stars, the appearance it would present to a spectator placed on a planet attendant on one of them eccentrically situated towards the north preceding quarter of the central mass, would be exactly similar to that of our Milky Way, traversing in a manner precisely analogous the firmament of large stars, into which the central cluster would be seen projected, and (owing to its distance) appearing, like it, to consist of stars much smaller than those in other parts of the heavens. Can it, then, be that we have here a brother-system bearing a real physical resemblance and strong analogy of structure to our own? Were it not for the subdivision of the ring, the most obvious analogy would be that of the system of Saturn, and the idea of Laplace respecting the formation of that system would be powerfully recalled by this object. But it is evident that all idea of symmetry caused by rotation on an axis must be relinquished, when we consider that the elliptic form of the inner subdivided portion indicates with extreme probability an elevation of that portion above the plane of the rest, so that the real form must be that of a ring split through half its circumference, and having the split portions set asunder at an angle of about 45 deg each to the plane of the other.”

Sketch of M51 by William Parsons, 3rd Earl of Rosse (Lord Rosse) in 1845. Credit: Public Domain

As with other Messier Objects, Admiral Smyth also had some insightful and poetic observations to add. As he wrote of this galaxy in September of 1836:

“We have then an object presenting an amazing display of the uncontrollable energies of the Omnipotence, the contemplation of which compels reason and admiration to yield to awe. On the outermost verge of telescopic reach we perceive a stellar universe similar to that to which we belong, whose vast amplitudes no doubt are peopled with countless numbers of percipient beings; for those beautiful orbs cannot be considered as mere masses of inert matter.

And it is interesting to know that, if there be intelligent existence, an astronomer gazing at our distant universe, will see it, with a good telescope, precisely under the lateral aspect which theirs presents to us. But after all what do we see? Both that wonderful universe, our own, and all which optical assistance has revealed to us, may be only the outliers of a cluster immensely more numerous.

The millions of suns we perceive cannot comprise the Creator’s Universe. There are no bounds to infinitude; and the boldest views of the elder Herschel only placed us as commanding a ken whose radius is some 35,000 times longer than the distance of Sirius from us. Well might the dying Laplace explain: “That which we know is little; that which we know not is immense.”

Lord Rosse would continue on in 1844 with his 6-feet (72-inch) aperture, 53-ft FL “Leviathan” telescope, but he was a man of fewer words.

“The greater part of the observations were made when the eye was affected by lamp-light, which made it difficult to estimate correctly the centre of the nucleus; it was of importance that no time should be unnecessarily spent, and after the lamp had been used a new measure was taken, as it was judged that the object was sufficiently seen. With the brighter stars this would frequently happen before the nucleus was well defined, as all impediments to vision seem to affect nebulae much more than stars the light of which would be estimated as of the same intensity. In the foregoing list the greatest discrepancies are in the measures of bright objects, and this is probably the proper account of it. No stars have been inserted in the sketch which are not in the table of the measurements. The general appearance of the object would have been better given if the minute stars had been put in from the eye-sketch, but it would have created confusion.”

May the stars from this distant island universe fill your eyes!

The Whirlpool Galaxy (Spiral Galaxy M51, NGC 5194), a classic spiral galaxy located in the Canes Venatici constellation, and its companion NGC 5195. Credit: NASA/ESA

Locating Messier 51:

Locating M51 isn’t too hard if you have dark skies, but this particular galaxy is very difficult where light pollution of moonlight is present. To find it, start with Eta UM, the star at the handle of the Big Dipper. In the finderscope or binoculars, you’ll clearly see 24 UM to the southwest. Now, center your optics there and move slowly southwest towards Cor Caroli (Alpha CVn) and you’ll find it!

In locations where skies are clear and dark, it is easy to see spiral structure in even small telescopes, or to make out the galaxy in binoculars – but even a change in sky conditions can hide it from a good location. Rich field telescopes with fast focal lengths to an outstanding job on this galaxy and companion and you may be able to make out the nucleus of both galaxies on a good night from even a bad location.

Object Name: Messier 51
Alternative Designations: M51, NGC 5194, The Whirlpool Galaxy
Object Type: Type Sc Galaxy
Constellation: Canes Venatici
Right Ascension: 13 : 29.9 (h:m)
Declination: +47 : 12 (deg:m)
Distance: 37000 (kly)
Visual Brightness: 8.4 (mag)
Apparent Dimension: 11×7 (arc min)

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

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

Sources:

 

Posted on by Tammy Plotner

Messier 50 – the NGC 2323 Open Star Cluster

The Messier 50 open star cluster, shown in proximity to the Seagull Nebula (IC 2177). Credit: Wikisky

Welcome back to Messier Monday! We continue our tribute to our dear friend, Tammy Plotner, by looking at the open star cluster of Messier 50. Enjoy!

In the 18th century, while searching the night sky for comets, French astronomer Charles Messier kept noting the presence of fixed, diffuse objects in the night sky. In time, he would come to compile a list of approximately 100 of these objects, with the purpose of making sure that astronomers did not mistake them for comets. However, this list – known as the Messier Catalog – would go on to serve a more important function.

One of these objects is the open star cluster known as Messier 50 (aka. NGC 2323). Located at a distance of about 3,200 light-years from Earth, this object sits near the border between the Monoceros and Canis Major constellations. It is described as a ‘heart-shaped’ figure, occupies an area about half the size of the full Moon, and is easy to find because of its proximity to Sirius (the brightest star in the night sky).

Description:

Located about 3,200 light years from our solar system, this stellar gathering could be perhaps as much as 20 light years across, but the central concentration is believed to only span across roughly 10 light years. While that doesn’t seem that large, it’s lit by the candlepower of what could be 200 stars! And picking such a group of stars out of a well-known OB1 association isn’t easy. It requires photometry. As J.J. Claria (et al) remarked in a 1997 study:

“UBV and DDO photoelectric photometry in the field of the open cluster NGC 2323 is presented. The analysis yields 109 probable members; one of them being a red giant, and 3 possible members. The basic cluster parameters are derived. NGC 2323 appears not to be physically connected to the CMa OB1 association.”

Close up of the Messier 50 open star cluster. Credit: Wikisky

In this region of the sky are vast molecular clouds compressing into star forming regions known as OB1 associations. The stars spawned by these vast clouds form into open clusters containing dozens to thousands of members and, over time, disassociate with not only the molecular cloud, but their sibling star clusters as well. Sure, it took 100-120 million years for it to happen, but as the group of stars cut away from the field, each member also aged differently.

By studying open clusters like M50 and its relative M35, we can learn more about the dynamics of star clusters which formed roughly at the same time in the same area. As Jasonjot Kalirai (et al) indicated in their 2003 study:

“The color-magnitude diagrams for the clusters exhibit clear main sequences stretching over 14 mag in the (V, B-V)-plane. Comparing these long main sequences with those of earlier clusters in the survey, as well as with the Hyades, has allowed for accurate distances to be established for each cluster. Analysis of the luminosity and mass functions suggests that, despite their young ages, both clusters are somewhat dynamically relaxed, exhibiting signs of mass segregation. This is especially interesting in the case of NGC 2323, which has an age of only 1.3 times the dynamical relaxation time. The present photometry is also deep enough to detect all of the white dwarfs in both clusters. We discuss some interesting candidates that may be the remnants of quite massive (M>=5Msolar) progenitor stars. The white dwarf cooling age of NGC 2168 is found to be in good agreement with the main-sequence turnoff age. These objects are potentially very important for setting constraints on the white dwarf initial-final mass relationship and the upper mass limit for white dwarf production.”

So, did age or movement produce the colorful display of stars we can observe in M50 – or was it simply the chemical ingredients responsible? According to a 2005 study conducted by Bragaglia and Monica:

“We describe a long-term project aimed at deriving information on the chemical evolution of the Galactic disk from a large sample of open clusters. The main property of this project is that all clusters are analyzed in a homogeneous way to guarantee the robustness of the ranking in age, distance, and metallicity. Special emphasis is devoted to the evolution of the earliest phases of the Galactic disk evolution, for which clusters have superior reliability with respect to other types of evolution indicators. The project is twofold: on one hand we derive the age, distance, and reddening (and indicative metallicity) by interpreting deep and accurate photometric data with stellar evolution models, and on the other hand, we derive the chemical abundances from high-resolution spectroscopy. The importance of quantifying the theoretical uncertainties by deriving the cluster parameters with various sets of stellar models is emphasized. Stellar evolution models assuming overshooting from convective regions appear to better reproduce the photometric properties of the cluster stars. The examined clusters show a clear metallicity dependence on the galactocentric distance and no dependence on age. The tight relation between cluster age and magnitude difference between the main-sequence turnoff and the red clump is confirmed.”

The M50 open cluster. Credit: Ole Nielsen

History of Observation:

While M50 was possibly discovered by G.D. Cassini 1711, it was independently recovered by Charles Messier on the night of April 5th, 1772. In his notes, he wrote of his discovery:

“Cluster of small stars, more or less brilliant, above the right loins of the Unicorn, above the star Theta of the ear of Canis Major, & near a star of 7th magnitude. It was while observing the Comet of 1772 that M. Messier observed this cluster. He has reported it on the chart of that comet, on which its trace has been drawn.”

It would later be observed by William Hershel, but not until his son John cataloged it before anyone began to notice colors in the stars. However, Admiral Smyth did!

“This is an irregularly round and very rich mass, occupying with its numerous outliers more than the field, and composed of stars from the 8th to the 16th magnitudes; and there are certain spots of splendour which indicate minute masses beyond the power of my telescope. The most decided points are, a red star towards the southern verge, and a pretty little equilateral triangle of 10th sizers, just below, or north of it. The double star here noted was carefully estimated under a full knowledge of the vertical and parallel lines of the field of view: this was made triple by H. [John Herschel], whose 2357 of the Fifth Series it is; but he must be mistaken in calling it Struve 748, which is Theta Orionis. It is sufficiently conspicuous as a double star, and though I perceive an infinitesimal point exactly om the vertical of A, I cannot ascertain whether it is H.’s C. This superb object was discovered by Messier in 1771 [actually 1772], and registered “a mass of small stars more or less brilliant.” It is 9 deg north-north-east of Sirius, and rather more than one-third of the distance between that star and Procyon.”

Locating Messier 50:

Because M50 is such a big and bright open star cluster, it’s relatively easy to find with complicated starhop instructions. Actually, the constellation of Monoceros is more difficult! Begin by identifying the brightest star in northern hemisphere skies – Alpha Canis Major – Sirius. Roughly a handspan to the northeast you’ll see another prominent bright star – Alpha Canis Minor – Procyon.

The location of Messier 50 in the Monoceros constellation. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

 

Between these two lay the faint and indistinguishable constellation of Monoceros, and slightly southwest of the center point is Messier 50. In small binoculars and a telescope finderscope, you’ll quickly spot a compression in the starfield, and may even be able to see it as a slight contrast change with the unaided eye. In larger binoculars and small telescopes, it blooms into a cloud of stars, well resolved against the grainy backdrop of fainter stars.

In large aperture telescopes, even more stars resolve and colors begin to appear. Because of magnitude and the nature of star clusters, Messier 50 makes an outstanding target for high light pollution areas, moonlit nights and even less than perfect sky conditions.

Enjoy your own “colorful” observations of this rich and beautiful star cluster!

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

Object Name: Messier 50
Alternative Designations: M50, NGC 2323
Object Type: Open Galactic Star Cluster
Constellation: Monoceros
Right Ascension: 07 : 03.2 (h:m)
Declination: -08 : 20 (deg:m)
Distance: 3.2 (kly)
Visual Brightness: 5.9 (mag)
Apparent Dimension: 16.0 (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, 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 Corvus Constellation

Celestial map of the constellation Corvus, the Raven. Credit and Copyright ©: Torsten Bronger

Welcome to another edition of Constellation Friday! Today, in honor of the late and great Tammy Plotner, we take a look at the “Raven” – the Corvus 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 constellation is the Corvus constellation, a southern constellation whose name in Latin means “the Raven”. Bordered by the constellations of Virgo, Crater and Hydra, it is visible at latitudes between +60° and -90° and is best seen at culmination during the month of May. Today, it is one of the 88 modern constellations recognized by the International Astronomical Union (IAU).

Name and Meaning:

In classical mythology, Corvus represents the Raven, and is both a charming and sad tale. Legend tells us that the constellation of Crater is the cup of the gods. This cup belonged to the god of the skies himself, the venerable archer-god Apollo. And who holds this cup, dressed in black? The Raven, Corvus.

“Noctua, Corvus, Crater, Sextans Uraniæ, Hydra, Felis, Lupus, Centaurus, Antlia Pneumatica, Argo Navis, and Pyxis Nautica”, plate 32 in Urania’s Mirror, by Sidney Hall. Credit: Library of Congress

The story of a creature sent to fetch water for his master, only to stop to eat figs. Corvus tarried too long, waiting on a fig to ripen. When he realized his mistake, the Raven returned to Apollo with his cup and brought along the serpent Hydra in his claws as well, claiming that the snake prevented him from filling the cup.

Realizing his feathered-friend’s lie, Apollo became angry and tossed the cup (Crater), the snake (Hydra) and the raven (Corvus) into the sky, where they became constellations for all eternity. He further punished the raven by making sure the cup would be out of reach, thus ensuring he would forever be thirsty.

History of Observation:

As with most of the 48 constellations recorded by Ptolemy, the Corvus constellation has roots that go back to ancient Mesopotamia. In the Babylonian star catalogues (dated to ca. 1100 BCE), Corvus was called the Babylonian Raven (MUL.UGA.MUSHEN), which sat on the tail of the Serpent – which was associated with Ningishzida, the Babylonian god of the underworld. This constellation was also sacred to the god of rains and storm (Adad).

By about 500 BCE, this constellation was introduced to the Greeks, along with Crater, Hydra, Aquila and Piscis Austrinus constellations. By the 2nd century CE, they were included by Ptolemy in his Almagest, which would remain the definitive source on astronomy and astrology to Medieval European and Islamic astronomers for many centuries.

In Chinese astronomy, the stars that make up Corvus are located within the Vermilion Bird of the South (Nán Fang Zhu Què). The four main stars depict a chariot (Zhen) while Alpha and Eta mark the linchpins for the wheels, and Zeta represents a coffin (Changsha).

In Indian astronomy, the first five stars in Corvus correspond to the Hast nakshatra – a lunar zodiacal constellation. This is one of is one of the 27 or 28 divisions of the sky, identified by the prominent stars in them, that the Moon passes through during its monthly cycle. While it is Hindu, it is still very similar to the divisions of the ecliptic plane referred to as the zodiac. The Moon takes approximately one day to pass through each nakshatra.

Notable Objects:

This small, box-like asterism has no bright star and consists of 11 stars which are visible to the unaided eye, yet Ptolemy only listed 7! There are 4 main stars and 10 which have Bayer/Flamsteed designations. For unaided eye observers, the Delta, Gamma, Epsilon and Beta (what appears to look like a figure 8, Y, E and B on the map) form an asterism that looks like a “sail”, and when connected seem to point to the bright star Spica.

The brightest star in Corvus is not even its alpha, but is Gamma Corvi. This giant star (which is thought to be a binary system) is located approximately 165 light years from Earth and is also known as Gienah, which comes from the Arabic phrase al-janah al-ghirab al-yaman (“the right wing of the crow”).

Antennae Galaxies – NGC 4038, NGC 4039. Credit: NASA, ESA, and the Hubble Heritage Team (STScI, AURA)-ESA, Hubble Collaboration

The second-brightest star, Beta Corvi, is a yellow-white G-type bright giant that is located about 140 light years from Earth. Its proper name, Kraz, was assigned to it in modern times, but the origin of the name is uncertain. Delta Corvi is a class A0 star in Corvus located approximately 87 light years distant from Earth whose traditional name (Algorab) comes from the Arabic word al-ghuraab – which means “the crow.”

Epsilon Corvi is a K2 III class star that is approximately 303 light-years from Earth. The star’s traditional name, (Minkar) comes from the Arabic word almánxar, which means “the nostril of the crow.” Alpha Corvi, which is only the fifth brightest star in the constellation, is a class F0 dwarf or subdwarf that is only 48.2 light years distant. The star’s traditional name (Alchiba) is derived from the Arabic al hibaa, which means “tent.”

Corvus is also home to many Deep Sky Objects. These include the Antennae Galaxies (NGC 4038/NGC 4039), a pair of interacting galaxies that were first discovered in the late 18th century. These colliding galaxies – which are located 45 million light years from Earth – are currently in the starburst stage, meaning they are experiencing an exceptionally high rate of star forming activity.

There’s also the NGC 4027 barred spiral galaxy, which is located about 83 million light years from Earth. This galaxy is peculiar, in that one of its spiral arm extends further than the other – possibly due to a past collision with another galaxy. Finally, there’s the large planetary nebula known as NGC 4361, which is located at the center of the constellation and resembled a faint elliptical galaxy.

The barred, spiral galaxy known as NGC 4027. Credit : ESO

Finding Corvus:

Let’s start with binoculars and look down at the southern corner, where we will find Alpha Corvi – aka. Alchiba. Alchiba belongs to the spectral class F0 and has apparent magnitude +4.00. This star is suspected of being a spectroscopic binary, although this has not yet been confirmed. Now take a look at Beta Corvi – aka. Kraz. Good old Kraz is approximately 140 light-years away and is a G-type bright giant star whose apparent visual magnitude varies between 2.60 and 2.66.

Head west and look at Epsilon. Although it doesn’t look any further away, spectral class K2 III – Minkar – is 303 light-years from Earth! Need a smile? Then take a look at Gamma, aka. Geinah. How about Delta? Algorab is a spectral class A0 and is about 87 light years from our solar system.

Now get out your telescope as we explore planetary nebula, NGC 4361 (RA 12 24 5 Dec -18 48). At around magnitude 10, this greenish disc is fairly easily spotted with smaller telescopes, but the 13th stellar magnitude central star requires larger aperture to be seen. It has a very symmetrical shape that is similar to a spiral galaxy.

For galaxy fans, have a look at interacting galaxy pair, NGC 4038 and NGC 4039 – the “Ringtail Galaxy” (RA 12 01 53 Dec -18 52-3). This peculiar galaxy (also referred to as the “Antennae Galaxies”) were both discovered by Friedrich Wilhelm Herschel in 1785. Even in relatively small telescopes, you can see two long tails of stars, gas and dust thrown out of the galaxies as a result of the collision that resemble the antennae of an insect.

Map of the Corvus Constellation. Credit: IAU and Sky&Telescope magazine

As explained by Vázquez (et al.) in a 1999 study:

“The morphology of this object is complex given the highly filamentary structure of the envelope, which is confirmed to possess a low mass. The halo has a high expansion velocity that yields incompatible kinematic and evolutionary ages, unless previous acceleration of the nebular expansion is considered. However, the most remarkable result from the present observations is the detection of a bipolar outflow in NGC 4361, which is unexpected in a PN with a Population II low-mass-core progenitor. It is shown that shocks resulting from the interaction of the bipolar outflow with the outer shell are able to provide an additional heating source in this nebula.”

Most galaxies probably undergo at least one significant collision in their lifetimes. This is likely the future of our Milky Way when it collides with the Andromeda Galaxy. Two supernovae have been discovered in the galaxy: SN 2004GT and SN 2007sr. A recent study finds that these interacting galaxies are closer to the Milky Way than previously thought – at 45 million light-years instead of 65 million light-years. Geez… What’s 20 million light years between friends?

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 49 – the NGC 4472 Elliptical Galaxy

The location of M49, in proximity to other Messier Objects and major stars. 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 elliptical galaxy known as Messier 49 (aka. NGC 4472). Located in the southern skies in the constellation of Virgo, this galaxy is one several members of the Virgo Cluster of galaxies and is located 55.9 million light years from Earth. On a clear night, and allowing for good light conditions, it can be seen with binoculars or a small telescope, and will appear as a hazy patch in the sky.

Description:

Messier 49 is the brightest of the Virgo Cluster member galaxies, which is pretty accurate considering it’s only about 60 million light years away and may span an area as large as 160,000 light years. It is a huge system of globular clusters, much less concentrated than Virgo cluster member M87 – but a giant none the less. As Stephen E. Zep (et al) wrote in a 2000 study:

“We present new radial velocities for 87 globular clusters around the elliptical galaxy NGC 4472 and combine these with our previously published data to create a data set of velocities for 144 globular clusters around NGC 4472. We utilize this data set to analyze the kinematics of the NGC 4472 globular cluster system. The new data confirm our previous discovery that the metal-poor clusters have significantly higher velocity dispersion than the metal-rich clusters in NGC 4472. The very small angular momentum in the metal-rich population requires efficient angular momentum transport during the formation of this population, which is spatially concentrated and chemically enriched. Such angular momentum transfer can be provided by galaxy mergers, but it has not been achieved in other extant models of elliptical galaxy formation that include dark matter halos. We also calculate the velocity dispersion as a function of radius and show that it is consistent with roughly isotropic orbits for the clusters and the mass distribution of NGC 4472 inferred from X-ray observations of the hot gas around the galaxy.”

This ground-based image shows the Small Magellanic Cloud. The area of the SMIDGE survey is highlighted, as well as the position of NGC 248. Credit: NASA/ESA/Hubble/Digitized Sky Survey 2

However, there was something going on in the mass structure of M49 that astronomers were curious about… Something they couldn’t quite explain. Was it dark matter? As M. Lowenstein wrote in a 1992 study:

“An attempt to constrain the total mass distribution of the well-observed giant elliptical galaxy NGC 4472 is realized by constructing simultaneous equilibrium models for the gas and stars using all available relevant optical and X-ray data. The value of <?>, the emission-weighted average value of kT, derived from the Ginga spectrum, <?> = 1.9 ± 0.2 keV, can be reproduced only in hydrostatic models where nonluminous matter comprises at least 90% of the total mass. However, in general, these mass models are not consistent with observed projected stellar and globular cluster velocity dispersions at moderate radii.”

The next thing you know, nuclear outburst were discovered – the product of interaction with a neighboring galaxy. As B. Biller (et al) indicated in a 2004 study:

“We present the analysis of the Chandra ACIS observations of the giant elliptical galaxy NGC 4472. The Chandra Observatory’s arcsec resolution reveals a number of new features. Specifically: 1) an ~8 arc min streamer or arm (this corresponds to a linear size of 36 kpc) extending southwest of the galaxy and an assymetrical, somewhat truncated streamer to the northeast. Smaller, morphologically similar structures are observed in NGC 4636 and are explained as shocks from a nuclear outburst in the recent past. The larger size of the NGC 4472 streamers requires a correspondingly higher energy input compared to the NGC 4636 case. The asymmetry of the streamers may be due to the interaction of NGC 4472 with the ambient Virgo cluster gas. 2) A string of small, extended sources south of the nucleus. These sources may stem from an interaction of NGC 4472 with the galaxy UGC 7637. 3) X-ray cavities corresponding to radio lobes, where expanding radio plasma has evacuated the X-ray emitting gas. We also present a luminosity function for the X-ray point sources detected within NGC 4472 which we compare to that for other early type galaxies.”

Chandra images showing 4 of the 9 galaxies discovered (left), and an artist’s impression on showing how gas falls towards a black hole and becomes a rapidly spinning disk of matter near the center (right). Credit: NASA/Chandra

But the very best was yet to come… the discovery of a black hole! According to NASA, the results from NASA’s Chandra X-ray Observatory, combined with new theoretical calculations, provide one of the best pieces of evidence yet that many supermassive black holes are spinning extremely rapidly. The images on the left show 4 out of the 9 large galaxies included in the Chandra study, each containing a supermassive black hole in its center.

The Chandra images show pairs of huge bubbles, or cavities, in the hot gaseous atmospheres of the galaxies, created in each case by jets produced by a central supermassive black hole. Studying these cavities allows the power output of the jets to be calculated. This sets constraints on the spin of the black holes when combined with theoretical models. The Chandra images were also used to estimate how much fuel is available for each supermassive black hole, using a simple model for the way matter falls towards such an object.

The artist’s impression on the right side of the main graphic shows gas within a “sphere of influence” falling straight inwards towards a black hole before joining a rapidly spinning disk of matter near the center. Most of the material in this disk is swallowed by the black hole, but some of it is swept outwards in jets (colored blue) by quickly spinning magnetic fields close to the black hole.

Previous work with these Chandra data showed that the higher the rate at which matter falls towards these supermassive black holes, the higher their power output is in jets. However, without detailed theory the implications of this result for black hole behavior were unclear. The new study uses these Chandra results combined with leading theoretical models for the production of jets, plus general relativity, to show that the supermassive black holes in these galaxies must be spinning at close to the maximum rate. If black holes are spinning at this limit, material can be dragged around them at close to the speed of light, the speed limit from Einstein’s theory of relativity.

Atlas Image obtained of Messier 49, taken by the Two Micron All Sky Survey (2MASS). Credit: NASA/UofMass/IPAC/Caltech/NASA/NSF/2MASS

History of Observation:

According to SEDS, M49 was the first member of the Virgo cluster of galaxies to be discovered, by Charles Messier, who cataloged it on February 19th, 1771. As he recorded in his notes at the time:

“Nebula discovered near the star Rho Virginis. One cannot see it without difficulty with an ordinary telescope of 3.5-feet [FL]. The Comet of 1779 was compared by M. Messier with this nebula on April 22 and 23: The comet and the nebula had the same light. M. Messier has reported this nebula on the chart of the route of the comet, which appeared in the volume of the Academy of the same year 1779. Seen again on April 10, 1781.” Eight years later, on April 22, 1779, on the occasion of following the comet of that year, and on the hunt for finding more nebulous objects in competition to other observers, Barnabas Oriani independently rediscovered this ‘nebula’: “Very pale and looking exactly like the comet [1779 Bode, C/1779 A1].”

In his Bedford Catalogue of 1844, Admiral William H. Smyth confused this finding with Messier’s discovery:

“A bright, round, and well-defined nebula, on the Virgin’s left shoulder; exactly on the line between Delta Virginis and Beta Leonis, 8deg, or less than half-way, from the former star. With an eyepiece magnifying 93 times, there are only two telescopic stars in the field, one of which is in the sp and the other in the sf quadrant; and the nebula has a very pearly aspect. This object was discovered by Oriani in 1771 [this is wrong: it was Messier who discovered it that year; Oriani found it only in 1779], and registered by Messier as a “faint nebula, not seen without difficulty,” with a telescope of 3 1/2 feet in length. It is a pity that this active and very assiduous astronomer could not have been furnished with one of the giant telescopes of the present days. Had he possessed efficient means, there can be no doubt of the augmentation of his useful and, in its day, unique Catalogue: a collection of objects for which sidereal astronomy must ever remain indebted to him.” This error was repeated by John Herschel in his General Catalogue of 1864 (GC), who also erroneously assigned this object to “1771 Oriani,” and also found its way into J.L.E. Dreyer’s NGC.

Let’s hope you don’t mistake it with the many other galaxies nearby!

The location of Messier 49 within the Virgo constellation. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

Locating Messier 49:

Galaxy hopping isn’t an easy chore and it takes some practice. Starting looking for M49 about halfway between Epsilon and Beta Virginis. Use Gamma to help triangulate your position. At near magnitude 8, Messier 49 is quite binocular possible and would show under dark sky conditions as a faint, very small egg shaped fog. However, it will not show in a finderscope of a telescope – but the nearby stars will.

Use their patterns to help guide you there. Because galaxies require dark skies, M49 cannot be found under urban conditions or during moonlit nights. In telescopes as small as 70mm, it will appear as a nebulous egg shape and become brighter – but no more resolved to larger instruments. To assist in location, begin with lowest magnification and increase magnification once found to darken background field.

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

Object Name: Messier 49
Alternative Designations: M49, NGC 4472
Object Type: Elliptical Galaxy
Constellation: Virgo
Right Ascension: 12 : 29.8 (h:m)
Declination: +08 : 00 (deg:m)
Distance: 60000 (kly)
Visual Brightness: 8.4 (mag)
Apparent Dimension: 9×7.5 (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 48 – the NGC 2548 Open Star Cluster

The position of Messier 48 in the Hydra constellation. Credit: Wikisky

Welcome back to Messier Monday! We continue our tribute to our dear friend, Tammy Plotner, by looking at the open star cluster of Messier 48. Enjoy!

In the 18th century, while searching the night sky for comets, French astronomer Charles Messier kept noting the presence of fixed, diffuse objects in the night sky. In time, he would come to compile a list of approximately 100 of these objects, with the purpose of making sure that astronomers did not mistake them for comets. However, this list – known as the Messier Catalog – would go on to serve a more important function.

One of these is the open star cluster known as Messier 48 (aka. NGC 2548). Located approximately 1,500 light years from Earth in the direction of the Hydra constellation, Charles Messier actually got the position of this cluster wrong, a mistake which was corrected by Caroline Herschel in 1783 (hence why she is sometimes credited with its discovery). This object is visible to the naked eye on a clear night, providing light conditions are favorable.

Description:

At a modest 300 million years old, this group of about 50 easily visible stars and 80 total members spans an area of space which covers 23 light years. By studying proper motion over time with an astrograph telescope, astronomers have determined it is roughly 1500 light years away from our solar system. But how are determinations like this made? By long term studies and painstaking photographic plates, which address which stars are moving, at what speeds, and in what direction.

The open star cluster Messier 48. Credit: Wikisky

As Z. Y. Wu of the Shanghai Astronomical Observatory indicated in a 2001 study:

“Absolute proper motions, their corresponding errors and membership probabilities of 501 stars in the intermediate-age open cluster NGC 2548 region are determined from MAMA measurements of 10 photographic plates. The plates have the maximum epoch difference of 82 years and they were taken with the double astrograph. The average proper motion precision is 1.18 mas yr -1. These proper motions are used to determine the membership probabilities of stars in the region. The number of stars with membership probabilities higher than 0.7 is 165.”

So now we understand how to determine distance, but how do astronomers determine age? As M. Hancock (et al) indicated in their 2008 study:

“We present an empirical assessment of the use of broad-band optical colours as age indicators for unresolved extragalactic clusters and investigate stochastic sampling effects on integrated colours. We use the integrated properties of Galactic open clusters (OCs) as models for unresolved extragalactic clusters. The population synthesis code Starburst99 (SB99) and four optical colours were used to estimate how well we can recover the ages of 62 well-studied Galactic OCs with published ages. We provide a method for estimating the ages of unresolved clusters and for reliably determining the uncertainties in the age estimates. Our results support earlier conclusions based on comparisons to synthetic clusters, namely the (U?B) colour is critical to the estimation of the ages of star-forming regions. We compare the observed optical colours with those obtained from SB99 using the published ages and get good agreement.”

Close up of the Messier 48 open star cluster. Credit: NASA

History of Observation:

According the SEDS, this open cluster was discovered by Charles Messier and cataloged by him on February 19, 1771. “Cluster of very small [faint] stars, without nebulosity; this cluster is at a short distance from the three stars that form the beginning of the Unicorn’s tail.”

However, as he did an error in data reduction, he gave a wrong position in his catalog so that the object was missing until Oswald Thomas identified it in 1934, and independently T.F. Morris in 1959. The identification of M48 by Oswald Thomas was confused by some historians, who have claimed erroneously instead that he had identified M47.

As M48 was lost, two independent rediscoveries occurred: First, Johann Elert Bode apparently found it in or before 1782, and second, Caroline Herschel independently rediscovered it in 1783;  “March 8th [17]83. At an equal distance from 29 [Zeta] and 30 Monocerotis, making an equilateral triangle with those two stars is a nebulous spot. By the telescope it appears to be a cluster of scattered stars. It is not in Messier catalogue.”

This latter discovery was published by Caroline’s famous brother, William Herschel, who included it in his catalog as H VI.22 on February 1, 1786. “A beautiful Cluster of much compressed stars, considerably rich. 10 or 12′ diameter. Caroline Herschel discovered it in 1783.”

John Herschel would visit Messier 48 often in his NGC cataloging efforts, describing it as, “A superb cluster which fills the whole field; stars of 9th and 10th to the 13th magnitude – and none below, but the whole ground of the sky on which it stands is singularly dotted over with infinitely minute points [stars]. Place that of a bright star, the southern of two which point into the concavity of an arc.”

Atlas Image mosaic of Messier 48 obtained as part of the Two Micron All Sky Survey (2MASS). Credit: UofMass/IPAC/Caltech/NASA/NSF

Once again, Messier’s mistake would be missed when it was re-observed by Admiral Smyth, who described this object as follows:

“A neat but minute double star, in a tolerably compressed cluster on the Unicorn’s flank, and lying 14deg south-east of Procyon. A 9 1/2 [mag], and B 10, both white. This object is in the middle of a splendid group, in a rich splashy region of stragglers, which fills the field of view, and has several small pairs, chiefly of the 9th magnitude. It was discovered by Miss Herschel in 1783, and was classed by WH [William Herschel] in February, 1783.”

Thanks to careful research done by Owen Gingerich in 1960, we now know exactly what happened:

“Although the circumstances of M48 are not so obvious, only one cluster of the size and brightness likely to be recorded by Messier is found in the region near “the three stars that form the beginning of the unicorn’s tail” (Zeta, 27, and 28 Monocerotis). Dr. Morris has pointed out that this cluster, NGC 2548, has the same right ascension as the position given for M48. (Allowance must, of course, be made for precession in comparing Messier’s figures with modern positions). The declination disagrees by about 5 degrees. Since no conspicuous star is located 2 1/2 degrees away in declination, we cannot account for this position by another error in sign. It seems unlikely that the comparison star was misidentified, since the right ascension is probably correct. Messier did not publish the name of the star used, and his original records are apparently no longer extant. Thus, a careful survey of the region described by Messier leads to the conclusion that NGC 2548 is the cluster the French observer intended as his 48th object, for lack of any cluster nearby that fits the description.”

May you find it a bit easier!!

Locating Messier 48:

The diamond-bright stars of winter help make locating M48 a little easier, as it is located just a little less than a hand span southeast of Procyon (Alpha Canis Minor) – or about 3 degrees southeast of Zeta Monocerotis. Like M44 in Cancer, M48 lies within the limits of unaided sight. It is quite large and will show several dozen stars easily to almost all binoculars and be well resolved in telescopes of any aperture. Be sure to use low magnification to see it best! Because Messier 48 is bright, it makes a fine object for urban sky conditions and moonlit nights.

The location of Messier 48 in the Hydra constellation. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

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

Object Name: Messier 48
Alternative Designations: M48, NGC 2548
Object Type: Open Galactic Star Cluster
Constellation: Hydra
Right Ascension: 08 : 13.8 (h:m)
Declination: -05 : 48 (deg:m)
Distance: 1.5 (kly)
Visual Brightness: 5.5 (mag)
Apparent Dimension: 54.0 (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, 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 47 – the NGC 2422 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 47 (NGC 2422), which is located in the constellation of Puppis roughly 1,600 light-years from Earth. Located in proximity to Messier 46, this star cluster is estimated to be 78 million years in age. It is also particularly bright, containing about 50 stars and occupying a region that is about the same size as that of the full Moon.

Description:

Spanning across about 12 light years of space, this clump of around 50 stars began their life around 78 million years ago. Now cruising through space some 1600 light years away from Earth, the group continues to distance itself from our solar system at a speed of 9 kilometers per second. For the most part, Messier 47 is a whole lot like the Pleiades star cluster – its brightest member shining just around magnitude 6 and holding a spectral class B2.

But, here you will also find two orange K giants with luminosity of about 200 times that of the Sun. At M47’s center you’ll find binary star, Sigma 1121, with components of magnitude 7.9 both and separated by 7.4 arc seconds. How do we know that M47 is a lot like the Pleiades? Let’s try X-ray sources and the advances of looking at open clusters far more differently than in optical wavelengths. As M. Barbera (et al) said in a 2002 study:

“We present the results of a ROSAT study of NGC 2422, a southern open cluster at a distance of about 470 pc, with an age close to the Pleiades. Source detection was performed on two observations, a 10-ks PSPC and a 40-ks HRI pointing, with a detection algorithm based on wavelet transforms, particularly suited to detecting faint sources in crowded fields. We have detected 78 sources, 13 of which were detected only with the HRI, and 37 detected only with the PSPC. For each source, we have computed the 0.2-2.0 keV X-ray flux. Using optical data from the literature and our own low-dispersion spectroscopic observations, we find candidate optical counterparts for 62 X-ray sources, with more than 80% of these counterparts being late type stars. The number of sources (38 of 62) with high membership probability counterparts is consistent with that expected for Galactic plane observations at our sensitivity. We have computed maximum likelihood X-ray luminosity functions (XLF) for F and early-G type stars with high membership probability. Heavy data censoring due to our limited sensitivity permits determination of only the high-luminosity tails of the XLFs; the distributions are indistinguishable from those of the nearly coeval Pleiades cluster.”

What else might be hiding inside Messier 47? Try new debris disk candidates. As Nadya Gorlova (et al) indicated in a 2004 study:

“Sixty-three members of the 100 Myr old open cluster M47 (NGC 2422) have been detected with the Spitzer Space Telescope. The Be star V 378 Pup shows an excess both in the near-infrared, probably due to free-free emission from the gaseous envelope. Seven other early-type stars show smaller excesses. Among late-type stars, two show large excesses. P1121 is the first known main-sequence star showing an excess comparable to that of Beta Pic, which may indicate the presence of an exceptionally massive debris disk. It is possible that a major planetesimal collision has occurred in this system, consistent with the few hundred Myr timescales estimated for the clearing of the solar system.”

Iof the star cluster Messier 47 taken by the Wide Field Imager camera on the 2.2-metre telescope at ESO’s La Silla Observatory in Chile. Credit: ESO

History of Observation:

Messier 47 was originally discovered before 1654 by Hodierna who described it as:

“[A] Nebulosa between the two dogs”… but it was an observation that wasn’t known about until long after Charles Messier independently recovered it on February 19, 1771. “Cluster of stars, little distant from the preceding; the stars are greater; the middle of the cluster was compared with the same star, 2 Navis. The cluster contains no nebulosity.”

However, it was one of those very rare circumstances when Messier actually made a mistake in his position calculations. Despite this error, the cluster was observed by Caroline Herschel and identified as M47 at least twice in early 1783.

As a consequence of Messier’s position mistake, Sir William Herschel also independently rediscovered it on February 4, 1785, and gave it the number H VIII.38. “A cluster of pretty compressed large [bright] and small [faint] stars. Round. Above [more than] 15′ diameter.” It would be John Herschel, on December 16, 1827, who would be the first to resolve Sigma 1121: “The chief star of a large, pretty rich, straggling cluster. It [the star] is double.”

Atlas Image mosaic obtained of Messier 47 as part of the Two Micron All Sky Survey (2MASS). Credit: UMass/IPAC/Caltech/NASA/NSF

The “Messy” mistake would haunt star catalogs – including both Herschel’s and Dreyer’s for years, until the whole clerical error was cleared up by Owen Gingerich in 1960:

“More explicit reasons for this identification [of M47 with NGC 2422] were given independently in 1959 by T.F. Morris, a member of the Messier Club of the Royal Astronomical Society of Canada’s Montreal Centre. Dr. Morris suggested that an error in signs in the difference between M47 and the comparison star could account for the position. Messier determined the declination of a nebula or cluster by measuring the difference between the object and a comparison star of known declination. The right ascension could be found by recording the times at which the object and the star drifted across a central wire in his telescope’s field; the time interval gives the difference in right ascension. The differences between Messier’s 1770 [actually 1771] position for M47 and his stated comparison star, 2 Navis (now 2 Puppis), if applied with opposite signs, leads to NGC 2422. Clearly, Messier made a mistake in computation!”

May you have Caroline Herschel’s luck finding it!

Locating Messier 47:

There is no simple way of finding Messier 47 in the finderscope of a telescope, but it’s not too hard with binoculars. Begin your hunt a little more than a fist width east/northeast of bright Sirius (Alpha Canis Majoris)… or about 5 degrees (3 finger widths) south of Alpha Monoceros. (It can sometimes by seen with the unaided eye under good conditions as a dim nebulosity.)  There you will find two open clusters that will usually appear in the same average binocular field of view.

Messier 47 location. Image: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

M47 is the westernmost of the pair. It will appear slightly brighter and the stars will be more fewer and more clearly visible. In the finderscope it will appear as if it is resolving, while neighboring eastern M46 will just look like a foggy patch. Because M47’s stars are brighter, it is better suited to less than perfect sky conditions, showing as a compression that begins to resolve in binoculars and will resolves almost fully even a small telescope.

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

Object Name: Messier 47
Alternative Designations: M47, NGC 2422
Object Type: Open Galactic Star Cluster
Constellation: Puppis
Right Ascension: 07 : 36.6 (h:m)
Declination: -14 : 30 (deg:m)
Distance: 1.6 (kly)
Visual Brightness: 5.2 (mag)
Apparent Dimension: 30.0 (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: