Posted on by Tammy Plotner

Messier 37 – the NGC 2099 Open Star Cluster

The open star cluster Messier 38, in proximity to Messier 36 and Messier 37. Credit: Wikisky

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the open star cluster known as Messier 37. Enjoy!

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 37 (aka M37 and NGC 2099). Located in the direction of the Auriga constellation, Messier 37 is one of three open star clusters (including Messier 36 and Messier 38) in this portion of the night sky – and also the brightest.

Description:

Of the trio of Messier star clusters in this area, M37 is by far the most stellar populated. It contains at least 150 stars that are around magnitude 12 and easily resolved by even small telescopes – and science is still counting actual members! At around 347 – 550 million years old, you’ll find at least a dozen red giants living here about 4,500 light years away from Earth… and they do it in a neighborhood that spans anywhere from 20 to 25 light years across!

The open star cluster Messier 37. Credit: Wikisky

Just how many stars might be inside this intermediate-aged cluster? As R. Sagar and Nilakshi of the Indian Institute for Astrophysics said in their 2002 study:

“The CCD observations of the rich open star cluster NGC 2099 and its surrounding field region have been carried out up to a limiting magnitude of V ~ 22 mag in B, V and I passbands for the first time. A total of ~ 12 000 stars have been observed in the area of about 24 arcmin x 34 arcmin in the cluster region, as well as ~ 2180 stars in the ~ 12arcmin x 12arcmin area of the field region located ~ 45arcmin away from the cluster center.”

Out of this huge number of stars, astronomers have been able to observe white dwarfs, too. This helps us to understand how they develop and what affects their helium or hydrogen content. Jasonjot Singh Kalirai et al. had the following to say in a 2004 study:

“Spectra have been obtained of 21 white dwarfs (WDs) in the direction of the young, rich open star cluster NGC 2099. This represents an appreciable fraction (>30%) of the cluster’s total WD population. The mean derived mass of the sample is 0.8 M—about 0.2 M larger than the mean seen among field WDs. A surprising result is that all of the NGC 2099 WDs have hydrogen-rich atmospheres (DAs); none exhibit helium-rich ones (DBs) or any other spectral class. We explore possible reasons for the lack of DBs in these clusters and conclude that the most promising scenario for the DA/DB number ratio discrepancy in young clusters is that hot, high-mass WDs do not develop large enough helium convection zones to allow helium to be brought to the surface and turn a hydrogen-rich WD into a helium-rich one.”

So, we’re setting the stage with number of stars and types. We have white dwarfs – but what about variables? Y.B. Kang (et al), put it this way in a 2007 study:

“Time-series CCD photometric observations of the intermediate-age open cluster NGC 2099 were performed to search for variable stars. We also carried out BV photometry to study physical properties of variables in the cluster. Using V-band time-series data, we carefully examined light variations of about 12,000 stars in the range of 10 < V < 22 mag. A total of 24 variable stars have been identified; seven stars are previously known variables and 17 stars are newly identified. On the basis of observational properties such as light curve shape, period, and amplitude, we classified the new variable stars as nine delta Scuti-type pulsating stars, seven eclipsing binaries, and one peculiar variable star. Judging from the position of delta Scuti-type stars in the color-magnitude diagram, only two stars are likely to have the cluster membership. One new variable KV10 shows peculiar light variations with a delta Scuti-type short period of about 0.044 day as well as a long period of 0.417 day.”

M37 (NGC 2099) open cluster. Credit: Wikipedia Commons

So what does knowing about these two types of stars help with our understanding of stellar evolution? That’s one of the goals of the RACE-OC project. As S. Messina (et al) said in 2008:

“Rotation and solar-type magnetic activity are closely related to each other in main-sequence stars of G or later spectral types. The presence and level of magnetic activity depend on star’s rotation, and rotation itself is strongly influenced by strength and topology of the magnetic fields. Open clusters represent especially useful targets to investigate the connection between rotation and activity. The open cluster NGC 2099 has been studied as a part of the RACE-OC project (Rotation and ACtivity Evolution in Open Clusters), which is aimed at exploring the evolution of rotation and magnetic activity in the late-type members of open clusters of different ages. We collected time series CCD photometric observations of this cluster in January 2004, and we determined the presence of periodicities in the flux variation related to the stellar rotation by Fourier analysis. We investigate the relations between activity manifestations, such as the light curve amplitude, and global stellar parameters. Results: We have discovered 135 periodic variables, 122 of which are candidate cluster members. Determination of rotation periods of G- and K-type stars has allowed us to better explore the evolution of angular momentum at an age of about 500 Myr. In our analysis, we have also identified 3 new detached eclipsing binary candidates among cluster members. A comparison with the older Hyades cluster (~625 Myr) shows that the newly-determined distribution of rotation periods is consistent with the scenario of rotational braking of main-sequence spotted stars as they age. However, a comparison with the younger M 34 cluster (~200 Myr) shows that the G8-K5 members of these clusters have the same rotation period distribution. That is, G8-K5 members in NGC 2099 seem to have experienced no significant braking in the age range from ~200 to ~500 Myr. Finally, NGC 2099 members have a smaller level of photospheric magnetic activity, as measured by light curve amplitude, than in younger stars of the same mass and rotation, suggesting that the activity level also depends on some other age-dependent parameters.”

History of Observation:

Although this great star cluster was originally recorded Giovanni Batista Hodierna before 1654, it would be 230 years before his records would be uncovered, so when Charles Messier first logged as Messier 37, it was believed to be an independent discovery.

“In the same night [September 2 to 3, 1764], I have observed a second cluster of small stars which were not very distant from the preceding, near the right leg of Auriga and on the parallel of the star Chi of that constellaiton: the stars there are smaller than that of the preceding cluster: they are also closer to each other, and contain a nebulosity. With an ordinary refractor of 3 feet and a half, one has difficulty to see these stars; but one distinguishes them with an instrument of greater effectivity. I have determined the position fo this cluster, which may have an extension of 8 to 9 minutes of arc: its right ascension was 84d 15′ 12″, and its declination 32d 11′ 51″ north.”

While William Herschel would return in later years to study Messier’s object, he did not publish his notes – but gives some great observing advice:

“A useful, coarse step; it will serve to learn to see nebulae, because it contains many small stars mixed with others in various magnitudes, many of which are not to be seen without great and long attention.” Messier 37 would be later given its NGC catalog designation by John Herschel who was the first to make a guess at its true stellar population: “Very fine large cluster, all resolved into stars of 10th to 13th magnitude. It fills 1 1/2 field, but the straggling stars extend very far. There may be 500 stars.”

As always, Admiral Smyth was the most poetical about his observing, and of M37 he writes:

“A magnificent object, the whole field being strewed as it were with sparkling gold-dust; and the group is resolvable into about 500 stars, from the 10th to the 14th magnitudes, besides the outliers. It was found and fixed by Messier in 1764, who described it as “a mass of small stars, much enveloped in nebulous matter.” This nebulous matter, however, yields to my telescope, and resolves into infinitely minute points of lucid light, among the distinct little individuals.”

The location of Messier 37 in the constellation Auriga. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

Locating Messier 37:

Locating Messier 37 is relatively easy once you understand the constellation of Auriga. Looking roughly like a pentagon in shape, start by identifying the brightest of these stars – Capella. Due south of it is the second brightest star which shares its border with Beta Tauri, El Nath. By aiming binoculars at El Nath, go north about 1/3 the distance between the two and enjoy all the stars! You will note two very conspicuous clusters of stars in this area, and so did Le Gentil in 1749.

Binoculars will reveal the pair in the same field, as will telescopes using lowest power. The dimmest of these is the M38, and will appear vaguely cruciform in shape. At roughly 4200 light years away, larger aperture will be needed to resolve the 100 or so fainter members. About 2 1/2 degrees to the southeast (about a finger width) you will see the much brighter M36.

More easily resolved in binoculars and small scopes, this “jewel box” galactic cluster is quite young and about 100 light years closer. If you continue roughly on the same trajectory about another 4 degrees southeast you will find open cluster M37. This galactic cluster will appear almost nebula-like to binoculars and very small telescopes – but comes to perfect resolution with larger instruments.

While all three open star clusters make fine choices for moonlit or light polluted skies, remember that high sky light means less faint stars which can be resolved – robbing each cluster of some of its beauty. Messier 37 is the brightest and easternmost of the trio and you’ll very much notice its density.

When you view this cluster with binoculars, you’ll be seeing it much as Messier did… But use the power of a telescope if you can. Because this cloud of stars is quite worth your time and attention!

Object Name: Messier 37
Alternative Designations: M37, NGC 2099
Object Type: Galactic Open Star Cluster
Constellation: Auriga
Right Ascension: 05 : 52.4 (h:m)
Declination: +32 : 33 (deg:m)
Distance: 4.4 (kly)
Visual Brightness: 6.2 (mag)
Apparent Dimension: 24.0 (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 36 – The Pinwheel Cluster

The open star cluster Messier 38, in proximity to Messier 36 and Messier 37. Credit: Wikisky

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Pinweel Cluster, otherwise known as Messier 36. Enjoy!

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.

Included in this list is the open star cluster Messier 36, also known as the Pinwheel Cluster. This cluster is so-named because of its association with the Auriga constellation (aka. “the Charioteer”). Though similar in size and make-up to the Pleiades Cluster (Messier 45), the Pinwheel Cluster is actually ten times farther away from Earth – and one of the most distant of any clusters catalogued by Messier.

What You Are Looking At:

Located a little more than 4000 light years from our solar system, this group of about 60 stars spans across about 14 light years of space. As you are studying it, you’ll notice one star which seems brighter than the rest… With good reason! Its a spectral type B2 and about 360 more luminous than our Sun. Many of the cluster members here are also B-type stars and rapid rotators.

Close-up of the central region of Messier 36. Credit: Wikisky

This means that 25 million year old Messier 36 shares a lot in common with another nearby star cluster, the Pleiades. By taking a deep look at young clusters with stars of varying ages, astronomers are able to how long circumstellar disks may last – giving us a clue as to whether or not planet-forming stars may lay within them.

As Karl E. Haisch, Jr. (et al), wrote in a 2001 study “Disk Frequencies and Lifetimes in Young Clusters“:

“We have completed the first systematic and homogeneous survey for circumstellar disks in a sample of young clusters that both span a significant range in age and contain statistically significant numbers of stars whose masses span nearly the entire stellar mass spectrum. Analysis of the combined survey indicates that the cluster disk fraction is initially very high and rapidly decreases with increasing cluster age, such that one-half the stars within the clusters lose their disks in 3 million years. Moreover, these observations yield an overall disk lifetime of ~6 million years in the surveyed cluster sample. This is the timescale for essentially all the stars in a cluster to lose their disks. This should set a meaningful constraint for the planet-building timescale in stellar clusters.”

ut, can M36 hold surprises? You betcha’. As Bo Reipurth stated in a 2008 study titled “Star Formation and Molecular Clouds towards the Galactic Anti-Center“:

“The open cluster M36 (NGC 1960), which apparently forms the center of the Aur OB1 association, has been the subject of numerous analyses, and of these the earliest studies are today of historical interest only. NGC 1960 has recently attracted attention as the most likely origin of a massive OB star that exploded about 40,000 yr ago, creating the supernova remnant Simeis 147, an old supernova remnant listed in the catalog compiled at Simeiz by Gaze & Shajn (1952). A pulsar, PSR J0538+2817, has been found near the center of Simeis 147.”

2MASS Atlas Image Mosaic of the open star cluster Messier 36. Credit: NASA/IPAC/Caltech/University of Massachusetts

And the search for planet-building stars within M36 hasn’t stopped yet. The Spitzer Space telescope will also be investigating it, thanks to a proposal made by George Rieke:

“We propose a deep IRAC/MIPS survey of NGC 1960, a ~20 Myr-old massive cluster unexplored in the mid infrared. This cluster is at a key stage in terrestrial planet formation. Our survey will likely detect infrared excess emission from debris disks and transition disks from ~ 100 intermediate-mass (1-3 solar mass) stars. Together with ground-based photometry/spectroscopy of this cluster, proposed observations of 10 Myr-old NGC 6871, scheduled cycle 4 observations of the massive 13 Myr old clusters h and chi Persei, and existing data on NGC 2547 at 30 Myr, this survey will yield robust constraints on the frequency of debris/transition disks as a function of spectral type, age, and cluster environment at a critical age range for planet formation. This survey will provide a benchmark study of the observable signatures of terrestrial planet formation that will inform James Webb Space Telescope observations of planet-forming disks a decade from now.”

History of Observation:

The presence of this awesome star cluster was first recorded by Giovanni Batista Hodierna before 1654 and re-discovered by Le Gentil in 1749. However, it was Charles Messier who took the time to carefully record its position for future generations:

“In the night of September 2 to 3, 1764, I have determined the position of a star cluster in Auriga, near the star Phi of that constellation. With an ordinary refractor of 3 feet and a half, one has difficulty to distinguish these small stars; but when employing a stronger instrument, one sees them very well; they don’t contain between them any nebulosity: their extension is about 9 minutes of arc. I have compared the middle of this cluster with the star Phi Aurigae, and I have determined its position; its right ascension was 80d 11′ 42″, and its declination 34d 8′ 6″ north.”

M36 Open Cluster. Credit: NOAO/AURA/NSF

It would be observed again by Caroline, William and John Herschel who would be the first to note the double star in M36’s center. Although none of their notes are particularly glowing on this awesome star cluster, Admiral Symth does come to the historic rescue!

“A neat double star in a splendid cluster, on the robe below the Waggoner’s left thigh, and near the centre of the Galaxy stream. A [mag] 8 and B 9, both white; in a rich though open splash of stars from the 8th to the 14th magnitudes, with numerous outliers, like the device of a star whose rays are formed by very small stars. This object was registered by M. [Messier] in 1764; and the double star, as H. [John Herschel] remarks, is admirably placed, for future astronomers to ascertain whether there be internal motion in clusters. A line carried from the central star in Orion’s belt, through Zeta Tauri, and continued about 13deg beyond, will reach the cluster, following Phi Aurigae by about two degrees.”

Locating Messier 36:

Locating Messier 36 is relatively easy once you understand the constellation of Auriga. Looking roughly like a pentagon in shape, start by identifying the brightest of these stars – Capella. Due south of it is the second brightest star which shares its border with Beta Tauri, El Nath. By aiming binoculars at El Nath, go north about 1/3 the distance between the two and enjoy all the stars!

You will note two very conspicuous clusters of stars in this area, and so did Le Gentil in 1749. Binoculars will reveal the pair in the same field, as will telescopes using lowest power. The dimmest of these is the M38, and will appear vaguely cruciform in shape. At roughly 4200 light years away, larger aperture will be needed to resolve the 100 or so fainter members. About 2 1/2 degrees to the southeast (about a finger width) you will see the much brighter M36.

The location of M36 in the Auriga constellation. Credit: IAU and Sky and Telescope Magazine (Roger Sinnott & Rick Fienberg)

More easily resolved in binoculars and small scopes, this “jewel box” galactic cluster is quite young and about 100 light years closer. If you continue roughly on the same trajectory about another 4 degrees southeast you will find open cluster M37. This galactic cluster will appear almost nebula-like to binoculars and very small telescopes – but comes to perfect resolution with larger instruments.

While all three open star clusters make fine choices for moonlit or light polluted skies, remember that high sky light means less faint stars which can be resolved – robbing each cluster of some of its beauty. Messier 36 is intermediate brightness of the trio and you’ll quite enjoy its “X” shape and many pairings of stars!

Has the central double changed with time? Why not observe for yourself and see!

Object Name: Messier 36
Alternative Designations: M36, NGC 1960, Pinwheel Cluster
Object Type: Galactic Open Star Cluster
Constellation: Auriga
Right Ascension: 05 : 36.1 (h:m)
Declination: +34 : 08 (deg:m)
Distance: 4.1 (kly)
Visual Brightness: 6.3 (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 35 – the NGC 2168 Open Star Cluster

The open star cluster Messier 35, with NGC 2158 and IC 2157 shown nearby. Credit: Wikisky

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the open star cluster known as Messier 35. Enjoy!

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 known as Messier 35, a large open star cluster located in the northern constellation Gemini. M35 is the only Messier Object located in Gemini, and lies near the border with the adjacent constellations of  Taurus, Auriga and Orion. It consists several hundred stars that are scattered over an area that is about the same size as a Full Moon.

What You Are Looking At:

Messier 35 is 2,800 light years away from Earth and is relatively young as star clusters go, having formed only about 100 million years ago. The cluster occupies a region of space that is roughly 24 light years in diameter, and an area of 28 arc minutes on the sky – which is roughly equal to the size of the full Moon.

Image of Messier 35 obtained by the Two Micron All Sky Survey (2MASS). Credit: NASA/2MASS

M35 has a central mass that spans 11.4 light years (3.75 parsecs), with an estimated mass of 1600 to 3200 solar masses. While most of the molecule cloud from which it formed has been blown away, some of the material resides in the immediate vicinity of its stars. This can be seen in the way that light from its particularly bright blue stars is scattered to create a diffuse glow.

These are the hottest main sequence stars in the cluster, which correspond to a spectral classification of B3. M35 also contains more evolved stars, including several orange and yellow giants, which have longer lifespans than the more-massive blue stars (only a few tense of millions of years).

As a result, these stars will likely die out in the near future while the smaller stars continue to evolve, drastically affecting the cluster’s luminosity and appearance. In short, it will become redder and dimmer over time.

History of Observation:

This wonderful star cluster was discovered by Philippe Loys de Chéseaux 1745-46 and recovered again by John Bevis before 1750. However, we know and love it best as Messier Object 35, when it was penned into being by Charles Messier. As he wrote of the cluster upon observing it for the first time:

“In the night of August 30 to 31, 1764, I have observed a cluster of very small stars, near the left foot of Castor, little distant from the stars Mu and Eta of that constellation [Gemini]. When examining this star cluster with an ordinary refractor of 3 feet, it seemed to contain nebulosity; but having examined it with a good Gregorian telescope which magnified 104 times, I have noticed that it is nothing but a cluster of small stars, among which there are some which are of more light; its extension may be 20 minutes of arc. I have compared the middle of this cluster with the star Eta of Castor; its right ascension has been concluded at 88d 40′ 9″, and its declination at 24d 33′ 30″ north.”

Close-up of the Messier 35 open star cluster, showing its blue stars. Credit: Wikisky

How long would it be before the companion cluster was observed as well? My guess is Sir William Herschel’s time. Although Herschel would not publish his notes on Messier objects, they do state while observing M35 that “There is no central condensation to denote a globular form.”

And what of Admiral Smyth? He observed the cluster in September of 1836, though he appeared to have missed its companion cluster. As he recorded of M35 at the time:

“A cluster, near Castor’s right foot, in the Galaxy, discovered and registered by Messier in 1764. It presents a gorgeous field of stars from the 9th to the 16th magnitudes, but with the center of mass less rich than the rest. From the small stars being inclined to form curves of three, four, and often with a large [bright] one at the root of the curve, it somewhat reminds one of the bursting of a sky-rocket.”

A nice description, but if you see the companion cluster, you’ll know it!

Locating Messier 35:

Locating M35 in binoculars is fairly easy once you recognize the constellation of Gemini. You’ll find it just a little more than the average field of view north of Eta – the center most of the three “foot” stars on the northernmost twin. In the finderscope of a telescope, begin with Eta and starhop north until you spot a faint fuzzy in the finderscope.

The location of Messier 35 in the norther n Gemini constellation. Credit: IAU/Sky & Telescope magazine/Roger Sinnott & Rick Fienberg

Because Messier 35 is large, you’ll need low magnification to appreciate the size of this cluster in a telecope. It stands up well to moonlight and light polluted skies – as well as less than perfect sky conditions, but you will need around a 10″ or larger telescope to really begin to notice its companion cluster, NGC 2158. In smaller telescopes with good conditions, it will appear as a faint nebulous patch.

And as always, here are the quick facts on M35 to get you started!

Object Name: Messier 35
Alternative Designations: M35, NGC 2168
Object Type: Galactic Open Star Cluster
Constellation: Gemini
Right Ascension: 06 : 08.9 (h:m)
Declination: +24 : 20 (deg:m)
Distance: 2.8 (kly)
Visual Brightness: 5.3 (mag)
Apparent Dimension: 28.0 (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 33 – The Triangulum Galaxy

M33, the Triangulum Spiral Galaxy, seen here in a 4.3 hour exposure image. Astronomers used JWST to examine a section of its south spiral arm to search out and find nearly 800 newly forming stars. Credit and copyright: John Chumack.
M33, the Triangulum Spiral Galaxy, seen here in a 4.3 hour exposure image. Astronomers used JWST to examine a section of its south spiral arm to search out and find nearly 800 newly forming stars. Credit and copyright: John Chumack.

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Triangulum Galaxy, also known as Messier 33. Enjoy!

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 Triangulum Galaxy, a spiral galaxy located approximately 3 million light-years from Earth in the direction of the Triangulum constellation. As the third-largest member of the Local Group of galaxies (behind the Andromeda Galaxy and the Milky Way), it is the one of the most distant objects that can be seen with the naked eye. Much like M32, M33 is very close to Andromeda, and is believed to be a satellite of this major galaxy.

Description:

At some 3 million light years away from Earth, the Triangulum Galaxy is the third largest galaxy in our Local Group and it may be a gravitationally bound companion of the Andromeda Galaxy. Its beautiful spiral arms show multitudes of red HII regions and blue clouds of young stars. The largest of these HII regions (NGC 604) spans nearly 1500 across and is the largest so far known.

The Triangulum Galaxy (M33), taken by the Swift Gamma-Ray Burst Mission. Credit: NASA/Swift

It has a spectrum similar to the Orion Nebula – our own Milky Way’s most celebrated starbirth region. “M33 is a gigantic laboratory where you can watch dust being created in novae and supernovae, being distributed in the winds of giant stars, and being reborn in new stars,” said University of Minnesota researcher and lead author Elisha Polomski. By studying M33, “you can see the Universe in a nutshell.”

Of course, our curiousity about our neighboring galaxy has driven us to try to understand more over the years. Once Edwin Hubble set the standard with Cepheid variables, we began measuring distance by discovering about 25 of them in M33. By 2004 we were studying the red giant star branch to peer even further. As A.W. McConnachie said in a 2004 study of the galaxy:

“The absolute bolometric luminosity of the point of core helium ignition in old, metal-poor, red giant stars is of roughly constant magnitude, varying only very slightly with mass or metallicity. It can thus be used as a standard candle. This technique then allows for the determination of realistic uncertainties which reflect the quality of the luminosity function used. Finally, we apply our technique to the Local Group spiral galaxy M33 and the dwarf galaxies Andromeda I and II, and derive distance. The result for M33 is in excellent agreement with the Cepheid distances to this galaxy, and makes the possibility of a significant amount of reddening in this object unlikely.”

By 2005, astronomers had detected two water masers on either side of M33 and for the first time ever – revealed what direction it as going in. According to Andreas Brunthaler (et al), who published a study about the distance and proper motion of the galaxy in 2005:

“We measured the angular rotation and proper motion of the Triangulum Galaxy (M33) with the Very Long Baseline Array by observing two H2O masers on opposite sides of the galaxy. By comparing the angular rotation rate with the inclination and rotation speed, we obtained a distance of 730 +/- 168 kiloparsecs. This distance is consistent with the most recent Cepheid distance measurement. This distance is consistent with the most recent Cepheid distance measurement. M33 is moving with a velocity of 190 +/- 59 kilometers per second relative to the Milky Way. These measurements promise a method to determine dynamical models for the Local Group and the mass and dark-matter halos of M31, M33, and the Milky Way.”

Composite image of the Triangulum Galaxy (Messier 33), taken at Mount Lemmon Observatory. Credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona

Yes, it’s moving toward the Andromeda Galaxy, much like how Andromeda is moving towards us! In 2006, a group of astronomers announced the discovery of an eclipsing binary star in M33. As A.Z. Bonanos, the lead author of the study that detailed the discovery, said:

“We present the first direct distance determination to a detached eclipsing binary in M33, which was found by the DIRECT Project. Located in the OB 66 association, it was one of the most suitable detached eclipsing binaries found by DIRECT for distance determination, given its 4.8938 day period.”

By studying the eclipsing binary, astronomers soon knew their size, distance, temperature and absolute magnitude. But more was yet to come! In 2007, the Chandra X-ray Observatory revealed even more when a black hole nearly 16 times the mass of the Sun was revealed. The black hole, named M33 X-7, orbits a companion star which it eclipses every 3.5 days. This means the companion star must also have an incredibly large mass as well….

Yet how huge must the parent star have been to have formed a black hole in advance of its companion? As Jerome Orosz, of San Diego State University, was quoted as saying in a 2007 Chandra press release:

“This discovery raises all sorts of questions about how such a big black hole could have been formed. Massive stars can be much less extravagant than people think by hanging onto a lot more of their mass toward the end of their lives. This can have a big effect on the black holes that these stellar time-bombs make.”

Artist’s rendering of the black hole found in orbit of the large blue star in M33 . Credit: Chandra/Harvard/HST

Stellar bombs? You bet. Gigantic stellar explosions even. Although no supernovae events have been detected in the Triangulum galaxy, it certainly doesn’t lack for evidence of supernova remnants. According to a 2004 study by F. Haberl and W. Pietsch of the Max-Planck-Institute:

“We present a catalogue of 184 X-ray sources within 50′ of the nucleus of the local group spiral galaxy M 33. The catalogue is derived from an analysis of the complete set of ROSAT archival data pointed in the direction of M 33 and contains X-ray position, existence likelihood, count rates and PSPC spectral hardness ratios. To identify the sources the catalog was correlated with previous X-ray catalogues, optical and radio catalogues. In addition sources were classified according to their X-ray properties. We find seven candidates for supersoft X-ray sources, of which two may be associated with known planetary nebulae in M 33. The majority of X-ray detected supernova remnants is also detected at radio frequencies and seen in optical lines. The low overall X-ray detection rate of optically selected SNRs can probably be attributed to their expansion into interstellar matter of low density.”

Or the creation of black holes…

History of Observation:

While the Triangulum Galaxy was probably first observed by Hodierna before 1654 (back when skies were dark), it was independently rediscovered by Charles Messier, and cataloged by him on August 25, 1764. As he recorded in his notes on the occasion:

“I have discovered a nebula between the head of the northern Fish and the large Triangle, a bit distant from a star which had not been known, of sixth magnitude, of which I have determined the position; the right ascension of that star was 22d 7′ 13″, and its declination 29d 54′ 10″ north: near that star, there is another one which is the first of Triangulum, described by the letter b. Flamsteed described it in his catalog, of sixth magnitude; it is less beautiful than that of which I have given the position, and one should set it to the rank of the stars of the eighth class. The nebula is a whitish light of 15 minutes in diameter, of an almost even density, despite a bit more luminous at two third of its diameter; it doesn’t contain any star: one sees it with difficulty with an ordinary refractor of one foot.”

The location of the Triangulum Galaxy in the night sky. Credit: Wikisky

While Sir William Herschel wouldn’t publish papers on Messier’s findings, he was an astronomically curious soul and couldn’t help but study M33 intently on his own, writing:

“There is a suspicion that the nebula consists of exceedingly small stars. With this low power it has a nebulous appearance; and it vanishes when I put on the higher magnifying powers of 278 and 460.” He would continue to observe this grand galaxy again and again over the years, cataloging its various regions with their own separate numbers and keeping track of his findings: “The stars of the cluster are the smallest points imaginable. The diameter is nearly 18 minutes.”

Yet it would take a very special observer, one named Bill Parsons – the third Earl of Rosse – to become the very first to describe it as spiral. As he wrote of it:

“September 16, 1849. – New spiral: Alpha the brighter branch; Gamma faint; Delta short but pretty bright; Beta pretty distinct; Epsilon but suspected; the whole involved in a faint nebula, which probably extends past several knots which lie about it in different directions. Faint nebula seems to extend very far following: drawing taken.”

Quite the description indeed, since it would eventually lead to Rosse’s description of M33 being “…full of knots. Spiral arrangement. Two similar curves like an “S” cross in the center”, and to other astronomers discovering that these “spiral nebulae” were extra-galactic!

The location of Messier 33 in the Triangulum constellation. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

Locating Messier 33:

While actually locating Messier 33 isn’t so difficult, seeing Messier 33 can be. Even though it is billed at nearly unaided eye magnitude, this huge, low surface brightness galaxy requires some experience with equipment and observing conditions or you may hunt forever in the right place and never find it. Let’s begin first by getting you in the proper area! First locate the Great Square of Pegasus – and its easternmost bright star, Alpha. About a hand span further east you will see the brightest star in Triangulum – Alpha.

M33 is just a couple of degrees (about 2 finger widths) west. Now, the most important part to understand is that you must use the lowest magnification possible, or you won’t be able to see the proverbial forest because of the trees. The image you see here at the top of the page is around a full degree of sky – about 1/3 the field of view of average binoculars and far larger than your average telescope eyepiece.

However, by using the least amount of magnification with a telescope you are causing M33 to appear much smaller – allowing it to fit within eyepiece field of view range. The larger the aperture, the more light it gathers and the brighter the image will be. The next thing to understand is M33 really is low surface brightness… Light pollution, a fine haze in the sky, moonlight… All of these things will make it difficult to find. Yet, there are places left here on Earth where the Triangulum Galaxy can be seen with no optical aid at all!

Enjoy your quest for M33. You may find it your first time out and it may be years before you see it in all its glory. But when you do, we guarantee you’ll never forget! Be sure to enjoy this video of the Triangulum galaxy too, courtesy of the European Southern Observatory:

Enjoy your quest for M33. You may find it your first time out and it may be years before you see it in all its glory. But when you do, we guarantee you’ll never forget!

And here are the quick facts on M33 to help you get started:

Object Name: Messier 33
Alternative Designations: M33, NGC 598, Triangulum Galaxy, Pinwheel Galaxy
Object Type: Type Sc, Spiral Galaxy
Constellation: Triangulum
Right Ascension: 01 : 33.9 (h:m)
Declination: +30 : 39 (deg:m)
Distance: 3000 (kly)
Visual Brightness: 5.7 (mag)
Apparent Dimension: 73×45 (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 32 – the “Le Gentil” Dwarf Elliptical Galaxy

Color view of M31 (The Andromeda Galaxy), with M32 (a satellite galaxy) shown to the lower left. Credit and copyright: Terry Hancock.

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at dwarf elliptical galaxy known as Messier 32. Enjoy!

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 dwarf elliptical galaxy known as Messier 32 (aka. NGC 221). Located about 2.65 million light-years from Earth, in the direction of the Andromeda constellation, this dwarf is actually a satellite galaxy of the massive Andromeda Galaxy (M31). Along with Andromeda, the Milky Way and the Triangulum Galaxy (M33) is a member of the Local Group.

Description:

M32 is an elliptical dwarf galaxy which contains about 3 billion solar masses. While it looks small compared to its massive neighbor, this little guy actually stretches across space some 8,000 light years in diameter. Once you pick it up, you’ll notice it’s really quite bright on its own – and with very good reason – its nucleus is almost identical to M31. Both contain about 100 million solar masses in rapid motion around a central supermassive object!

The dwarf elliptical galaxy Messier 32 (Le Gentil). Credit: Wikisky

As Alister W. Graham wrote in his 2002 study – titled “Evidence for an outer disk in the Prototype `Compact Elliptical’ Galaxy M32“:

“M32 is the prototype for the relatively rare class of galaxies referred to as compact ellipticals. It has been suggested that M32 may be a tidally disturbed r1/4 elliptical galaxy or the remnant bulge of a disk-stripped early-type spiral galaxy reveals that the surface bightness profile, the velocity dispersion measurements, and the estimated supermassive black hole mass in M32 are inconsistent with the galaxy having, and probably ever having had, an r1/4 light profile. Instead, the radial surface brightness distribution of M32 resembles an almost perfect (bulge+exponential disk) profile; this is accompanied by a marked increase in the ellipticity profile and an associated change in the position angle profile where the “disk” starts to dominate. Compelling evidence that this bulge/disk interpretation is accurate comes from the best-fitting r1/n bulge model, which has a Sersic index of n=1.5, in agreement with the recently discovered relation between a bulge’s Sersic index and the mass of a bulge’s supermassive black hole.”

By probing deeply into Messier 32, we’ve learned this little galaxy is home to mainly mature red and yellow stars. And they’re good housekeepers, too… because there’s practically no dust or gas to be found. While this seems neat and tidy, it also means there isn’t any new star formation going on either, but there are signs of some lively doings in the not too distant past.

Because M32 has shared “space” with neighboring massive M31, the strong tidal field of the larger galaxy may have ripped away what once could have been spiral arms – leaving only its central bulge and triggering starburst in the core. As Kenji Bekki (et al) wrote in their 2001 study:

“The origin of M32, the closest compact elliptical galaxy (cE), is a long-standing puzzle of galaxy formation in the Local Group. Our N-body/smoothed particle hydrodynamics simulations suggest a new scenario in which the strong tidal field of M31 can transform a spiral galaxy into a compact elliptical galaxy. As a low-luminosity spiral galaxy plunges into the central region of M31, most of the outer stellar and gaseous components of its disk are dramatically stripped as a result of M31’s tidal field. The central bulge component on the other hand, is just weakly influenced by the tidal field, owing to its compact configuration, and retains its morphology. M31’s strong tidal field also induces rapid gas transfer to the central region, triggers a nuclear starburst, and consequently forms the central high-density and more metal-rich stellar populations with relatively young ages. Thus, in this scenario, M32 was previously the bulge of a spiral galaxy tidally interacting with M31 several gigayears ago. Furthermore, we suggest that cE’s like M32 are rare, the result of both the rather narrow parameter space for tidal interactions that morphologically transform spiral galaxies into cE’s and the very short timescale (less than a few times 109 yr) for cE’s to be swallowed by their giant host galaxies (via dynamical friction) after their formation.”

Messier 31 (the Andromeda Galaxy), along with Messier 32 and Messier 110. Credit: Wikisky

History of Observation:

M32 was discovered by Guillaume Le Gentil on October 29th, 1749 and became the first elliptical galaxy ever observed. Although it wasn’t cataloged by Charles Messier until August 3rd, 1764, he had also seen it some seven years earlier while studying at the Paris Observatory, but his notes had been suppressed. But no matter, for he made sure to include it in his notes with a drawing! As he wrote of the object:

“I have examined in the same night [August 3 to 4, 1764], and with the same instruments, the small nebula which is below and at some [arc] minutes from that in the girdle of Andromeda. M. le Gentil discovered it on October 29, 1749. I saw it for the first time in 1757. When I examined the former, I did not know previously of the discovery which had been made by M. Le Gentil, although he had published it in the second volume of the Memoires de Savans erangers, page 137. Here is what I found written in my journal of 1764. That small nebula is round and may have a diameter of 2 minutes of arc: between that small nebula and that in the girdle of Andromeda one sees two small telescopic stars. In 1757, I made a drawing of that nebula, together with the old one, and I have not found and change at each time I have reviewed it: One sees with difficulty that nebula with an ordinary refractor of three feet and a half; its light is fainter than that of the old one, and it doesn’t contain any star. At the passage of that new nebula through the Meridian, comparing it with the star Gamma Andromedae, I have determined its position in right ascension as 7d 27′ 32″, and its declination as 38d 45′ 34″ north.”

Later, Messier 32 would be examined again, this time by Admiral Symth who said:

“An overpowering nebula, with a companion about 25′ in the south vertical M32 … The companion of M31 was discovered in November, 1749, by Le Gentil, and was described by him as being about an eighth of the size of the principal one. The light is certainly more feeble than here assigned. Messier – whose No. 32 it is – observed it closely in 1764, and remarked, that no change had taken place since the time of its being first recorded. In form it is nearly circular. The powerful telescope of Lord Rosse is a reflector of three feet in diameter, of performance hitherto unequalled. It was executed by the Earl of Rosse, under a rare union of skill, assiduity, perseverance, and muniference. The years of application required to accomplish this, have not worn his Lordship’s zeal and spirit; like a giant refreshed, he has returned to his task, and is now occupied upon a metallic disc of no less than six feet in diameter. Should the figure of this prove as perfect as the present one, we may soon over-lap what many absurdly look upon as the boundaries of the creation.”

The location of Messier 32 location in the Andromeda constellation. Credit: Roberto Mura

Locating Messier 32:

Locating M32 is as easy as locating the Andromeda Galaxy, but it will require large binoculars or at least a small telescope to see. Even under moderately light polluted skies the Great Andromeda Galaxy can be easily be found with the unaided eye – if you know where to look. Seasoned amateur astronomers can literally point to the sky and show you the location of M31, but perhaps you have never tried to find it.

Believe it or not, this is an easy galaxy to spot even under the moonlight. Simply identify the large diamond-shaped pattern of stars that is the Great Square of Pegasus. The northernmost star is Alpha, and it is here we will begin our hop. Stay with the northern chain of stars and look four finger widths away from Alpha for an easily seen star.

The next along the chain is about three more finger widths away… And we’re almost there. Two more finger widths to the north and you will see a dimmer star that looks like it has something smudgy nearby. Point your binoculars there, because that’s no cloud – it’s the Andromeda Galaxy!

Now aim your binoculars or telescope its way… Perhaps one of the most outstanding of all galaxies to the novice observer, M31 spans so much sky that it takes up several fields of view in a larger telescope, and even contains its own clusters and nebulae with New General Catalog designations. If you have larger binoculars or a telescope, you will be able to pick up M31’s two companions – M32 and M110. Messier 32 is the elliptical galaxy to the south.

Why not stretch your own boundaries? Go observing! Halton Arp included Messier 32 as No. 168 in his Catalogue of Peculiar Galaxies. It’s bright, easy and fun! And here are the quick facts on this Messier Object to help you get started:

Object Name: Messier 32
Alternative Designations: M32, NGC 221
Object Type: Type E2, Elliptical Galaxy
Constellation: Andromeda
Right Ascension: 00 : 42.7 (h:m)
Declination: +40 : 52 (deg:m)
Distance: 2900 (kly)
Visual Brightness: 8.1 (mag)
Apparent Dimension: 8×6 (arc min)

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

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

Sources:

Posted on by Tammy Plotner

The Canes Venatici Constellation

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

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

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

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

Name and Meaning:

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

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

History of Observation:

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

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

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

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

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

Notable Features:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Finding Canes Venatici:

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

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

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

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

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

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

Sources:

Posted on by Tammy Plotner

Messier 24 – the Sagittarius Star Cloud

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

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

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

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

Description:

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

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

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

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

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

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

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

History of Observation:

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

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

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

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

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

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

Locating Messier 24:

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

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

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

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

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

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

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

Sources:

Posted on by Tammy Plotner

Messier 21 (M21) – The NGC 6531 Open Star Cluster

The Messier 21 open star cluster and the Trifid Nebula. Credit: Wikisky

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Messier 21 open star cluster. Enjoy!

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

One of these objects is Messier 21 (aka. NGC 6531), an open star cluster located in the Sagittarius constellation. A relatively young cluster that is tightly packed, this object is not visible to the naked eye. Hence why it was not discovered until 1764 by Charles Messier himself. It is now one of the over 100 Deep Sky Objects listed in the Messier Catalog.

Description:

At a distance of 4,250 light years from Earth, this group of 57 various magnitude stars all started life together about 4.6 million years ago as part of the Sagittarius OB1 stellar association. What makes this fairly loose collection of stars rather prized is its youth as a cluster, and the variation of age in its stellar members. Main sequence stars are easy enough to distinguish in a group, but low mass stars are a different story when it comes to separating them from older cluster members.

Messier 21 (NGC 6531). Atlas Image mosaic obtained as part of the Two Micron All Sky Survey (2MASS), a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.
Atlas mosaic image of Messier 21 (NGC 6531) obtained as part of the Two Micron All Sky Survey (2MASS). Credit: 2MASS/UofM/IPAC/Catech/NASA/NSF

As Byeong Park of the Korean Astronomy Observatory said in a 2001 study of the object:

“In the case of a young open cluster, low-mass stars are still in the contraction phase and their positions in the photometric diagrams are usually crowded with foreground red stars and reddened background stars. The young open cluster NGC 6531 (M21) is located in the Galactic disk near the Sagittarius star forming region. The cluster is near to the nebula NGC 6514 (the Trifid nebula), but it is known that it is not associated with any nebulosity and the interstellar reddening is low and homogeneous. Although the cluster is relatively near, and has many early B-type stars, it has not been studied in detail.”

But study it in detail they did, finding 56 main sequence members, 7 pre-main sequence stars and 6 pre-main sequence candidates. But why did this cluster… you know, cluster in the way it did? As Didier Raboud, an astronomer from the Geneva Observatory, explained in his 1998 study “Mass segregation in very young open clusters“:

“The study of the very young open cluster NGC 6231 clearly shows the presence of a mass segregation for the most massive stars. These observations, combined with those concerning other young objects and very recent numerical simulations, strongly support the hypothesis of an initial origin for the mass segregation of the most massive stars. These results led to the conclusion that massive stars form near the center of clusters. They are strong constraints for scenarii of star and stellar cluster formation.” say Raboud, “In the context of massive star formation in the center of clusters, it is worth noting that we observe numerous examples of multiple systems of O-stars in the center of very young OCs. In the case of NGC 6231, 8 stars among the 10 brightest are spectroscopic binaries with periods shorter than 6 days.”

Credit: earthsky.org
Achernar, the flattest star known, is classified as be star. Credit: earthsky.org

But are there any other surprises hidden inside? You bet! Try Be-stars, a class of rapidly rotating stars that end up becoming flattened at the poles. As Virginia McSwain of Yale University’s Department of Astronomy wrote in a 2005 study, “The Evolutionary Status of Be Stars: Results from a Photometric Study of Southern Open Clusters“:

“Be stars are a class of rapidly rotating B stars with circumstellar disks that cause Balmer and other line emission. There are three possible reasons for the rapid rotation of Be stars: they may have been born as rapid rotators, spun up by binary mass transfer, or spun up during the main-sequence (MS) evolution of B stars. To test the various formation scenarios, we have conducted a photometric survey of 55 open clusters in the southern sky. We use our results to examine the age and evolutionary dependence of the Be phenomenon. We find an overall increase in the fraction of Be stars with age until 100 Myr, and Be stars are most common among the brightest, most massive B-type stars above the zero-age main sequence (ZAMS). We show that a spin-up phase at the terminal-age main sequence (TAMS) cannot produce the observed distribution of Be stars, but up to 73% of the Be stars detected may have been spun-up by binary mass transfer. Most of the remaining Be stars were likely rapid rotators at birth. Previous studies have suggested that low metallicity and high cluster density may also favor Be star formation.”

History of Observation:

Charles Messier discovered this object on June 5th, 1764. As he wrote in his notes on the occassion:

“In the same night I have determined the position of two clusters of stars which are close to each other, a bit above the Ecliptic, between the bow of Sagittarius and the right foot of Ophiuchus: the known star closest to these two clusters is the 11th of the constellation Sagittarius, of seventh magnitude, after the catalog of Flamsteed: the stars of these clusters are, from the eighth to the ninth magnitude, environed with nebulosities. I have determined their positions. The right ascension of the first cluster, 267d 4′ 5″, its declination 22d 59′ 10″ south. The right ascension of the second, 267d 31′ 35″; its declination, 22d 31′ 25″ south.”

Messier 21. Credit: Wikisky
Close up of the Messier 21 star cluster. Credit: Wikisky

While Messier did separate the two star clusters, he assumed the nebulosity of M20 was also involved with M21. In this circumstance, we cannot fault him. After all, his job was to locate comets, and the purpose of his catalog was to identify those objects that were not. In later years, Messier 21 would be revisited again by Admiral Smyth, who would describe it as follows:

“A coarse cluster of telescopic stars, in a rich gathering galaxy region, near the upper part of the Archer’s bow; and about the middle is the conspicuous pair above registered, – A being 9, yellowish, and B 10, ash coloured. This was discovered by Messier in 1764, who seems to have included some bright outliers in his description, and what he mentions as nebulosity, must have been the grouping of the minute stars in view. Though this was in the power of the meridian instruments, its mean apparent place was obtained by differentiation from Mu Sagittarii, the bright star about 2 deg 1/4 to the north-east of it.”

Locating Messier 21:

Once you have become familiar with the Sagittarius region, finding Messier 21 is easy. It’s located just two and a half degrees northwest of Messier 8 – the “Lagoon Nebula” – and about a half a degree northeast of Messier 20 – the “Trifid Nebula“. If you are just beginning to astronomy, try starting at the teapot’s tip star (Lambda) “Al Nasl”, and starhopping in the finderscope northwest to the Lagoon.

Credit IAU/Sky & Telescope magazineRoger Sinnott & Rick Fienberg
The location of M21 in the Sagittarius constellation. Credit: IAU/Sky & Telescope magazineRoger Sinnott & Rick Fienberg

While the nebulosity might not show in your finder, optical double 7 Sagittari, will. From there you will spot a bright cluster of stars two degrees due north. These are the stars embedded withing the Trifid Nebula, and the small, compressed area of stars to its northeast is the open star cluster M21. It will show well in binoculars under most sky conditions as a small, fairly bright concentration and resolve well for all telescope sizes.

And here are the quick facts, for your convenience:

Object Name: Messier 21
Alternative Designations: M21, NGC 6531
Object Type: Open Star Cluster
Constellation: Sagittarius
Right Ascension: 18 : 04.6 (h:m)
Declination: -22 : 30 (deg:m)
Distance: 4.25 (kly)
Visual Brightness: 6.5 (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 16 (M16) – The Eagle Nebula

Composite image of the Eagle Nebula (Messier 16, or NGC 6611), based on images obtained with the Wide-Field Imager camera on the MPG/ESO 2.2-metre telescope at the La Silla Observatory. Credit: ESO

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Messier 16 open star cluster – aka. The Eagle Nebula (and a slew of other names). Enjoy!

In the 18th century, while searching the night sky for comets, French astronomer Charles Messier began noticing a series of “nebulous objects” in the night sky. Hoping to ensure that other astronomers did not make the same mistake, he began compiling a list of these objects,. Known to posterity as the Messier Catalog, this list has come to be one of the most important milestones in the research of Deep Sky objects.

One of these objects it he Eagle Nebula (aka. NGC 661. The Star Queen Nebula and The Spire), a young open cluster of stars located in the Serpens constellation. The names “Eagle” and “Star Queen” refer to visual impressions of the dark silhouette near the center of the nebula. The nebula contains several active star-forming gas and dust regions, which includes the now-famous “Pillars of Creation“.

Description:

Located some 7,000 light years away in the next inner spiral arm of the Milky Way galaxy, the Eagle Nebula spans some 70 by 50 light years across. Born around 5.5 million years ago, this glittering swarm marks an area about 15 light years wide, and within the heart of this nebula is a cluster of stars and a region that has captured our imaginations like nothing else – the “Pillars of Creation”.

Here, star formation is going on. The dust clouds are illuminated by emission light, where high-energy radiation from its massive and hot young stars excited the particles of gas and makes them glow. Inside the pillars are Evaporating Gaseous Globules (EGGs), concentrations of gas that are emerging from the “womb” that about to become stars.

M16 Stars, Pillars, and the Eagle's EGGs
Wide-field IR view of the Eagle Nebula, showing its Stars, the Pillars, and the Eagle’s EGGs. Credit: ESO

These pockets of interstellar gas are dense enough to collapse under their own weight, forming young stars that continue to grow as they accumulate more and more mass from their surroundings. As their place of birth contracts gravitationally, the interior gas reaches its end and the intense radiation of bright young stars causes low density material to boil away.

These regions were first photographed by the Hubble Space Telescope in 1995. As Jeff Hester – a professor at Arizona State University and an investigator with the Hubble’s Wide Field and Planetary Camera 2 (WFPC2) – said of the discovery:

“For a long time astronomers have speculated about what processes control the sizes of stars – about why stars are the sizes that they are. Now in M16 we seem to be watching at least one such process at work right in front of our eyes.”

The Hubble has shown us what happens when all the gas boils away and only the EGGs are left. “It’s a bit like a wind storm in the desert,” said Hester. “As the wind blows away the lighter sand, heavier rocks buried in the sand are uncovered. But in M16, instead of rocks, the ultraviolet light is uncovering the denser egg-like globules of gas that surround stars that were forming inside the gigantic gas columns.”

The Eagle Nebula's pillars of creation taken in 1995 (right) and 2015. The new image was obtained with the Wide Field Camera 3, installed by astronauts in 2009. Credit: Left: NASA, ESA/Hubble and the Hubble Heritage Team. Right: NASA, ESA/Hubble, STScI, J. Hester and P. Scowen (Arizona State University)
The Eagle Nebula’s pillars of creation taken in 1995 (right) and 2015. The new image was obtained with the Wide Field Camera 3, installed by astronauts in 2009. Credit: Left: NASA, ESA/HST/Hubble Heritage Team/STScI, J. Hester and P. Scowen (Arizona State University).

And some of these EGGs are nothing more than what would appear to be tiny bumps and teardrops in space – but at least we are looking back in time to see what stars look like when they were first born. “This is the first time that we have actually seen the process of forming stars being uncovered by photoevaporation,” Hester emphasized. “In some ways it seems more like archaeology than astronomy. The ultraviolet light from nearby stars does the digging for us, and we study what is unearthed.”

History of Observation:

The star cluster associated with M16 (NGC 6611) was first discovered by Philippe Loys de Chéseaux in 1745-6. However, it was Charles Messier who was the very first to see the nebulosity associated with it. As he recorded in his notes:

“In the same night of June 3 to 4, 1764, I have discovered a cluster of small stars, mixed with a faint light, near the tail of Serpens, at little distance from the parallel of the star Zeta of that constellation: this cluster may have 8 minutes of arc in extension: with a weak refractor, these stars appear in the form of a nebula; but when employing a good instrument one distinguishes these stars, and one remarks in addition a nebulosity which contains three of these stars. I have determined the position of the middle of this cluster; its right ascension was 271d 15′ 3″, and its declination 13d 51′ 44″ south.”

A new look at M16, the Eagle Nebula in this composite from the Herschel telescope in far-infrared and XMM-Newton in X-ray. Credits: far-infrared: ESA/Herschel/PACS/SPIRE/Hill, Motte, HOBYS Key Programme Consortium; X-ray: ESA/XMM-Newton/EPIC/XMM-Newton-SOC/Boulanger
Composite image of M16 from the Herschel telescope in far-infrared and XMM-Newton in X-ray. Credits: ESA/Herschel/PACS/SPIRE/Hill, Motte, HOBYS Key Programme Consortium/XMM-Newton/EPIC/XMM-Newton-SOC/Boulanger

Oddly enough, Sir William Herschel, who was famous for elaborating on Messier’s observations, didn’t seem to notice the nebula at all (according to his notes). And Admiral Smyth, who could always be counted on for flowery prose about stellar objects, just barely saw it as well:

“A scattered but fine large stellar cluster, on the nombril of Sobieski’s shield, in the Galaxy, discovered by Messier in 1764, and registered as a mass of small stars in the midst of a faint light. As the stars are disposed in numerous pairs among the evanescent points of more minute components, it forms a very pretty object in a telescope of tolerable capacity.”

But of course, the nebula isn’t an easy object to spot and its visibility on any given night depends greatly on sky conditions. As historical evidence suggest, only one of the two masters (Messier) caught it. So take a lesson from history and return to the sky many times. One day you’ll be rewarded!

Locating Messier 16:

One of the easiest ways to find M16 is to identify the constellation of Aquila and begin tracing the stars down the eagle’s back to Lambda. When you reach that point, continue to extend the line through to Alpha Scuti, then southwards towards Gamma Scuti. Aim your binoculars or image correct finderscope at Gamma and put it in the 7:00 position.

The location of M16 in the Serpens constellation. Credit: constellation-guide.com
The location of M16, relative to the “Teapot” asterism in the Sagittarius constellation. Credit: constellation-guide.com

For those using a finderscope, M16 will easily show up as a faint haze. Even those using binoculars won’t miss it. If Gamma is in the lower left hand corner of your vision – then M16 is in the upper right hand. For all optics, you won’t be able to miss the open star cluster and the faint nebulosity of IC 4703 can be seen from dark sky locations.

Another way to find M16 is by first locating the “Teapot” asterism in Sagittarius constellation (see above), and then by following the line from the star Kaus Australis (Epsilon Sagittarii) – the brightest star in Sagittarius – to just east of Kaus Media (Delta Sagittarii). Another way to find the nebula is by extending a line from Lambda Scuti in Scutum constellation to Alpha Scuti, and then to the south to Gamma Scuti.

Those using large aperture telescopes will be able to see the nebula well, but sky conditions are everything when it comes to this one. The star cluster which is truly M16 will always be easy, but the nebula is a challenge.

And as always, here are the quick facts on M16 to help you get started:

Object Name: Messier 16
Alternative Designations: M16, NGC 6611, Eagle Nebula (IC 4703)
Object Type: Open Star Cluster and Emission Nebula
Constellation: Serpens (Cauda)
Right Ascension: 18 : 18.8 (h:m)
Declination: -13 : 47 (deg:m)
Distance: 7.0 (kly)
Visual Brightness: 6.4 (mag)
Apparent Dimension: 7.0 (arc min)

And be sure to enjoy this video of the Eagle Nebula and the amazing photographs of the “Pillar of Creation”:

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.

Posted on by Tammy Plotner

Messier 15 (M15) – The Great Pegasus Cluster

The M15 Globular Cluster (aka. Great Hercules Cluster). Astronomers suspect the existence of one or more intermediate-mass black holes at its heart. Credit: NASA/ESA/HST
The M15 Globular Cluster (aka. Great Hercules Cluster). Astronomers suspect the existence of one or more intermediate-mass black holes at its heart. Credit: NASA/ESA/HST

Welcome back to Messier Monday! Today, in our ongoing tribute to Tammy Plotner, we take a look at the M15 globular cluster, one of the oldest and best known star clusters in the night sky. Enjoy!

In the 18th century, French astronomer Charles Messier began noticing a series of “nebulous objects” in the night sky while looking for comets. Not wanting other astronomers to make the same mistake, he began compiling a list of these objects into a catalog. In time, this list would include 100 objects, and came to be known by future astronomers as the Messier Catalog.

One of these objects is the globular cluster known as M15. Located in the northern constellation Pegasus, it is one of the brightest clusters in the night sky (with a visual brightness that is roughly 360,000 times that of our Sun). It is also one of the finest globular clusters in the northern section of the sky, the best deep-sky object in the constellation of Pegasus, and one of the oldest and best known globular clusters.

Description:

Messier 15 is probably the most dense globular cluster in our entire Milky Way galaxy – having already undergone a process of contraction. What does that mean to what you’re seeing? This ball of stars measures about 210 light years across, yet more than half of the stars you see are packed into the central area in a space just slightly more than ten light years in size.

By looking for single stars within globular clusters, the Hubble Space Telescope was either looking for a massive black hole or evidence of a “core collapse” – the intense gravity of so many stars so close together. Although it was peeking nearly 37,000 light-years away, the Hubble was able to resolve hundreds of stars converging on M15’s core. Like magnetism, their gravity would either cause them to attract or repel one another – and a black hole may have formed at some point in the cluster’s 12-billion-year life.

The globular cluster known as Messier 15, located some 35 000 light-years away in the Pegasus constellation. Credit: Mount Lemmon SkyCenter/University of Arizona
The globular cluster known as Messier 15, located some 35 000 light-years away in the Pegasus constellation. Credit: Mount Lemmon SkyCenter/University of Arizona

The study which addressed this data – which appeared in the January 1996 issue of the Astronomical Journal, was led by Puragra Guhathakurta of UCO/Lick Observatory, UC Santa Cruz – asked the question of whether or not the speed of the cluster’s stars could tell us if M15’s dense core was caused by a single huge object, or just mutual attraction. As Guhathakurta stated in the study:

“It is very likely that M15’s stars have concentrated because of their mutual gravity. The stars could be under the influence of one giant central object, although a black hole is not necessarily the best explanation for what we see. But if any globular cluster has a black hole at its center, M15 is the most likely candidate.”

John Bahcall and astrophysicist Jeremiah Ostriker of Princeton University were the first to forward the idea that Messier 15 might be hiding a black hole. While it is distinct from many other globular clusters by having such a dense core, it really isn’t that much different than all the rest of the globular clusters we see. Yet, no where else in our galaxy, except at its core, are the stars that dense!

It is estimated that 30,000 distinct stars exist in the inner 22 light-years of the cluster alone. The closer the Hubble telescope looked, the more stars it found. This increase in stellar density continued all the way to within 0.06 light-years of the center – about 100 times the distance between our Sun and Pluto. “Detecting separate stars that close to the core was at the limit of Hubble’s powers,” says Brian Yanny of the Fermi National Accelerator Laboratory.

The location of M15, within the Pegasus Constellation. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)
The location of M15, within the Pegasus Constellation. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

At this point, even the great Hubble could not distinguish individual stars, or locate the exact position of the core. Guhathakurta and is colleagues theorized that the stars crowd even closer inside the radius, so they plotted the distribution of the stars as a function of distance from the core. When the results came back, they had two answers – either a black hole was responsible, or a gravothermal catastrophe called core collapse was the culprit.

“It’s a catastrophe in the sense that once it starts, this process can run away very quickly,” said Guhathakurta. “But other processes could cause the core to bounce back before it collapses all the way.”

At an estimated 13.2 billion years old, it is one of the oldest known globular clusters, but it isn’t done throwing some surprises at us. M15 was the first globular cluster in which a planetary nebula, Pease 1 or K 648 (“K” for “Kuster”), could be identified – and can be seen with larger aperture amateur telescopes. Even stranger is the fact that Messier 15 contains 112 variable stars, and 9 known pulsars – neutron stars which are the leftovers of ancient supernovae. And one of these is a double neutron star system – M15 C.

History of Observation:

M15 was discovered by Jean-Dominique Maraldi on September 7, 1746 while he was looking for a comet. Says he:

“On September 7 I noticed between the stars Epsilon Pegasi and Beta Equulei, a fairly bright nebulous star, which is composed of many stars, of which I have determined the right ascension of 319d 27′ 6″, and its northern declination of 11d 2′ 22”. About 25 years later, Charles Messier would independently rediscover it to add to his own catalog, describing it as: “In the night of June 3 to 4, 1764, I have discovered a nebula between the head of Pegasus and that of Equuleus it is round, its diameter is about 3 minutes of arc, the center is brilliant, I have not distinguished any star; having examined it with a Gregorian telescope which magnifies 104 times, it had little elevated over the horizon, and maybe that observed at a greater elevation one can perceive stars.”

Camera SBIG STX16803 CCD Camera Filters Astrodon Gen II Dates December 2015 Location Mount Lemmon SkyCenter Exposure RGB = 2 : 2 : 2 Hours Acquisition Astronomer Control Panel (ACP), Maxim DL/CCD (Cyanogen), FlatMan XL (Alnitak) Processing CCDStack, Photoshop, PixInsight Credit Line & Copyright Adam Block/Mount Lemmon SkyCenter/University of Arizona
Deep Broadband (RGB) image of M15, taken from the Mount Lemmon SkyCenter. Credit and Copyright: Adam Block/Mount Lemmon SkyCenter/University of Arizona

Sir William Herschel would be the first to resolve some of its stars, but not the core. It would be his son John who would later pick up structure. However, like the dutiful and colorful observer that he was, Admiral Smyth will leave us with this lasting impression:

“Although this noble cluster is rated as globular, it is not exactly round, and under the best circumstances is seen as in the diagram, with stragglers branching from a central blaze. Under a moderate magnifying power, there are many telescopic and several brightish stars in the field; but the accumulated mass is completely insulated, and forcibly strikes the senses as being almost infinitely beyond those apparent comets. Indeed, it may be said to appear evidently aggregated by mutual laws, and part of some stupendous and inscrutable scheme of involution; for there is nothing quiescent throughout the immensity of the vast creation.”

Considering Smyth’s observations were made nearly two centuries before we really began to understand what was going on inside Messier 15, you’ll have to admit he was a very good observer!

Locating Messier 15:

Surprisingly enough, globular cluster M15 is easy to find. Once you’ve located the “Great Square” of Pegasus, simply choose its brightest and southwesternmost star – Alpha. Now identify the small, kite shape of the constellation of Delphinus. Roughly halfway between these two (and slightly south), you’ll spy a slightly reddish star – Epsilon Peg (Enif).

By placing Enif in your binoculars or image correct finderscope at the 7:00 position, you can’t miss this bright, compact beauty. Even the smallest of optics will reveal the round glow and telescopes starting at 4″ will begin resolution – while large telescopes will simply amaze you. However, don’t expect to open this globular up to the core region. As already noted, its pretty dense in there!

And here are the quick facts for Messier 15, for your convenience:

Object Name: Messier 15
Alternative Designations: M15, NGC 7078
Object Type: Class IV Globular Cluster
Constellation: Pegasus
Right Ascension: 21 : 30.0 (h:m)
Declination: +12 : 10 (deg:m)
Distance: 33.6 (kly)
Visual Brightness: 6.2 (mag)
Apparent Dimension: 18.0 (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