Messier 34 – the NGC 1039 Open Star Cluster

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 objects is known as Messier 34, an open star cluster located in the northern Perseus constellation. Located at a distance of about 1,500 light years from Earth, it is one of the closest Messier objects to Earth, and is home to an estimated 400 stars. It is also bright enough to be seen with the naked eye or binoculars, where light conditions permit.

What You Are Looking At:

This cluster of stars started its journey off together through our galaxy some 180 million years ago as part of the “Local Association”… groups of stars like the Pleiades, Alpha Persei Cluster and the Delta Lyrae Cluster that share a common origin, but have become gravitationally unbound and are still moving together through space. We know the stars are related by their common movement and ages, but what else do we know about them?

The core region of the Messier 34 open star cluster. Credit: Wikisky

Well, one thing we do know is that out of the 354 stars in the region survey, 89 of them are actual cluster members and that all six of the visual binaries and three of the four known Ap stars are members of the cluster. There’s even a giant among them! But like almost all stars out there, we know they usually aren’t singles and actually have companions. As Theodore Simon wrote in his 2000 study regarding NGC 1039 and NGC 3532:

“Roughly half the sources detected in both images have likely optical counterparts from earlier ground-based surveys. The remainder are either prospective cluster members or foreground/background stars, which can be decided only through additional photometry, spectroscopy, and proper-motion studies. There is some indication (at the 98% confidence level) that solar-type stars may lack the extreme rotation and activity levels shown by those in the much younger Pleiades and alpha Persei clusters, but a detailed assessment of the coronal X-ray properties of these clusters must await more sensitive observations in the future. If confirmed, this finding could help to rule out the possibility that stellar dynamo activity and rotational braking are controlled by a rapidly spinning central core as stars pass through this phase of evolution from the Pleiades stage to that represented by the Hyades.”

If there’s companion stars to be discovered, what else might be in the field that we just can quite “see”? Try white dwarfs. As Kate Rubin (et al.) published in the May 2008 issue of the Astronomical Journal:

“We present the first detailed photometric and spectroscopic study of the white dwarfs (WDs) in the field of the ~225 Myr old (log ?cl = 8.35) open cluster NGC 1039 (M34) as part of the ongoing Lick-Arizona White Dwarf Survey. Using wide-field UBV imaging, we photometrically select 44 WD candidates in this field. We spectroscopically identify 19 of these objects as WDs; 17 are hydrogen-atmosphere DA WDs, one is a helium-atmosphere DB WD, and one is a cool DC WD that exhibits no detectable absorption lines. Of the 17 DAs, five are at the approximate distance modulus of the cluster. Another WD with a distance modulus 0.45 mag brighter than that of the cluster could be a double-degenerate binary cluster member, but is more likely to be a field WD. We place the five single cluster member WDs in the empirical initial-final mass relation and find that three of them lie very close to the previously derived linear relation; two have WD masses significantly below the relation. These outliers may have experienced some sort of enhanced mass loss or binary evolution; however, it is quite possible that these WDs are simply interlopers from the field WD population.”

Close-up image of M34 showing its white dwarf population, taken by the Sloan Digital Sky Survey. Credit: SDSS

While it sounds a little confusing, it’s all about how star clusters evolve. As David Soderblom wrote in a 2001 study:

“We analyze Keck Hires observations of rotation in F, G, and K dwarf members of the open cluster M34 (NGC 1039), which is 250 Myr old, and we compare them to the Pleiades, Hyades, and NGC 6475. The upper bound to rotation seen in M34 is about a factor of two lower than for the 100 Myr-old Pleiades, but most M34 stars are well below this upper bound, and it is the overall convergence in rotation rates that is most striking. A few K dwarfs in M34 are still rapid rotators, suggesting that they have undergone core-envelope decoupling, followed by replenishment of surface angular momentum from an internal reservoir. Our comparison of rotation in these clusters indicates that the time scale for the coupling of the envelope to the core must be close to 100 Myr if decoupling does, in fact, occur.”

History of Observation:

M34 was probably first found by Giovanni Batista Hodierna before 1654, and independently rediscovered by Charles Messier in on August 25, 1764. As he described it in his notes:

“I have determined the position of a cluster of small stars between the head of the Medusa and the left foot of Andromeda almost on the parallel of the star Gamma of that letter constellation. With an ordinary refractor of 3 feet, one distinguishes these stars; the cluster may have 15 minutes in extension. I have determined its position with regard to the star Beta in the head of the Medusa; its right ascension has been concluded at 36d 51′ 37″, and its declination as 41d 39′ 32″ north.”

Image of Messier 34 taken by the Two Micron All-Sky Survey (2MASS) of Messier 34 (also known as M34 or NGC 1039). Credit: 2MASS/UMass/IPAC-Caltech/NASA/NSF

Over the years, a great many historic observers would turn a telescope its way to examine it – also looking for more. Said Sir William Herschel: “A cluster of stars; with 120, I think it is accompanied with mottled light, like stars at a distance.” Yet very little more can be seen except for the fact that most of the stars seem to be arranged in pairs – the most notable being optical double in the center – h 1123 – which was cataloged by Sir John Herschel on December 23rd, 1831.

Charles Messier discovered it independently on August 25th, 1764, and included it in the Messier Catalog. As he wrote in the first edition of the catalog:

“In the same night of [August] 25 to 26, I have determined the position of a cluster of small stars between the head of the Medusa [Algol] & the left foot of Andromeda almost on the parallel of the star Gamma of that letter constellation. With an ordinary [non-achromatic] refractor of 3 feet [FL], one distinguishes these stars; the cluster may have 15 minutes in extension. I have determined its position with regard to the star Beta in the head of the Medusa; its right ascension has been concluded at 36d 51? 37?, & its declination as 41d 39? 32? north.”

But as always, it was Admiral William Henry Smyth who described the object with the most florid prose. As he wrote in his notes when observing the cluster in October 1837, he noted the following:

“A double star in a cluster, between the right foot of Andromeda and the head of Medusa; where a line from Polaris between Epsilon Cassiopeiae and Alpha Persei to within 2deg of the parallel of Algol, will meet it. A and B, 8th magnitudes, and both white. It is in a scattered but elegant group of stars from the 8th to the 13th degree of brightness, on a dark ground, and several of them form into coarse pairs. This was first seen and registered by Messier, in 1764, as a “mass of small stars;” and in 1783 was resolved by Sir W. Herschel with a seven-foot reflector: with the 20-foot he made it “a coarse cluster of large stars of different sizes.” By the method he applied to fathom the galaxy, he concluded the profundity of this object not to exceed the 144th order.”

The location of Messier 34 in the northern Perseus constellation. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

Locating Messier 34:

M34 is easily found in binoculars about two fields of view northwest of Algol(Beta Persei). You will know when you have found this distinctive star cluster because “X” marks the spot! In a telescope finderscope, it will appear as a faint, hazy spot and will fully resolve to most average telescopes. Messier 34 makes an excellent target for moonlit nights or light polluted areas and will stand up well to less than perfect sky conditions.

It can even be seen unaided from ideal locations! Enjoy your observations!

And as always, we’ve included the quick facts on this Messier Object to help you get started:

Object Name: Messier 34
Alternative Designations: M34, NGC 1039
Object Type: Galactic Open Star Cluster
Constellation: Perseus
Right Ascension: 02 : 42.0 (h:m)
Declination: +42 : 47 (deg:m)
Distance: 1.4 (kly)
Visual Brightness: 5.5 (mag)
Apparent Dimension: 35.0 (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:

Messier 29 – The NGC 6913 Open Star Cluster

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 29. 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 would come to include 100 of the most fabulous objects in the night sky.

One of these objects is Messier 29, an open star cluster located in the northern skies in the direction of the Cygnus constellation. Situated in a highly crowded area of the Milky Way Galaxy, about 4,000 light-years from Earth, this star cluster is slowly moving towards us. Though somewhat isolated in the night sky, it can be easily spotted using binoculars and small telescopes.

Description:

While Messier Object 29 might appear a little bit boring compared to some of its more splashy catalog companions, it really isn’t. This little group of stars is part of the Cygnus OB1 association which just happens to be heading towards us at a speed of 28 kilometers per second (17.4 mps) . If it weren’t obscured by Milky Way dust, the light of its stars would be 1000 times brighter!

Messier 29 and Gamma Cygni (Sadr). Credit: Wikisky
Messier 29 and Gamma Cygni (Sadr). Credit: Wikisky

All in all, M29 has around 50 member stars, but this 10 million year old star cluster still has some surprises. The five brightest stars you see are are all giant stars of spectral class B0, and if we were to put one next to our own Sol, it would shine 160,000 times brighter. Image just how “lit up” any planet might be that would reside inside that 11 light year expanse!

Astronomers were curious about Messier 29, too, so they went in search of binary stars. As C. Boeche (et al) wrote in a 2003 study:

“Between 1996 and 2003 we obtained 226 high resolution spectra of 16 stars in the field of the young open cluster NGC 6913, to constrain its main properties and study its internal kinematics. Twelve of the program stars turned out to be members, one of them probably unbound. Nine are binaries (one eclipsing and another double lined) and for seven of them the observations allowed us to derive the orbital elements. All but two of the nine discovered binaries are cluster members. In spite of the young age (a few Myr), the cluster already shows signs that could be interpreted as evidence of dynamical relaxatin and mass segregation.

“However, they may be also the result of an unconventional formation scenario. The dynamical (virial) mass as estimated from the radial velocity dispersion is larger than the cluster luminous mass, which may be explained by a combination of the optically thick interstellar cloud that occults part of the cluster, the unbound state or undetected very wide binary orbit of some of the members that inflate the velocity dispersion and a high inclination for the axis of possible cluster angular momentum. All the discovered binaries are hard enough to survive average close encounters within the cluster and do not yet show signs of relaxation of the orbital elements to values typical of field binaries.”

So why is finding binary stars important? Evolution is the solution, the hunt for Be stars. As S.L. Malchenko of the Crimean Astrophysical Observatory wrote in a 2008 study on Be stars:

“The phenomenon of Be stars has been known for over a century. The fact that at least 20% of B stars have an emission spectrum supports that the definition that this phenomenon is not special but it is rather typical from a large group of objects at a certain stage of evolution. The vagueness of the concept of the Be phenomenon suggests that this definition encompasses a broad group of objects near the main sequence that includes binary systems with different rate of mass exchange. This young open cluster in the Cyg OB1 association, is also know as M29, contains a large number of luminous stars with spectral types around B0. An extreme variation of extinction is found across the young open cluster NGC 6913, extinction in the cluster center is relatively homogeneous, but very large. We observed 10 spectra for 7 B stars and one known Be star in the blue region.”

Close-up of the core region of Messier 29. Credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona
Close-up of the core region of Messier 29. Credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona

Although you won’t be able to detect it visually, there is also some nebulosity associated with M29, which is another important clue to this star cluster’s evolution. As B. Bhavya of Cochin University of Science and Technology wrote in a 2008 study:

“The Cygnus region is a region of recent star formation activity in the Milky Way and is rich in massive early type stars concentrated in OB associations. The presence of nebulosity and massive stars indicate that the stars have been forming till very recently and the young clusters found here are the result of the recent star formation event. Though the above fact is known, what is not known is that when this star formation process started and how it proceeded in the region. Though one assumes that all the stars in a cluster have the same age, this assumption is not valid when the candidate cluster is very young. In the case of young clusters, there is a chance for a spread in the age of the stars, depending on the duration of star formation. An estimation of this formation time-scale in the clusters formed in a star forming complex, will indicate the duration of star formation and its direction of propagation within the complex. In principle, duration of star formation is defined as the difference between the ages of the oldest and the youngest star formed in the cluster. In practice, the age of the oldest star is assumed as the age of that star which is about to turn-off from the main-sequence (MS) (turn-off age) and the age of the youngest star is the age of the youngest pre-MS star (turn-on age). The turn-off age of many clusters are known, but the turn-on age is not known for most of the clusters.”

History of Observation:

This cool little star cluster was an original discovery of Charles Messier, who first observed it in 1764. As he wrote of the object in his notes at the time:

“In the night of July 29 to 30, 1764, I have discovered a cluster of six or seven very small stars which are below Gamma Cygni, and which one sees with an ordinary refractor of 3 feet and a half in the form of a nebula. I have compared this cluster with the star Gamma, and I have determined its position in right ascension as 303d 54′ 29″, and its declination of 37d 11′ 57″ north.”

Gammy Cygni (the brightest object in the center) and neighboring regions. Credit: Wikipedia Commons/Erik Larsen
Gammy Cygni (the brightest object in the center) and neighboring regions. Credit: Wikipedia Commons/Erik Larsen

In the case of this cluster, it was independently recovered again by Caroline Herschel, who wrote: “About 1 deg under Gamma Cygni; in my telescope 5 small stars thus. My Brother looked at them with the 7 ft and counted 12. It is not in Mess. catalogue.”

William would also return to the cluster as well with his own observations: “Is not sufficiently marked in the heavens to deserve notice, as 7 or 8 small stars together are so frequent about this part of the heavens that one might find them by hundreds.”

So why the confusion? In this circumstance, perhaps Messier was a bit distracted, for it would appear that his logged coordinates were somewhat amiss. Leave it to Admiral Symth to set the records straight:

“A neat but small cluster of stars at the root of the Swan’s neck, and in the preceding branch of the Milky Way, not quite 2deg south of Gamma; and preceding 40 Cygni, a star of the 6th magnitude, by one degree just on the parallel. In the sp [south preceding, SW] portion are the two stars here estimated as double, of which A is 8, yellow; B 11, dusky. Messier discovered this in 1764; and though his description of it is very fair, his declination is very much out: worked up for my epoch it would be north 37d 26′ 15″. But one is only surprised that, with his confined methods and means, so much was accomplished.”

Kudos to Mr. Messier for being able to distinguish a truly related group of stars in a field of so many! Take the time to enjoy this neat little grouping for yourself and remember – it’s heading our way.

Locating Messier 29:

Finding M29 in binoculars or a telescope is quite easy once you recognize the constellation of Cygnus. Its cross-shape is very distinctive and the marker star you will need to locate this open star cluster is Gamma – bright and centermost. For most average binoculars, you will only need to aim at Gamma and you will see Messier 29 as a tiny grouping of stars that resembles a small box.

Messier 29 location. Image: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)
The location of Messier 29, in the direction of the Cygus constellation. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

For a telescope, begin with your finderscope on Gamma, and look for your next starhop marker star about a finger width southwest. Once this star is near the center of your finderscope field, M29 will also be in a low magnification eyepiece field of view. Because it is a very widely spaced galactic open star cluster that only consists of a few stars, it makes an outstanding object that stands up to any type of sky conditions.

Except, of course, clouds! Messier 29 can easily be seen in light polluted areas and during a full Moon – making it a prize object for study for even the smallest of telescopes.

As always, here are the quick facts to help you get started:

Object Name: Messier 29
Alternative Designations: M29, NGC 6913
Object Type: Open Galactic Star Cluster
Constellation: Cygnus
Right Ascension: 20 : 23.9 (h:m)
Declination: +38 : 32 (deg:m)
Distance: 4.0 (kly)
Visual Brightness: 7.1 (mag)
Apparent Dimension: 7.0 (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:

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

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: