Starship Asterisk*

Open Star Clusters M35 and NGC 2158

Explanation: Open clusters of stars can be near or far, young or old, and diffuse or compact. Found near the plane of our Milky Way galaxy, they contain from 100 to 10,000 stars, all of which formed at nearly the same time. Bright blue stars frequently distinguish younger open clusters. M35, on the upper left, is relatively nearby at 2800 light years distant, relatively young at 150 million years old, and relatively diffuse, with about 2500 stars spread out over a volume 30 light years across. An older and more compact open cluster, NGC 2158, is at the lower right. NGC 2158 is four times more distant than M35, over 10 times older, and much more compact with many more stars in roughly the same volume of space. NGC 2158's bright blue stars have self-destructed, leaving cluster light to be dominated by older and yellower stars. Both clusters are seen toward the constellation of Gemini.

<< Previous APOD

This Day in APOD

Next APOD >>

[/b]

Another fantastic open cluster pairing! I love the power of illusions in how the mind perceives things, it tells us instinctively that things that are located next to one another are likely to be related. Regarding our 2D view of 3D space, this is quite often not the case and many hundreds or thousands of light years separate things. Some of my favourite "cosmic deceptions" include M46 and the planetary nebula NGC 2438 and the nebula "pair" of NGC 3576 and NGC 3603. Also maybe some of the golden reddened colour of NGC 2158 is due to greater dust extinction.

P.S. It's my 232nd post!!

Today’s apod is eye opening as to the relativity of the open-ness of “open” clusters. I had thought that open clusters were called that to distinguish them from globular clusters in which the stars are permanently bound together gravitationally, whereas in open clusters the stars all eventually disperse. I didn’t know that an “open” cluster could be both old and compact as NGC 2158 clearly is. This cluster looks like a wimpy version of a globular cluster. My question is, will NGC 2158's stars disperse given enough time, or is it gravitationally bound?

in spite of being born in an open cluster ,the stars of NGC 2158 survived together for over 1.5 BILLION+ years ..force is strong with this one !!

BDanielMayfield wrote:Today’s apod is eye opening as to the relativity of the open-ness of “open” clusters. I had thought that open clusters were called that to distinguish them from globular clusters in which the stars are permanently bound together gravitationally, whereas in open clusters the stars all eventually disperse. I didn't know that an “open” cluster could be both old and compact as NGC 2158 clearly is. This cluster looks like a wimpy version of a globular cluster. My question is, will NGC 2158's stars disperse given enough time, or is it gravitationally bound?

There is no sharp dividing line between globular clusters and open clusters. Check out this very interesting page which contains information and pictures of M53 and NGC 5053. (Check out this page, too.) M53 and NGC 5053 are both globular clusters, and they are at similar distances from us, about 60,000 light-years. They are probably of similar ages, likely 10-12 billion years old. Both are typically metal-poor and contain blue horizontal stars, the kind of stars that are only found in metal-poor populations. Both are made of gas that has probably been recycled through only one or two previous generations of massive stars. Yet they are very different, because M53 is a typically rich globular cluster, whereas NGC 5053 is loose and star-poor.

Why do they look so different? There can be only one answer. M53 was the product of a truly amazing blaze of star formation, much, much bigger and fiercer than, say, R136 in the Large Magellanic Cloud. NGC 5053, by contrast, probably started out no bigger or heavier than R136. Over billions of years, NGC 5053 has mostly but not completely evaporated, in the same way that R136 will have shrunk to an unimpressive faint loose cluster ten billion years from now. M53 has also lost a lot of stars, but because of the much higher gravity it started out with, it has held on to its constituent stars much more efficiently than NGC 5053.

How can we say, then, that rich cluster NGC 2158 is an open cluster, when faint loose cluster NGC 5053 is a globular? It has to do with the age and metallicity of these clusters. NGC 2158 is only about one billion years old, according to Wikipedia. At this age, it is sure to be much, much more metal-rich than NGC 5053; that is, the gas that the cluster was born from had been enriched by heavy elements produced by many generations of massive stars and supernovae. Also, the color-magnitude diagram is sure to look different for NGC 2158 than for NGC 5053. NGC 2158 is sure to have hydrogen-fusing stars that are more massive than the Sun, while NGC 5053 is sure to lack them. And while NGC 5053 is sure to have blue horizontal branch stars, NGC 2158 is sure to lack them.

Here is the color-magnitude diagram for NGC 5053. Note at upper left a small but definite population of relatively bright blue horizontal stars. Here is a color-magnitude diagram for NGC 2158, although it must be pointed out that it is not a B-V diagram but a J-H diagram, which I am not so good at reading. Two B-V diagrams would have made the comparison easier.

There is one anomalously blue star in NGC 2158, which is seen at 7 o'clock in NGC 2158 in today's APOD. Personally I can't help wondering if this star might not be a foreground star, perhaps even a star that belongs to cluster M35.

Ann

P.S. Happy 232 (233?), starsurfer!

Very nice!!!

I have the APOD Wallpaper software that brings up each day's APOD image on my PC automatically. Can anyone tell me how to get the day's explanation on the screen automatically at the same time? According to some reports, there is an item on the right-click menu of the wallpaper icon, but I don't see it. Thanks! AdrianLister@blueyonder.co.uk

BDanielMayfield wrote:
Today’s apod is eye opening as to the relativity of the open-ness of “open” clusters. I had thought that open clusters were called that to distinguish them from globular clusters in which the stars are permanently bound together gravitationally, whereas in open clusters the stars all eventually disperse. I didn’t know that an “open” cluster could be both old and compact as NGC 2158 clearly is. This cluster looks like a wimpy version of a globular cluster. My question is, will NGC 2158's stars disperse given enough time, or is it gravitationally bound?

NGC 2158's stars will disperse given enough (1-2 billion years?) time by passing close to or through a molecular cloud.

Globular clusters (which generally avoid such clouds) may last for tens of billions of years.

http://en.wikipedia.org/wiki/Open_cluster wrote:
<<An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy, and many more are thought to exist. They are loosely bound to each other by mutual gravitational attraction and become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic center, resulting in a migration to the main body of the galaxy as well as a loss of cluster members through internal close encounters. Open clusters generally survive for a few hundred million years. In contrast, the more massive globular clusters of stars exert a stronger gravitational attraction on their members, and can survive for many billions of years. Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring.

Young open clusters may still be contained within the molecular cloud from which they formed, illuminating it to create an H II region. Over time, radiation pressure from the cluster will disperse the molecular cloud. Typically, about 10% of the mass of a gas cloud will coalesce into stars before radiation pressure drives the rest of the gas away.

Many open clusters are inherently unstable, with a small enough mass that the escape velocity of the system is lower than the average velocity of the constituent stars. These clusters will rapidly disperse within a few million years. In many cases, the stripping away of the gas from which the cluster formed by the radiation pressure of the hot young stars reduces the cluster mass enough to allow rapid dispersal. Clusters which have enough mass to be gravitationally bound once the surrounding nebula has evaporated can remain distinct for many tens of millions of years, but over time internal and external processes tend also to disperse them. Internally, close encounters between stars can increase the velocity of a member beyond the escape velocity of the cluster. This results in the gradual 'evaporation' of cluster members.

Externally, about every half-billion years or so an open cluster tends to be disturbed by external factors such as passing close to or through a molecular cloud. The gravitational tidal forces generated by such an encounter tend to disrupt the cluster. Eventually, the cluster becomes a stream of stars, not close enough to be a cluster but all related and moving in similar directions at similar speeds. After a cluster has become gravitationally unbound, many of its constituent stars will still be moving through space on similar trajectories, in what is known as a stellar association, moving cluster, or moving group. Several of the brightest stars in the 'Plough' of Ursa Major are former members of an open cluster which now form such an association, in this case, the Ursa Major moving group. Eventually their slightly different relative velocities will see them scattered throughout the galaxy. A larger cluster is then known as a stream, if we discover the similar velocities and ages of otherwise unrelated stars.>>

Man, I love this place! Thanks for all the great info/knowledge you guys are so generous to share with us.

If these open clusters tend to fall along our galaxy's edge, is there a way their location could be predicted as precession changes over time? In the link to this picture in Gemini there seems a number of named objects that fall along the ecliptic.

I wish I knew a manufacture that would make a model that shows all the various rotations/planes we experience as our planet/solar system/galaxy moves through the universe. I think that is one "big-picture" I would love to see as a display. Ron

Generally you "Connect the dots" to see things. Or your mind associates a "form" to see something. In this case, the vacant spots between the stars show things to my eyes. At the lower center, above the word CLUSTERS is a Heart shape in the black of space. In the top slightly upper right, above 2158, is an Octopus looking region. There is an outline of a Rose in the upper left. There is a "W" to the left of M35. If you look long enough you can start to see "words" and letters. In the area of the Heart, I see 6$68...****

****- Your eyes may vary, objects noticed are from a personal perspective only...void where prohibited.

A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters, which are found in the halo of a galaxy, contain considerably more stars and are much older than the less dense galactic, or open clusters, which are found in the disk.

This is essentially the difference between Globular and Open Clusters, even Compact Open Clusters. Where they are situated in, or around, the Galaxy. Globulars formed at around the same time as the galaxy. Open and other clusters formed later in star forming regions with more and heavier elements.

Population 2 and 1 stars. Globular Clusters, being out in the Halo region of a galaxy are Population 2 stars, and the galactic and inner galactic stars are heavier element Population 1 stars.

It is ODD TO ME that Population 2 stars still burn good out there. They have been around since the beginning of the galaxy and still shine on. But then the Sun has been going for 5 billion years, and has about 5 billion or more to go. But these are at and even beyond that point. Amazing they can do so with less heavy elements, but then maybe it is a gravity thing. But then again. The heavier elements are an ASH. And ash may glow like an ember, and be hot and smolder for along time, but not burn well. So, I guess it does make some sense.

SHINE ON STARS!!!!

:---[===] *

Ron-Astro Pharmacist wrote:If these open clusters tend to fall along our galaxy's edge, is there a way their location could be predicted as precession changes over time? In the link to this picture in Gemini there seems a number of named objects that fall along the ecliptic.

I wish I knew a manufacture that would make a model that shows all the various rotations/planes we experience as our planet/solar system/galaxy moves through the universe. I think that is one "big-picture" I would love to see as a display. Ron

Hi Ron. The best resource I have found for gaining a three-dimensional understanding of the sky and the cosmos is Guy Ottewell's Astronomical Companion, available from Ottewell's Universal Workshop.

The Astronomical Companion is a large-format folio-sized book that starts with side-by-side overviews of how things appear in the sky and how things "really" work. The book is organized around a series of illustrations of progressively larger spheres: the orbit of the Moon around the Earth, the Earth's orbit around the Sun, our solar system, the nearest stars, our corner of the galaxy, the whole galaxy, the galaxies in our local group, the nearest galaxy clusters, and on out to quasars and the edge of the known universe.

Ottewell is a brilliant polymath with a wry sense of humor and a very humane perspective. I start each morning by consulting his Astronomical Calendar with my morning tea, and I recommend his work to everyone interested in the sky and the universe.

Thanks - Finding my place in the universe has been quite a journey. I'm sure this will help Ron

Boomer12k wrote:
A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters, which are found in the halo of a galaxy, contain considerably more stars and are much older than the less dense galactic, or open clusters, which are found in the disk.

Indeed, globulars are halo objects, almost all of them 10-12 billion years old, and open clusters are in most cases found in the disk. Doesn't this suggest to you that the globular clusters formed before the Milky Way had a disk? Back when it was just a bunch of huge gas clouds smashing into one another?

According to this recent result from NASA, galaxies of almost all sizes have gradually grown more well-ordered over the last eight billion years. They have become ever less chaotic, ever more dominated by disks and ever faster-rotating.

But imagine way back when, when the Milky Way was only a proto-galaxy, completely chaotic. Enormous gas clouds smashed into one another and set off wildfires of incredible star formation, the likes of which have not been seen since the early days of the universe. Back then there were few stars and tremendous helpings of gas. So much of that gas has since become locked up inside small red dwarf stars, where it will stay at least until the universe has grown to several times its present age. Today, there are many stars and comparatively little gas. Much of the gas that still remains is funneled into the fast-rotating disk of disk galaxies, which is why so much of the star formation in the present-day universe happens in galactic disks. But the amount of gas available is only good for making modest open clusters.

Some time in the future, the well-ordered disk galaxies of the Milky Way and Andromeda are predicted to collide. That collision will destroy their disks and make their gas clouds collide and set off major bursts of star formation. Even so, that coming star formation will only be a pale reminder of those galactic wildfires that were set off ten to twelve billion years ago.

And when the well-ordered disks have been destroyed, and no more open clusters will form, the globulars will remain, swarming like bees around their elliptical galaxy "hive". The globular "insects" are sure buzzing in great numbers around giant elliptical galaxy M87.

Ann

How would the sky look like if our solar system were located in an open cluster (a big one)?

Ann,

I guess it would indeed show that Globulars formed before the main disk of a galaxy. The Blobs of gas formed into Population 2's and later in the disk Pop 2 and 1's, and later still Pop 1's. As the Mass increased, the galaxy settled down over time as more and more material was added. (second picture)

But doesn't this create a problem for incoming dust and gas, and other smaller galactic collisions? They have to "get passed" the globulars. Would they not tend to be captured by the globulars? (1st picture)

Thanks.

:---[===] *

Sinan İpek wrote:How would the sky look like if our solar system were located in an open cluster (a big one)?

Try this place Sinan....
http://astrobob.areavoices.com/2012/04/ ... r-cluster/

:---[===] *

Boomer12k wrote:

Sinan İpek wrote:How would the sky look like if our solar system were located in an open cluster (a big one)?

Try this place Sinan....
http://astrobob.areavoices.com/2012/04/ ... r-cluster/

:---[===] *

Oh, thanks! It boosted my imagination.

Boomer12k wrote:
But doesn't this create a problem for incoming dust and gas, and other smaller galactic collisions? They have to "get passed" the globulars. Would they not tend to be captured by the globulars?

That's an interesting problem. We have to consider the processes by which the Milky Way gains mass.

1) Collisions. They must have been important in the past. A major collision will create new globulars, if there is enough gas available.

2) Capturing and tearing apart passing dwarf galaxies. The link shows galaxy 5907 feeding on a small orbiting dwarf galaxy, which has been "stretched" into a long star stream. The stars will probably not be able to escape the gravitational grip of NGC 56907. The Milky Way is surrounded by several star streams from shredded dwarf galaxies.

3) Capturing gas clouds from intergalactic space. These gas clouds falling onto the Milky Way appear to be a reality, or at least two astronomers think so.

Personally I don't think that globular clusters will "block" or capture gas and dust falling onto the Milky Way. There are two reasons for my doubts. The globulars are tiny, and they contain very little mass compared with the Milky Way. Globular clusters are almost all stars. They contain no gas, very little dust, and little dark matter. The biggest globular, Omega Centauri, contains a few million stars, whereas the Milky Way is believed to contain at least 200 billion stars. So the Milky Way contains many thousands of times as many stars as the largest Milky Way globular cluster, and our galaxy also contains gas, dust and vast amounts of dark matter. Bear in mind, too, that while the radius of the Milky Way is about 100,000 light-years, the radius of Omega Centauri is less than a hundred light-years. Also there are probably no more than, say, 200 globulars in the Milky Way, leaving vast numbers of cubic light-years of Milky Way halo completely unaffected by globulars. The puny gravity, small sizes and small numbers of globular clusters in our galaxy are not likely to deter gas clouds from falling onto the Milky Way!

Ann

Ann wrote:1) Collisions. They must have been important in the past. A major collision will create new globulars, if there is enough gas available.

Why? I'm not aware of any theory of globular cluster formation that suggests galactic collisions can produce them. They seem tied to the initial formation of galaxies, and all later collisions should be able to do is disrupt them and hasten their evaporation.

Personally I don't think that globular clusters will "block" or capture gas and dust falling onto the Milky Way.

There is good reason to think that the process of globulars capturing gas from passing molecular clouds has lead to waves of star formation in those globulars, possibly explaining why some appear to show stars of different ages.

Chris Peterson wrote:

Ann wrote:1) Collisions. They must have been important in the past. A major collision will create new globulars, if there is enough gas available.

Why? I'm not aware of any theory of globular cluster formation that suggests galactic collisions can produce them. They seem tied to the initial formation of galaxies, and all later collisions should be able to do is disrupt them and hasten their evaporation.

I was thinking of collisions like the one between NGC 4038 and 4039, which has produced some impressive star clusters. There might also possibly be some new young "globulars", or at least "populous blue clusters", in Stephan's Quintet.

Chris Peterson wrote:

Personally I don't think that globular clusters will "block" or capture gas and dust falling onto the Milky Way.

There is good reason to think that the process of globulars capturing gas from passing molecular clouds has lead to waves of star formation in those globulars, possibly explaining why some appear to show stars of different ages.

That's a good point, Chris.

Ann

Ann wrote:I was thinking of collisions like the one between NGC 4038 and 4039, which has produced some impressive star clusters. There might also possibly be some new young "globulars", or at least "populous blue clusters", in Stephan's Quintet.

Well... as long as you place "globular" in quotes. But I think it's best to avoid that term completely. The clusters that can form when galaxies collide are usually called "super star clusters" and are quite different from globular clusters.

I couldn't resist posting Adam Block's wonderful new image of NGC 2158 here. Thanks to Adam's mastery of true-color photography, we can see that NGC 2158 is made up of rather bright red giants and unevolved hydrogen-fusing dwarfs, most of which are likely of spectral classes F and G. There are a few likely A-type stars there, too, which are bluish and moderately bright. However, the bright blue star seen in the lower part of the cluster is almost certainly a foreground object, quite likely a member of young cluster M35.

Ann

Anthony - Just got my Astronomical Companion. It will keep me busy for quite some time. Perfect reference for what I was looking for. A Great suggestion!! Thanks Ron