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Getting to grips with Globulars


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Hi all,

I am fast developing a fascination for globular clusters; partly because they are such impressive sights; even thru my 4" scope in a light-polluted back garden.

I hear there are about 150 of them swarming around our galaxy like bees around a hive. What is it about globulars? Why do they circle our galaxy? How did they form? What is holding them together ... and and am I the only one fascinated by them!?

Thanks,

Steve :)

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Globular clusters are, indeed, fascinating objects - and usually very rewarding to look at through most decent-sized telescopes.

I'm no expert on globulars, but I know that they are quite unlike open clusters. Whereas an open cluster may contain a few hundred stars, globulars tend to contain thousands or even millions of stars. I assume the main thing holding them together is gravity.

Globulars are also very different insofar as they contain very old stars. Open clusters tend to be young and, like M45, are often still associated with nebulosity. I've yet to hear of a globular with any nebulosity associated with it.

I can't really comment on how they were formed. I'm sure that a more learned person on this forum like Astroman will have something positive to contribute if/when he catches up with this thread.

As you say, Steve, there are only about 150+ (I believe) globulars known in our entire galaxy. They are nowhere as plentiful as open clusters. However there are some good examples of globulars visible to us in the northern hemisphere. These include M13 & M92 in Hercules, M15 in Pegasus, M3 in Canes Venatici, M22 in Sagittarius (good southern horizon required here), M80 & M4 in Scorpius (also good southern horizons required) & M5 in Serpens.

Yes, globulars are a very rewarding target for amateur scopes. Interesting thread! :)

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This is the first thing that I've found - doesn't answer your question but backs up what Andy said - I'll keep looking...

Globular clusters are gravitationally bound concentrations of approximately ten thousand to one million stars. They populate the halo or bulge of the Milky Way and other galaxies with a significant concentration toward the Galactic Center. Spectroscopic study of globular clusters shows that they are much lower in heavy element abundance than stars such as the Sun that form in the disks of galaxies. Thus, globular clusters are believed to be very old and formed from an earlier generation of stars (Population II). More recent estimates yield an age of 12 to 20 billion years; the best value for observation is perhaps 14 to 16 billion (see e.g. the discussion at M92). As their age is crucial as a lower limit for the age of our universe, it was subject to vivid and continuous discussion since decades. The age of globular clusters is determined by investigating their H-R diagrams, as discussed in our globular cluster page.

The disk stars, by contrast, have evolved through many cycles of starbirth and supernovae, which enrich the heavy element concentration in star-forming clouds and may also trigger their collapse.

Our galaxy has about 200 globular clusters, most in highly eccentric orbits that take them far outside the Milky Way. Most other galaxies have globular cluster systems as well, in some cases (e.g., for M87) containing several thousands of globulars!

Open (or galactic) clusters are physically related groups of stars held together by mutual gravitational attraction. They are believed to originate from large cosmic gas/dust clouds in the Milky Way, and to continue to orbit the galaxy through the disk. In many clouds visible as diffuse nebulae star formation takes still place at this moment, so that we can observe the formation of new young open star clusters (composed of young Population I stars). Open clusters populate about the same regions of the Milky Way and other galaxies as diffuse nebulae, notably spiral arms in disk galaxies, and irregular galaxies, and are thus found along the band of the Milky Way in the sky.

Most open clusters have only a short life as stellar swarms. As they drift along their orbits, some of their members escape the cluster, due to velocity changes in mutual closer encounters, tidal forces in the galactic gravitational field, and encounters with field stars and interstellar clouds crossing their way. An average open cluster has spread most of its member stars along its path after several 100 million years; only few of them have an age counted by billions of years. The escaped individual stars continue to orbit the Galaxy on their own as field stars: All field stars in our and the external galaxies are thought to have their origin in clusters.

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Globulars are the best! They formed along with our galaxy and are about the same age. If you visualize the formation of a whirlpool, or the currents in a river, the main vortex would be the Milky Way, while the smaller eddy's would be the globulars. As the whole system formed, the globulars took a more polar orbit about the center of the MW. Along their journey, they pass repeatedly through the disk stars which, depending on their proximity to the center, distrupt them in different ways over the eons. Some have their central stars compressed, causing them to age faster or go supernova. Some have their cores dispersed, making them appear more like open clusters. There's a lot more, but I don't have much time at the moment.

One globular you'll have to check out is M53 in Coma Berenices. It's pretty easy to find at the opposite corner of the Coma triangle. It's a great one.

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I'll second that Astroman, Globs are my favourite objects, they are amazing to observe and knowing what they are gives you a "WOW!" feeling.

Plus the fact you can be a tight Bottom with your EPs (like me!) with Globs as long as you have the aperture you don't need the FOV!

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Yeah, I heard that one too.  It's a possible explanation for globulars that have odd orbits around the core.  Most are highly elliptical and more or less centered.  Some, the exception to the rule if you like, are excessively oblate or otherwise nonconformist.  Also, overall compositions may differ from those known to belong to the MW.  From an orbital standpoint, it may make sense.  From a density standpoint, it's more problematical.  There are several known "dwarf" galaxies that were consumed by the MW.  Their trajectories through the MW indicate their foreign origin.  Most are diffuse, unlike a globular, and a complete orbit has not been determined, unlike the rogue globulars in the theory.

I guess what I'm saying is, the consumption theory holds for a couple to a few globulars, but not enough to warrant full explanation.  (This could be said about many, many theories though, so don't take it too hard.:) )

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Thats one that I hadn't heard off.

So would you get one globular per dwarf galaxy? I know that GC's are comprised of older stars but if the age of these stars are fairly uniform this would imply that the MW was extremely hungry over a short period of time.

Ant

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One globular per dwarf?  That's an interesting question.  It depends on the motions within the dwarf at "contact" with the MW.  The problem with calculating it is the two would be tidally affected long before stuff would start to break up.  The core of the dwarf is the key, since anything in a disk would be stripped pretty quickly.

That's not much of an answer I know, but truthfully, I'd sort of dismissed the theory and hadn't thought about it much. >shrugs<

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It depends on what's happening IN the dwarf as it is consumed. The core could remain intact if dense enough. There could be other globulars generated IF there were dense enough areas within the dwarf, but that's a big if. As I said, it'd be tough to figure or calculate because of all the variables-mass, velocity, density, trajectory.

The problem I have with this theory is it doesn't seem to take into account the very strong gravitational effects of a large galaxy upon a smaller one. Most of the stars would be stripped by tides long before the dwarf was assimillated. This effect is seen countless times in colliding galaxies, (Google "Arp Interacting Galaxy Catalog"). It seems too much like a lucky shot for a small galaxy to survive intact enough to produce a globular. Of course, if it happens, it must be possible, (Dr. Chip Megan's second law).

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  • 1 year later...

I know this post is old, but I couldn't help bringing up something that I'd read this evening and wanted to check if it was correct. Is it true that globulars present more detail through the eyepiece than as an image? The explanation that I read was that photographs make the cores of GC's appear as quite brilliant and this makes the stars at their centre combine together into a bright mass, whereas the naked eye sees the stars as individual points of light and can therefore pick out lots more individual stars. I ask as I'm someone who wants eventually to do imaging but realises that means a big investment in time as well as cash. I was therefore pleased to hear that there might be some objects that are better viewed (i.e. more detail can be seen) though a telescope than as an image.

Thanks for any replies.

Cheers, Martin

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I think it's a matter of opinion, but my opinion is, it's true. Newbies may not think much of GC's, since they tend to be very far away and difficult to resolve at a glance. When I show them to the public, I go through a regular routine to describe what they're seeing. Of course, some globs are easier than others, but the schpiel goes like this,

"The cluster is centered in the EP. Take your time. See the fuzziness? That's close to X-hundred thousand stars. Start at the edges. See the individual stars there? Now, keep looking, keep looking, keep looking. Begin to move the eye closer to the middle. Keep looking, keep looking, keep looking. See the stars begin to resolve? Good. Move toward the center a little more, not a lot. Keep looking, keep looking, keep looking. Almost to the core? Good. Keep looking, keep looking, keep looking. Now move to the core and concentrate, but don't strain. You should see individual stars there, too. Notice the 3d effect? Some stars are definitely closer to us, some are behind and overlapping. Now, back off your view a little. Take in the whole cluster. Maybe now, you'll believe there are X-hundred thousand stars in there!"

I do this for about every other person or group. Make them slow down and observe, not just look. Dark skies and good seeing help a lot, but some clusters look great in smaller scopes. M13, M92, in Hercules, M53 in Coma Berenices, M5 in Pegasus. Try this method yourself. (Read slowly, and keep looking....)

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I agree, the views of some GCs in my 14" Dob are easily as good as the best images. I can't really say that about any other group of objects.

GCs are my favourite objects and, as Astroman says, knowing what they are completely blows me away when I'm observing them.

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Fascinating stuff.

I'd guessed that, with all those stars so very close together, the whole glob. would be swirling like a cup of freshly stirred coffee as they all orbit rapidly round each other to prevent the glob. from collapsing. So I had a Google, read this http://relativity.livingreviews.org/open?pubNo=lrr-2002-2&page=articlesu10.html

and now I'm going to go and soak my head. There didn't used to be smoke coming out of my ears! The orbit periods appear to be in the 100,000 year area if I understood the maths, so no thrashing about to speak of.

Loads of stuff to learn about Globs. out there on the wibbly wobbly web.

To get back to the notion of looking versus imaging, I'd go for ease of viewing. Imaging them is dead easy, as they have a high surface brightness, but imaging them well enough to resolve the core and retain the peripheral stars is challenging to say the least.

Captain Chaos

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