Globular Clusters
Published on Dec 10, 2005 at 7:51 pm.
1 Comment.
Filed under galaxies, star clusters, stars.
Over at Tom’s Astronomy Blog he has a posting about an image that he took of the globular cluster M71. It is actually a very good image. One of my favorite globular clusters is M13, probably because it was the first one that I saw. Globular clusters are really very interesting objects. They are often really not as appreciated as they should be as astrophysical objects. In amateur telescopes, they seldom appear as more than fuzzy balls to the eye. A camera (either film or CCD) will show some of the stars. Even so, you don’t get the real picture.
They are roughly spherical balls of stars, often with hundreds of thousands of stars, bound gravitationally to one another. Globular clusters are generally much farther away than open clusters, such as the Pleiades, also known as M45 (I was originally going to explain all this M-stuff, but I changed my mind, so that I will save for a later blog). M13 is located about 25,000 lightyears distant, as opposed to only 440 lightyears for the Pleiades. This is because globular clusters lie outside the plane of the galaxy’s disk. Visually, our galaxy looks like a giant pinwheel. The space between the spiral arms, though, is filled with material, so the galaxy is really more like a fat pancake. Most everything that you see in the sky with your naked eye is in that pancake. Globular clusters orbit around the pancake in a vast swarm. They are part of what we refer to as the halo of the galaxy. In fact, this fact allowed Harlow Shapley in the early part of the 20th Century to determine that the Sun is not located in the center of the galaxy, but is rather thousands of lightyears from the center! A little under 200 globular clusters are now known around our galaxy. We have found them around other galaxies, too. A few of the Andromeda Galaxy’s globulars are even within the grasp of many amateur’s telescopes.
Globular clusters are generally very ancient objects. In fact, globular clusters were among the first objects to form in the galaxy. Most globulars formed as the galaxy formed, it seemed. There was a second period of globular cluster formation a few billion years later, for reasons that are not clear — perhaps a collision or near collision of the Milky Way with another galaxy? Interestingly, one of the satellite galaxies of our Milky Way Galaxy, the Large Magellanic Cloud, has globular clusters that are much younger than our own.
Given that globular clusters are relics of the days of the formation of our galaxy, the study of globulars can lead to an interesting insight into the early Milky Way. Which came first, the oldest globulars or the Milky Way? What are they doing in a big swarm around the disk of the galaxy?
How do we know that the globular clusters are so old? One way is to study the types of stars in the globular clusters. The high mass stars don’t live long. The lower the mass, the longer the star lives (another blog topic here!). We can look at the highest mass stars in a star cluster, and this gives us a minimum age for the star cluster. Of course, we are assuming that the stars formed with a distribution of masses at the same time. There is strong evidence to suggest that this is what did happen. Another clue is the metallicity of the stars. Astronomers consider anything other than hydrogen and helium to be a “metal.” Following the Big Bang, there were almost no other types of atoms but hydrogen and helium. All of the metals are formed in the nuclear fusions that occur in the cores of stars, or through the nuclear reactions that occur during a supernova explosion (which is how really massive stars die). Thus, the earliest stars would not contain metals. The later a star is formed, then the more metal enriched the gas that formed it would be. Globular clusters are very metal poor, suggesting great age. This is not as ironclad an argument as the stellar mass data that I mentioned, since there are few other stars in the halo to create metals. Presumably, a cloud of gas left over from the formation of the galaxy might remain fairly metal poor in the halo for much longer than a cloud of gas in the disk of the galaxy. Still, most globulars are very poor in metals because they are very old.
A rather startling finding, at least for me, was that one of the extrasolar planets that we’ve recently found is located in the globular cluster M4. What makes this remarkable is that the low metallicity of stars in globular clusters means that there was very little of the heavier elements to act as seeds for planets. One of the theories of planetary formation requires seeds of heavier elements in order to form gas giants. Apparently this is not how the planet in M4 formed. Until this planet was found, I had felt that the only stars that would have planets would be younger stars that had more metals. Now, the M4 planet is NOT a rocky planet like our Earth. It is a gas giant, similar to Jupiter. Except that it can’t be just like Jupiter, because part of what makes Jupiter the way that it is is it’s rocky core, which is over ten times the mass of our Earth. So, this suggests that there may be more than one mechanism by which planets form.
So, that is probably more than you wanted to know about globular clusters, but I find them interesting.
-Astroprof
jay on September 2, 2009 at 10:34 pm: 1
trying to find out name of the Globular Star Cluster seen in the last week of August 2009, visible quite close to Moon on lower right perhaps within one Moon diameter as seen from Maine USA around August 25th. Using a freeware astronomy program I was able to identify the object as a Globular Star Cluster but have not found the specific name of the cluster yet. If you can tell me this, I would appreciate it. A kindergarten kid saw the cluster and asked his dad what it was. I was quite amazed when I looked thru my beginner’s telescope and could tell it was unusual, then was even more amazed to find out that the boy had seen a Globular Cluster without any binoculars or telescope and knew it was unusual! Thank you.