Globular clusters are interesting. Surprisingly interesting, actually. Globular clusters are dense clumps of stars, usually hundreds of thousands of stars--much more than a typical open cluster. Also, obviously, they are packed into a tight glob. That is how you visually tell an open cluster from a globular cluster. (Duh:-) But the most interesting thing about them is where they are found.
But first, this is a picture of the globular cluster Messier 5:
|Messier 5 / 20" refractor (Rachael) / Stephen Williams|
This picture was taken through the 20" refractor (Rachael) at the Chabot Space and Science Center. As usual, I used a Nikon D80 camera set up at the prime focus. Whereas the planetary pictures I've posted in the past are assembled from dozens of frames stacked together, this image is actually only 2 30second exposures, stacked to get the equivalent of 60seconds of exposure time. I actually took dozens of shots that night, then culled them down to just the two best images, and stacked them.
So the interesting thing about these globular clusters is as much where they are as what they are. All the globular clusters are in the halo that surrounds galaxies. Spiral galaxies like our own Milky Way are surrounded by an extended, faint halo made up mostly of these globular clusters. All of the globular clusters are in the halo, none are found in the main body of the host galaxy. And these are a common feature of large galaxies as we can see these globular clusters in the halos around other galaxies as well.
Another interesting thing is the age of these clusters, or more specifically the age of the stars in the clusters. The stars in a globular are mostly ancient--as old as the host galaxy itself. New stars are not being born in these clusters since all the free gas has been sucked up into the existing stars. Furthermore, the stars in a globular cluster like M5 are mostly "Population II" stars, meaning they contain very little of the elements heavier then helium, which suggests that the stars are made from primitive material of the universe, and not from material recycled from other stars. The globular clusters are therefore probably formed along with the galaxy, are likely a consequence of the process that form galaxies in general, and are not typically sharing material with the host galaxy. In particular, the Milky Way has a much higher proportion of heavier elements (i.e. carbon, silicon, oxygen, etc.) so while the material within the Milky Way is being recycled and remade into new stars as the older ones die, The globular clusters have made their stars and are not recycling them.
These globular clusters are like fossils of the ancient universe, from a time when our own galaxy was busy forming. This is pretty interesting.