aesmael

    But since I'm not, why not elaborate? The two deep sky objects I saw Monday night, the Jewel Box and Omega Centauri, are both star clusters. This means that unlike, say, most of the stars in a constellation or a random telescope field, these stars are actually associated with each other and formed together, from the same nebula. The Jewel Box is an example of an open cluster, which are typically found in star-forming regions of space (such as the spiral arms of galaxies like our own) and normally contain a few dozen to a few hundred stars. They do not tend to last long on the cosmic scale, falling apart as the stars gradually seperate in their orbits about the galactic centre until they no longer appear to have anything to do with one anothe, or else grow old and die. This is what is happening to the Jewel Box; it received its name due to the spectacular contrast between its hot, blue main sequence members and the cooler redder stars which have entered their giant phase. One way to tell the age of a cluster is to identify the most massive members which still reside on the main sequence. So there you have it, dear reader: the Jewel Box owes its fame and beauty to fact it is dying.
    Omega Centauri is a globular cluster, the other kind. Those contain anything from thousands to millions of stars and they tend to be, as the name suggests, shaped like an enormous blob of stars. They get incredibly dense toward their centres with stars believed seperated by less than a light year on average (by contrast, the normal seperation in our neighbourhood is more like four light years) and their orbits tend to be highly elliptical and may have little to do with the plane of their host galaxy (yes, I am speaking about spirals - we humans can be parochial like that(. I tend to think of them as miniature satellite galaxies.
    Unlike open clusters, globulars are not normally formed in the present universe. Their stars are old, some of the oldest in the universe. The only stars in most globular clusters still on the main sequence are significantly less massive than our sun. If the solar system had been formed in a globular cluster the Sun would be dead and the Earth ashes by now - except an earthlike world would probably not have formed in an environment like that because the stuff to make it from did not exist back then. There is at least one way new globular clusters can be made, though. When galaxies collide so do their giant molecular clouds, triggering an intense burst of star formation and you can get the thousands-to-millions of stars condensing from the interstellar medium all at once that are needed to make a globular cluster. Huge doses of ionising radiation being tossed about too, and with increased star formation comes a heightened rate of supernovae. These are the dangers that make galactic mergers so hazardous to the health, the actual odds of two stars colliding are infinitesimal (though methinks 'twould be fun to watch from a safe distance). We may get to see this closeup for ourselves in two billion years, but probably not from Earth.
    Back to Omega Cen, I think one of the more massive of the Milky Way's globulars. It bears the designation 'Omega' because to the unaided eye and at small magnifications it appears to be a fuzzy star, and so it was miss taken. With a bigger scope, like the one I was using, it looks more like a sea or cloud of stars, a swarm filling that space. The gaps between the stars are occupied by other stars. Although it appears somewhat larger in the eyepiece than the Jewel Box, I would guess Omega Centauri is ten times as distant so it is actually much bigger.

    In fact, I went to fetch some facts and some illustrations and became briefly confused. I mistook the actual Jewel Box for a different but very spectacular cluster (NGC 290) described the same way but more than ten times as distant as Omega Centauri. it actually lies in our neighbouring galaxy the Small Magellanic Cloud, 200,000 light years away. Omega Centauri is still more than twice its size at 150 light years across and contains many, many more stars even though NGC 290 is quite large for an Open Cluster.
    If you wish to see what I am talking about, have a look at the Jewel Box and Omega Centauri for yourself. These pictures are of course of much higher quality than what I saw; the human eye has only a short exposure time compared to what can be done with a camera. And since I am feeling generous, here is the cluster that confused me: NGC 290.

Peace, Tricia.