Credit: M. Bessell
The description of the stellar evolution is based on calculations. However, no star completes its main life sequence or evolution into a red giant fast enough to be able to observe these structural changes as they occur. Fortunately, nature gives us an indirect way to test our calculations.
Instead of observing the evolution of a single star, we can observe a group or cluster of stars. We look for a group of stars that are very close together in space, held together by gravity, and often move around a common center. It can therefore be assumed that the individual stars in this group formed almost simultaneously, from same cloud & have the same composition. We expect these stars to differ only in mass, and their mass determines how fast they move in each stage of life.
Since stars with higher masses evolve faster, we can find star clusters in which massive stars have already completed their main sequence evolution phase and have become red giants, while stars with lower masses in the same star cluster are still in the main sequence, or even if the cluster is very-young, go through a gravitational contraction before the main sequence. We can see many stages of stellar evolution in members of a single group, and we can see if our models can explain why HR diagrams of different age groups look the way they do.
The three basic types of star clusters that astronomers have discovered are globular clusters, open star clusters, & stellar associations. Its properties are summarized in the table below. Globular clusters contain only very old stars, while open clusters and stellar associations. the associations contain young stars.
| Characteristic | Globular Clusters | Open Clusters | Associations |
| Number in the Galaxy | 150 | Thousands | Thousands |
| Location in the Galaxy | Halo and central bulge | Disk(and spiral arms) | Spiral arms |
| Diameter (in light years) | 50-450 | <30 | 100-500 |
| Mass M sun | 104-106 | 102-103 | 102-103 |
| Number of stars | 104-106 | 50-1000 | 102-104 |
| Color of brightest stars | Red | Red or blue | Blue |
| Luminosity of cluster (L sun) | 104-106 | 102-106 | 104-107 |
| Typical ages | Billions of years | A few hundred million years to, in the case of unusually large clusters, more than a billion years | Up to about 107 years |
Globular Clusters
Globular clusters were given this name because they are almost symmetrical round systems of typically hundreds of thousands of stars. The most massive globular cluster in our own galaxy is Omega Centauri, which is about 16,000 light years away and contains several million stars. Note that the brightest stars in this cluster, which are red giants that have already completed the main sequence phase of their evolution, are red orange in color. These stars have typical surface temperatures around 4000 K. As we shall see, globular clusters are among the oldest parts of our Milky Way.
credit : modification of work by NASA, ESA and the Hubble Heritage Team
(STScI/AURA
How about living in a globular cluster? In the dense central regions, the stars would be about a million times closer together than in our own neighborhood. If the earth orbited one of the inner stars in a globular cluster, the nearest stars would be light months away, not light years, they would still appear as points of light, but they would be brighter than any of the stars we see in our sky. The Milky Way would likely be difficult to see through the bright haze of starlight that the star cluster creates.
There are about 150 known globular clusters in our galaxy, most of them in a spherical halo (or cloud) that surrounds the flat disk formed by most of the stars in our galaxy. All globular clusters are very far from the Sun and some are 60,000 light years or more from the main disk of the Milky Way. Globular clusters range in diameter from 50 light years to more than 450 light years.
Open Clusters
Open clusters are found in the disk of the galaxy. They have different ages, some as old or even older than our Sun. The youngest open star clusters are still connected to the interstellar matter from which they were formed. Open star clusters are smaller than globular clusters usually have a diameter of less than 30 light years and usually contain only a few dozen to several hundred stars. Stars in open star clusters usually appear well separated from each other even in central regions, which is why they are also named “open” .Our galaxy contains thousands of open clusters, but we can only see a small fraction of them. Interstellar dust, which is also concentrated in the disk, weakens the light of more distant clusters so much that they can no longer be detected.
Although individual stars can survive billions of years in an open cluster, they typically only stay together as a cluster for a few million years or at most a few hundred million years. There are many reasons for that. In small open clusters, the average velocity of the member stars within the cluster can be greater than the escape velocity of the cluster, and the stars gradually “evaporate” from the cluster. Close encounters of member stars can also increase the speed of one of the members beyond the escape speed. Every few hundred million years, the cluster can get close to a giant molecular cloud, and the gravitational force exerted by the cloud can tear the cluster apart.
Multiple open clusters are visible to the naked eye. The most famous of these are the Pleiades, which appear as a small cluster of six stars (some people can even see more than six, and the Pleiades are sometimes called the Seven Sisters). This star cluster is arranged like a small spoon and can be seen in the constellation Taurus. Good-pairs of binoculars will show dozens of stars in clusters, and a telescope will show hundreds of stars.
The Hyades are another famous open star cluster in Taurus. To the naked eye, it appears as a collection of faint V-shaped stars that mark the face of the taurus. Telescopes show that Hyades actually contains more than 200 stars.
Stellar Associations
An association is a group of extremely young stars, typically containing 5-50 hot and bright O and B stars, scattered over a region of space about 100 to 500 light years in diameter. For example, most of the stars in the constellation Orion form one of the closest stellar associations. The associations also contain hundreds or thousands of low-mass stars, but they are much fainter & less noticeable. The presence of really hot glowing stars indicates that star formation in association has occurred over the past few million years or so. Since O stars spend their entire lives in just a million years, they would not continue to exist unless star formation recently occurred. Therefore, it is not surprising that associations are found in regions rich in gas and dust, which are necessary for the formation of new stars. It’s like a new building that is still surrounded by some of the building materials from which it was built and where the landscape still shows signs of construction. On the other hand, since associations such as ordinary open star clusters occur in regions occupied by dusty interstellar matter, many are hidden from our eyes..
