We have already discussed binary systems, systems of two stars in orbit around a common center. But what about systems with more than two stars? Groups of stars, generally 10 or more, are referred to as clusters and sometimes as associations. Let us turn our attention to these systems.

Associations are groups of stars boasting somewhere between 10 and 100 members. The association, as a group, is generally short-lived. This is because Kepler's laws dictate that the stars closer to the center of the Galaxy will orbit with a somewhat shorter period than stars farther from the center. Therefore the location of the stars cause them to eventually separate. (Think of 2 runners on a track. They start off together, but the faster inside runner will eventually leave the slower outside runner behind.) Because associations are short-lived, the stars within them formed relatively quickly. Therefore the stars are high mass stars and generally very bright. Associations are therefore easily distinguished when viewed in distant galaxies.

Groups of stars in excess of 100 in number are referred to as clusters. Clusters come in two types. The first type is an open (or galactic) cluster. Like associations, open clusters are relatively short-lived. This is true for the same reason as it was true for associations. The gravitational forces of the galaxy are eventually stronger than the gravitational attraction of the cluster members. It is important to understand however, that the cluster is short-lived, the members are not. So even after the members of the cluster are no longer grouped together, they still exist as stars.

Open clusters have the following characteristics: they contain hundreds to thousands of members, they contain few red giants and oftentimes many protostars, the stars are loosely gathered with little to no structure, the members generally show a high percentage of metals in their spectra, they are generally found within the disk of our Galaxy.

M7: A Open Cluster

The second type of cluster is a globular cluster. Globular clusters have a much higher stellar population. The member stars may number in excess of 100,000. This large population gives the cluster a stronger gravitational attraction to each other, therefore, the group remains intact for billions of years. This will yield the following characteristics: they contain thousands of members, they contain many red giants and no protostars, the stars are clustered in a spherical shape with a heavy concentration at the center, the members are very old, forming before the galaxy contained very many metals, they are generally found all around the galaxy including outside the disk in a formation referred to as the halo of the galaxy.

M4: A Globular Cluster

The HR diagrams for these types of clusters helped to distinguish and characterize the stars within. By determining the location of the oldest main sequence star astronomers are able to determine the age of the cluster. The idea here is that all the stars began formation at around the same time and therefore all have the same age. The highest mass star on the main sequence is ready to cease hydrogen burning. By determining that star's age we can therefore determine the age of the entire cluster. This star is at a location on the H-R diagram referred as the turnoff point. With that in mind astronomers have determined the age of most globular clusters to be in excess of 10 billion years. This makes globular clusters some of the oldest objects in the universe.

The next step in groupings of stars is the galaxy. But we should first focus on our own galaxy. Our galaxy is referred to as the Milky Way. This is because it can be seen stretched across the sky on a clear dark evening looking like milk spilled across the curtain of darkness.

7,000 Stars And The Milky Way
Credit: Knut Lundmark (Copyright: Lund Observatory)

When attempts were first made to map our position within the Milky Way it was concluded that we were in the center of this large disk-like grouping of stars. Astronomers came to this conclusion because the density of stars was the same no matter which direction we looked through the disk of the Milky Way. It turns out that this is not in fact the case. The error in our conclusion was due to dust and gas obscuring our vision beyond a certain distance within the galaxy. This dust and gas acted much like fog, not allowing us to see the entire galaxy. By observing the distribution of globular clusters above and below the disk of the Milky Way astronomers were able to determine the overall size of the Galaxy and our position within it.

The overall shape of our galaxy can be modeled as a disk roughly 100,000 light years in diameter. The central portion of this disk contains a large nuclear bulge about 10,000 light years in diameter. The concentration of stars in the nuclear bulge is very high and the bulge resembles a large globular cluster in shape and characteristics. The disk of the galaxy is roughly 2,000 light years in thickness elsewhere. Surrounding the disk of the galaxy is the collection of globular clusters referred to as the halo. The halo may extend more than 200,000 light years in diameter.

It has been determined that the Milky Way is probably a spiral galaxy (possibly a barred spiral). The terms spiral comes from the structural distribution of the stars within the disk. The stars are grouped at higher densities in arms curving out from the center to give a spiral shape. A complete mapping of these arms is not possible in the visible portion of the spectrum. It was only possible by detecting the distribution of hydrogen gas in the radio portion of the spectrum.

The radio signal generated by neutral hydrogen gas has a characteristic wavelength of 21 cm. It is therefore referred to as 21 cm radiation. This radiation comes about when the electron orbiting the nucleus alters its spin from what physicists call spin up to spin down. When this transition takes place energy is given off in the form of radiation with this characteristic wavelength of 21 cm. These radio waves can penetrate the gas and dust that hinders observations in the visible portion of the spectrum. Radio waves are therefore critical in determining the overall structure of the entire galactic disk.

It is legitimate to ask, "How do the spiral arms hold their shape since stars near the center of the galaxy orbit in a shorter period than stars near the outside of the galaxy?" The answer to this question lies with a concept referred to as a "density wave." A density wave is created when the stars compress to smaller separations and then expand to larger separations. These compressions and expansions (often called rarefactions) travel around the disk of the galaxy as spiral arms. The waves travel with a velocity different than the stars within the wave.

Another example of a density wave is a sound wave. When sound travels, the gas molecules in the air compress together and expand apart. The sound waves travel through the air. The air molecules are not traveling with the wave. Consider the molecules vibrating in your throat when talking. Those are not the same molecules that vibrate your ear drum. Similarly, the stars in the wave now will not be in the wave later.

This page was last updated on 06/28/01 .