Introductory Astronomy: The Disk

The disk component includes all of the material (gas, dust, stars, and star clusters) which are confined to the plane of rotation of the galaxy. The diameter of the disk is about 30,000 parsecs. It has a thickness of about 600 parsecs for the cold components (like neutral gas) and about 2000 parsecs for the hot components (like ionized gas). (The difference arises because the cold components have less energy and therefore don't move as far or as fast as the hot components.) The Sun is located at about 8,500 parsecs from the center, roughly in the middle of the disk.

The disk includes the spiral arms , which are long patterns of bright stars, star clusters, gas, and dust in the shape of arms. It also contains two kinds of star clusters: open clusters, and associations.

Open clusters are collections of stars which are held together by their own gravity. They are not as dense as Globular clusters; they typically contain about 100 - 1000 stars in a region approximately 3-30 parsecs in diameter. They survive about a few billion years and are concentrated along the band of the Milky Way. A well known example of an open cluster is the Pleiades, shown right.

Associations are groups of 10 - 100 stars which are so widely scattered in space that their mutual gravity cannot hold them together. We find them clustered together only because they all formed from the same cloud and have not had time to go far from their birthplace. Associations thus contain very young stars (O and B stars). They are located along the spiral arms (sites of star formation) and are short-lived.

Disk stars are Population I stars. This means that they have circular orbits in the plane of the galaxy and are relatively young stars. They are generally metal rich.

The disk contains lots of gas and dust and is therefore the site of active star formation. Star formation occurs in the spiral arms of the galaxy. It can be initiated by a density wave or by a self-sustaining process. Self-sustaining star formation occurs when the birth or death of a massive star compresses neighboring clouds of gas. For example, the outward rush of heated gas produced when a massive star becomes radiative (when it is born) or the expansion of a supernova remnant (when it dies) could compress the surrounding gas. This compression would trigger star formation in the neighboring regions. If these neighboring regions then produce massive stars, the cycle will continue. This means that once star formation begins, it tends to continue on its own. If the massive stars that begin the cycle are already in the spiral arms, this process could sustain the spiral arm pattern. If the massive stars are located randomly, the process will not produce a spiral pattern. Astronomers believe that, in many galaxies, star formation is a combination of spiral density waves and self-sustaining star formation.