MadSci Network: Astronomy |
The Milky Way is divided into three distinct components based on age and a couple of other characteristics. Our sun is part of the disk of the galaxy. This region is moderately dense, both in terms of number of stars and number of diffuse gas clouds. Disk stars are characterized by their youth and by their predictable rotation about the galactic center: they all orbit in the same direction as the sun at about the same rate. The disk is sometimes divided into the "thin disk," containing very young stars and star-forming regions, and the "thick disk," whose stars aren't quite so young. The disk has a diameter of about 150,000 light years and a height of about 4,000 light years. The second region is the bulge. This is a more or less peanut-shaped region in the center of the galaxy. It has the highest star density of all the regions, and it contains young stars as well. It is, however, quite small, with a diameter of only 6,000 light years.
The third region is known as the halo: it is a large, spherical, very diffuse group of stars surrounding the disk. The halo has by far the smallest density of stars and has virtually no gas clouds. Its stars are also the oldest of all. They also do not necessarily orbit the galactic center in the direction of the disk's rotation. Some orbit vertically, and some even orbit backwards. The halo has a diameter of about 300,000 light years. You can think of the galaxy as a globe. The equator has most of the stars in the disk, the bulge is in the center, and the halo stars take up the rest of the globe, although they are very spread out.
Figuring out the age of each region isn't easy. One good method is to look at the spectrum of a star. The spectrum is the amount of light at each frequency. You can see the spectrum of a light by looking at it through a prism; the prism separates the light into its component frequencies. It turns out that each chemical element emits light at certain very well-specified frequencies. So by looking at the spectrum, you can determine which elements are in the star.
Why does that help us find out a star's age? It turns out that the Big Bang created only hydrogen and helium. All other elements were formed in early generations of stars; when they died, they expelled those elements into the universe. Later generations of stars incorporated these "polluted" regions of gas when they formed. So, the more heavy elements (like carbon, calcium, or oxygen) a star has, the younger it is. By making these sorts of studies, astronomers have determined the age of the components.
How these separate components form is another interesting question. In the beginning, the galaxy was just a giant ball of rotating hydrogen and helium gas. As it rotated, the sphere of gas flattened out because gas molecules banged into each other and lost angular momentum. Meanwhile, some of the gas condensed into stars. These early stars will not be affected by collisions with the diffuse gas, so they remain in the halo (which is why we see old stars there). But all the gas flattens out into the disk. Because stars form out of the gas, subsequent generations of stars will all form in the disk or in the bulge. The bulge forms as the disk continues to rotate and lose angular momentum.
There are two basic types of galaxies in the universe. Most galaxies (about 60% of them) are like our own: spiral galaxies. These all have the same general form as the Milky Way, although the sizes of the bulges vary widely. Other galaxies are known as ellipticals, because they lack disks and all their stars are distributed in a football or spherical shape. Ellipticals tend to be quite large; current theories predict that ellipticals form when two or more spiral galaxies collide and merge. About 30% of galaxies are elliptical. The rest of the galaxies are "irregular," with no coherent structure. The closest galaxies to us (the Magellanic clouds) are of this type.
You can find more information on all of these issues in any introductory astronomy textbook
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