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Unformatted text preview: Astronomy 101 - Test 4 Review THE MILKY WAY The Milky Way is a galaxy, which can be defined as a system of 10 7 to 10 13 stars, interstellar gas and dust, and “dark matter” bound together by the combined gravity of these. In the Milky Way’s case, the number of stars is roughly 400 billion. The main structural components of the Milky Way are the disk, bulge and halo. The disk is where we live, and contains most of the stars of the Milky Way, and the vast majority of the interstellar gas and dust. It is about 30,000 pc (or 30 kpc) in diameter (this is about 100,000 light years, so light takes 100,000 years or so to cross the Milky Way – galaxies are big). The stars in the disk are both young and old – some are forming right now out of interstellar gas clouds. Almost all star formation in the Milky Way is in the disk. The disk also has a spiral structure. This is hard to perceive from within the disk but is discerned because we can measure distances to objects in the disk and see that they are concentrated in arms. The Sun is not on one of the main arms but on a small spur, called the Orion spur, coming off one of the arms. A side view of the disk would show dust obscuring the central parts of the stellar layer, because most dust (and gas) is in a thinner layer than the stars. The halo is a spherical shape component that surrounds the disk. It is at least 30,000 pc across. It contains the globular clusters (about 150 in all), each of which contains up to 1 million stars. There are also stars between the globular clusters in the halo. Halo stars are very old and probably the first ever to form in the Milky Way. The bulge is a spherical component at the center of the disk, about 1 kpc across. It also contains mostly old stars. There is a “supermassive” black hole at the center of the galaxy of about three million times the Sun’s mass, which is a site of unusual activity. Shapley in 1917 was the first to make a good estimate of the size of the Milky Way. He noticed that there were more globular clusters in one direction in the sky than in the opposite direction. By estimating distances to some Horizontal Branch stars in them, he knew they were far away (many kpc), and he guessed that they formed a system centered on the center of the Milky Way. From all this, he was able to infer a distance of the Sun from the center of about 16 kpc. The modern value is 8 kpc. The orbits of stars in the halo and bulge are generally very elongated and random in orientation, like bees in a hive. The disk rotates, but not like a rigid object. The Sun’s speed around the center is about 225 km/sec, which means it takes 240 million years to go around once (its orbital period)....
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- Fall '09
- The Milky Way