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Even our unaided eyes tell us that we live in some kind of disk structure. We see the Milky Way in the summer time as we look toward the center (white arrow). We see a thinner Milky Way in the winter time as we look opposite the center (blue arrow) and we see fewer stars in the fall and spring as we look out of the disk (red arrows). What we think we live in What we really see
Goals • Describe the overall structure of the Milky Way • Explain how we know its shape and size • Describe the different parts of the Milky Way • Describe the evidence for “dark’ matter • Describe the evidence for a massive black hole in the center
Examples of different spiral galaxies seen from different orientations.
While we can deduce the overall shape of our galaxy based on visible star counts, it is difficult to be specific because many of the stars are obscured by gas and dust. For example, the actual center of our galaxy is not the brightest region in the sky because of this fact. William Herschel’s 18th century “map” of the galaxy
Because it is difficult to accurately determine the spectral type of a star and its absolute luminosity we can’t use 1/d2 law very accurately. A class of stars called RR Lyrae and Cepheid variables have the unique property of having periodic pulsations which are related to their absolute brightness. Thus we can tell their absolute brightness by simply measuring their periods. Then we can use 1/d2 law to get their distance.
RR Lyrae and Cepheid variables have now increased our ability to measure distance to galactic sizes.
With variables stars as a new measuring technique, astronomers could map out their three dimensional distribution. Since many variables stars are found in globular clusters, we map their distribution and found they map out a spherical distribution nearly 30 kpc across. The center was offset from our sun by 8 kpc which shows the distance the Sun is from the galactic center.
Graphic showing parts of the Milky Way. How we see our galaxy in the infrared
Close to the Sun the motions of stars appear random. On larger scales, the stars in the disk revolve around the center differentially while the Halo is still random. It takes the Sun 225 million years to revolve once. Thus, it has been around the galaxy only 20 times.
