Chapter 14: The Milky Way Galaxy

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 really see What we think we live in

Chapter 14: The Milky Way Galaxy Examples of different spiral galaxies seen from different orientations. Satellite Galaxy

Chapter 14: The Milky Way Galaxy 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

Chapter 14: The Milky Way Galaxy Because it is difficult to accurately determine the spectral type of a star and its absolute luminosity we can’t use 1/d 2 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/d 2 law to get their distance.

Chapter 14: The Milky Way Galaxy RR Lyrae and Cepheid variables have now increased our ability to measure distance to galactic sizes.

Chapter 14: The Milky Way Galaxy 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.

Chapter 14: The Milky Way Galaxy What we think our galaxy looks like from above How we see our galaxy in the infrared

Chapter 14: The Milky Way Galaxy 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.

Chapter 14: The Milky Way Galaxy