Our Galaxy. Milky Way Galaxy = Sun + ~100 billion other stars + gas and dust. Held together by gravity! The Milky Way with the Naked Eye

Transcription

1 Our Galaxy Milky Way Galaxy = Sun + ~100 billion other stars + gas and dust Held together by gravity! The Milky Way with the Naked Eye We get a special view of our own galaxy because we are part of it! The Milky Way with a Telescope What can we deduce about the shape of our galaxy? 1

2 It appears as a thin band! It must have the shape of a flattened disk! When we look in the plane of the disk we see more stars than out of the plane! The Milky Way completely encircles the sky! It is brighter in some directions than others We must be inside the disk! 2

3 The Shape of the Milky Way We are not at the center of the disk! NGC 4565 In the disk of our galaxy we find open star clusters A Globular Cluster in the halo of our galaxy Stars in the halo have weaker lines due to heavy elements than stars in the disk! Two stars of identical temperature and luminosity should have the same spectra! 3

4 Why? They are older! They were the first stars to form in the Milky Way when heavy elements were less abundant! Stellar Populations Population I Young, heavy element-rich stars in the disk Population II Old, heavy element-poor stars in the halo Halo stars are used to date the age of the galaxy: 13.6 billion years The Universe is 13.7 billion years old so the Milky Way formed only a 100 million years after the Universe! Formation of the Milky Way The Milky Way looks like a giant protoplanetary disk! The Milky Way formed from a giant cloud of protogalactic hydrogen and helium gas It must have formed in a similar manner to the solar system! 4

5 Halo and bulge stars start to form as the protogalactic cloud starts to collapse Conservation of angular momentum causes remaining gas to flatten into a spinning disk Star formation continues in the disk forming Population I stars while old Population II stars remain in the halo Halo and bulge stars retain the random motion of the original cloud while the disk stars move about the center on a flattened plane Problem: Dense gas and dust prevents us from seeing very far into the disk at visible wavelengths The Internal Structure of the Milky Way 5

6 21 cm Radiation The Alignment of Magnets Radio radiation from hydrogen atoms Clouds of hydrogen gas are Doppler shifted by their motion in the galaxy The Spiral Structure of the Milky Way From another perspective Spiral arms are sites of current star formation 6

7 Spiral Density Wave Theory A traffic jam on a freeway A galactic traffic jam! Galactic Rotation Rotation of the Milky Way Galactic Rotation Curve Plot of orbital speed vs. distance from center Stars rotate about the center of the Milky way 7

8 Expected Actual 8

9 Conclusion? There must be a source of gravity beyond the disk keeping the orbital speeds high! Problem: Nothing is seen beyond the disk! What is it? Emits little or no radiation DARK Exerts gravity MATTER The Milky Way s Dark Matter Halo 90% of the mass of the galaxy is dark matter! 9

10 There appear to be two components Massive Compact Halo Objects (MACHO s) Baryonic dark matter made of ordinary atoms, probably in the form of dim, low mass stars and brown dwarfs MACHO s can be detected by microlensing Weakly Interacting Massive Particles (WIMP s) Non-baryonic dark matter made of exotic particles like neutrinos, or perhaps some which are currently unknown Only account for 50% of dark matter halo VIRGOHI21 a dark galaxy? The Galactic Center 10

11 Studied at infrared, radio and X-ray wavelengths due to the blocking of visible radiation by interstellar matter At the center of our galaxy is a very intense radio source Stars are extremely crowded at the center Stars near the radio source are traveling very rapidly One star comes to within 120 AU! Analysis of their motions indicates the mass of the central object to be 4-5 million solar masses! It must be very small! 11

12 What can be so massive but so small? A supermassive black hole! 12