The Milky Way Galaxy

Transcription

1 57 The Milky Way Galaxy The Milky Way appears as a diffuse band of light encircling the celestial sphere. Early attempts to probe the size of the galaxy were unsuccessful due to intervening dust. In the early 1900s, Harlow Shapley devised a method to determine the distance to the galactic center.

2 58 The Milky Way galaxy can be described by Disk: Halo: Nuclear Bulge:

3 59 Determining mass of Milky Way galaxy: Distance between Sun and galactic center 28,000 lt yrs 1.8x10 9 AUs V 220 km/s Period orbit 2.4 x 10 8 years Apply Newton s version of Kepler s third law: M inner galaxy + M ʘ = a 3 /p 2 a = distance to galactic center in AUs p = orbital period of Sun in years M inner galaxy M ʘ Assuming the average star has mass of 0.5 M ʘ, 200 billion stars in galaxy (interior to Sun s orbit) The outer disk of our galaxy rotates much faster than can be accounted for by the gravity of the visible mass. Dark Matter:

4 60 Galaxies Early astronomers often referred to these fuzzy patches of light as spiral nebulae. They were believed to be either small, nearby objects within the Milky Way (Shapley) or large objects like the MW that are very far away and outside our galaxy Island Universes (Curtis) Controversy reached its peak in 1920 with the Curtis- Shapley debate. Issues to be resolved: What are the distances to the spirals? Are spirals composed of stars or gas? Why do spirals avoid the plane of the MW?

5 : controversy finally resolved by Edwin Hubble Used Cepheid variable stars to determine the distance to the Great Spiral in Andromeda There are millions of galaxies seen in all directions, and density counts indicate there are billions in the observable universe. Clusters: just as stars form in groups, so do galaxies The Milky Way and Andromeda galaxies are part of a poor cluster called The Local Group

6 62 A typical rich cluster contains thousands of galaxies and is tens of millions of light years in diameter. Clusters are also members of even larger associations called Superclusters. Represent largest scale of structure in the universe Typically hundreds of millions of light years across containing million of galaxies Galaxies are not evenly distributed throughout the universe. Distribution best described as frothy with galaxies on the surfaces of enormous bubbles The velocities of galaxies within clusters often exceed the escape velocities calculated from the observed mass more evidence of dark matter

7 63 Cosmology Cosmology: the study of the origin, evolution, and ultimate fate of the universe. Cosmological principle: on a large scale, the universe should look essentially the same from any vantage point. No edge and no center! There are two testable predictions of the cosmological principle. The universe should be: 1) Homogeneous: at the largest scale, matter is spread uniformly throughout the universe. 2) Isotropic: In general, the universe looks the same in every direction (applies to expansion).

8 64 These two predictions (and the assumption that the universe was infinitely old) originally lead to a problem. Olber s Paradox: Why does the sky get dark at night? Resolution to Olber s paradox: universe is not static in time or space. The universe is expanding, which implies it had a beginning

9 65 Expansion of the universe: most profound cosmological discovery of the 20 th century actually predicted in Einstein s original equations for General Relativity Expansion discovered by Edwin Hubble in 1929 By measuring the redshift of a galaxy, can determine its radial velocity using the Doppler equation direct relationship between distance and velocity indicates expansion Hubble s Law: V = H D

10 66 H = V km/s km and Mpc are both units of distance D Mpc Can estimate the age of the universe from H Age 1 H (after canceling units of distance) Gives time elapsed since all galaxies were in the same place at the same time If H = 100 age of universe 10 billion years If H = 50 age of universe 20 billion years Recently, H determined to be 72 km/s/mpc, which yields an age of 14 billion years