The Milky Way Galaxy and Hubble’s Law
Messier Catalogue • Charles Messier made the first catalog of non-stellar “fuzzy” objects during the 1770's. • He made a list of 109 such objects that he did not want to mistake for comets. • These objects turned out to be the following: • galaxies • star clusters • nebulae
Questions • How many stars are in the Milky Way? • About 200 billion • How many galaxies are there? • billions and billions
Questions • How old is the Milky Way Galaxy? • It is 15 billion years old and will remain active for a another 10 billion years. • Recall: The Sun is 4.5 billion years old.
Are all stars members of the Milky Way? • No, most stars in the universe are in other galaxies.
Where in the Milky Way is our solar system located? • The solar system is located in a spiral arm about 28,000 light years from the center of the Milky Way.
Sketch of the Milky Way Galaxy • Disk • Bulge • Halo • Spiral Arms • Sun • Globular Clusters • Open Clusters and Nebulae
How do we know that we are located in the spiral arm? • In 1917 Harlow Shapley discovered that the globular clusters form a huge spherical system that is not centered on the Earth.
Finding Our Place Globular Clusters evenly distributed Early view Sun at center Sun Globular Clusters unevenly distributed Harlow Shapley
Harlow Shapley Hebert Curtis The Great Debate April 26, 1920 A galaxy is a nebula within the Milky Way! A galaxy is an island universes!
M51 M104 M81
Spiral Galaxies • galaxies like the Milky Way with arcing structures lying in a plane and emanating from the nuclear bulge
M32 - E2 NGC 4125 M87 - E1
Elliptical Galaxies • galaxies with an elliptical shape, no spiral arms, and little interstellar matter
NGC 1097 M91 NGC 4123 NGC 4477
Barred Spiral Galaxies • galaxies with a bar of stars running through the nuclear bulge
Centaurus A M87
Irregular Galaxies • galaxies that are asymmetrical and are sometimes just two or more galaxies colliding
Galactic Collisions Simulation
Galaxy Observations • During the 1920's Edwin Hubble and Milton Humason photographed the spectra of many galaxies with the 100 inch telescope at Mount Wilson. • They found that most of the spectra contained absorption lines with a large redshift.
Red Shift and Distance 24 Mpc 1200 km/s 300 Mpc 15,000 km/s 780 Mpc 39,000 km/s 1220 Mpc 61,000 km/s
Galaxy Observations • Using the Doppler effect, Hubble calculated the velocity at which each galaxy is receding from us. • Using the period and brightness of Cepheid variables in distant galaxies, Hubble estimated to distances to each of the galaxies.
Hubble’s Law • Hubble noticed that there was a linear relationship between the recessional velocity and the distance to the galaxies. • This relationship is know as Hubble’s Law: V = H D recessional velocity = Hubble’s Constant Distance
Hubble’s Law • H is known as the Hubble constant and is about 75km/s/Mpc. • This means that a galaxy that is 1 megaparsec from Earth will be moving away from us at a speed of 75km/s.