Chapter 16 The Milky Way Galaxy

16.1 Overview n How many stars are in the Milky Way? – About 200 billion n How many galaxies are there? – billions and billions n How old is the Milky Way Galaxy? – It is 15 billion years old and will remain active for a another 10 billion years. n Are all stars members of the Milky Way Galaxy?

What are some of the major features of our galaxy? n Disk n Nuclear Bulge n Halo n Spiral Arms n Sun n Globular Clusters n Open Clusters and Nebula

Where in the Milky Way is our solar system located? n The solar system is located in a spiral arm about 28,000 light years from the center of the Milky Way.

How do we know that we are located in the spiral arm? n In 1917 Harlow Shapley discovered that the globular clusters form a huge spherical system that is not centered on the Earth.

Finding Our Place Sun Early view Sun at center Harlow Shapley Globular Clusters evenly distributed Globular Clusters unevenly distributed

Zone of Avoidance n a band running around the sky in which few galaxies are visible n caused by dust within the Milky Way Galaxy

16.2 Differential Galactic Rotation n Material closer to the galactic center orbits with a shorter period than the material farther from the galactic center.

Rotation and Mass Distribution n To the chalk board...

Chapter 17 Normal Galaxies

The Great Debate Harlow Shapley Heber Curtis April 26, 1920 A galaxy is an island universes! A galaxy is a nebula with the Milky Way!

M104 M81 M51

Spiral Galaxies n galaxies like the Milky Way with arcing structures lying in a plane and emanating from the nuclear bulge

M32 - E2 M87 - E1 NGC 4125

Elliptical Galaxies n galaxies with an elliptical shape, no spiral arms, and little interstellar matter

NGC 1097 NGC 4477 M91 NGC 4123

Barred Spiral Galaxies n galaxies with a bar of stars running through the nuclear bulge

Hubble Classification of Galaxies n Ellipticals – From E0 (round) to E7 (oblate) n Spirals – SO have nuclei but no spiral arms – From Sa (large nuclei and tight arms) to Sc (small nuclei and the most open arms) n Barred Spirals – From SBa to SBc

M87 Centaurus A

Irregular Galaxies n galaxies that are asymmetrical and are sometimes just two or more galaxies colliding

Galactic Collisions

Chapter 18 Clusters of Galaxies

Edwin Hubble

Galaxy Observations n During the 1920's Edwin Hubble and Milton Humason photographed the spectra of many galaxies with the 100 inch telescope at Mount Wilson. n They found that most of the spectra contained absorption lines with a large redshift.

Red Shift and Distance 24 Mpc1200 km/s 300 Mpc15,000 km/s 780 Mpc39,000 km/s 1220 Mpc61,000 km/s

Galaxy Observations n Using the Doppler effect, Hubble calculated the velocity at which each galaxy is receding from us. n Using the period and brightness of Cepheid variables in distant galaxies, Hubble estimated to distances to each of the galaxies.

Hubble’s Law n Hubble noticed that there was a linear relationship between the recessional velocity and the distance to the galaxies. n This relationship is know as Hubble’s Law: v = H o d recessional velocity = Hubble’s Constant  Distance

Hubble’s Law n H o is known as the Hubble constant and is about 75km/s/Mpc. n This means that a galaxy that is 1 megaparsec from Earth will be moving away from us at a speed of 75km/s.

Edwin Hubble

Cosmology n The study of the origin, structure and evolution of the universe. n What does the Hubble law tells us about our universe? Our universe is expanding.

Raisin Cake Model Like raisins in rising raisin cake, galaxies move away away from each other in our expanding universe.

Cosmology n Hubble Time n The age of the universe if the expansion has been constant. n t = 1/H o = ? n The expanding universe probably originated in an explosion called the Big Bang between 12 and 18 billion years ago.

Cosmology n Will the universe end? – Present observations suggest that it will expand forever. n What caused the Big Bang? n Where did the energy come from? n Why did it happen? Unknown…

Chapter 19 Active Galaxies and Quasars

Structure of Our Universe n Universe - all space – Galaxies and Quasars n Virgo Cluster of Galaxies n Milky Way Galaxy n Local Star Cluster n Our Solar System n The Earth