Chapter 16The Milky Way Galaxy

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

What are some of the major features of our galaxy?DiskNuclear BulgeHaloSpiral ArmsSunGlobular ClustersOpen Clusters and Nebula

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.

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 PlaceSunEarly viewSun at centerHarlow ShapleyGlobular Clusters evenly distributedGlobular Clusters unevenly distributed

Zone of Avoidancea band running around the sky in which few galaxies are visiblecaused by dust within the Milky Way Galaxy

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

16.2.1 Rotation and Mass DistributionTo the chalk board...

Chapter 17Normal Galaxies

The Great DebateApril 26, 1920A galaxy is anisland universes!A galaxy is a nebula with the Milky Way!

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

Elliptical Galaxiesgalaxies with an elliptical shape, no spiral arms, and little interstellar matter

Barred Spiral Galaxiesgalaxies with a bar of stars running through the nuclear bulge

Hubble Classification of GalaxiesEllipticalsFrom E0 (round) to E7 (oblate)SpiralsSO have nuclei but no spiral armsFrom Sa (large nuclei and tight arms) to Sc (small nuclei and the most open arms)Barred SpiralsFrom SBa to SBc

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

Galactic Collisions

Chapter 18Clusters of Galaxies

Edwin Hubble

Galaxy ObservationsDuring 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 Distance24 Mpc1200 km/s300 Mpc15,000 km/s780 Mpc39,000 km/s1220 Mpc61,000 km/s

Galaxy ObservationsUsing 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.

Hubbles LawHubble noticed that there was a linear relationship between the recessional velocity and the distance to the galaxies.

This relationship is know as Hubbles Law:v = Ho drecessional velocity = Hubbles Constant Distance

Hubbles LawHo 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.

Edwin Hubble

CosmologyThe study of the origin, structure and evolution of the universe.

What does the Hubble law tells us about our universe?Our universe is expanding.

Raisin Cake ModelLike raisins in rising raisin cake, galaxies move awayaway from each other in our expanding universe.

CosmologyHubble TimeThe age of the universe if the expansion has been constant.t = 1/Ho = ?The expanding universe probably originated in an explosion called the Big Bang between 12 and 18 billion years ago.

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

Chapter 19Active Galaxies and Quasars

Structure of Our UniverseUniverse - all spaceGalaxies and QuasarsVirgo Cluster of GalaxiesMilky Way GalaxyLocal Star ClusterOur Solar SystemThe Earth