20.2 The Milky Way and Other Galaxies I can: -explain that gravity is the force that binds together dust and gas to form stars, star systems, and star clusters -recognize that there are billions of galaxies in the Universe and they can be classified by size and shape. -classify the Milky Way as a spiral galaxies. -describe the Milky Way galaxy as having more than 100 billion stars and a diameter of more than 100 thousand light years -recognize that at the center of the Milky Way  there is a bulge stars, from which are spiral arms of gas, dust and most of the young stars. -recognize that the solar system is part of the Milky Way galaxy. K -demonstrate that Hubble’s law that galaxies that are farther away have a greater red shift, so the speed at which a galaxy is moving away is proportional to its distance from the Earth. S -explain that the phenomenon due to Doppler shifting, so the shift of light from a galaxy to the red end of the spectrum indicates that the galaxy and the observer are moving farther away from one another

Galaxies Collection of stars, dust, and gas bound together by gravity. Held together by gravity Cluster: a group of stars or galaxies bound together by gravity Milky Way and Andromeda belong to LOCAL CLUSTER. Superclusters Can be 100 million light years across.

Types of Galaxies

The Milky Way is a spiral galaxy Interstellar matter: the gas and dust located between the stars in a galaxy.