GALAXIES A Place where stars hang out

Galaxy A large collection of stars held together by gravity Contain Dust Gas Astronomers believe that there are about 50 billion (50,000,000,000) in the universe Each galaxy contains billions/trillions of stars Galaxy

http://www.nasa.gov/mission_pages/hubble/science/xdf.html

Spiral galaxies # shaped like pin wheel 2nd most common type of galaxy #Centeral bulge (nucleus) that stars revolve around filled with old stars and little material for stars to form # Arms that extend from central area filled with gasses and dust for stars to form It is now believed that in the center of all spiral galaxies is a Super Massive Black Hole Spiral galaxies

Spiral galaxies

Pin wheel Galaxy

Sombrero galaxy - spiral

Elliptical galaxies smooth and featureless with round or oval shapes Stars seem to move around a central location very little gas and dust Filled with older stars Most galaxies in the universe are elliptical Elliptical galaxies http://hubblesite.org/videos/video_details/9-active-galaxy-hercules-a-visible-radio

Courtesy Hubble web site

Irregular galaxies Do not have a uniform (no regular shape) appearance Filled with a lot of gas and dust Lots or star formation Young hot stars

Two Irregular companions: The Large and Small Magellinic Clouds

Evolution of galaxies Galaxies change over time 1. Irregular galaxies are believed to be forming Filled with huge amounts of gas and dust and new young stars 2. Spiral Galaxies are believed to have formed from Irregulars Area in center has little gas and dust a lot of older stars arms are filled with regions of gas and dust and new stars forming 3. Elliptical galaxies seem to be the oldest Filled with old stars and little evidence that any new stars can form the gases and dust have been used up http://safeshare.tv/w/NvbWCaAPPW

Colliding galaxies http://hubblesite.org/videos/video_details/17-galaxy-collisions-simulation-vs-observations Small galaxy believed to be colliding with the milky way http://www.daviddarling.info/encyclopedia/S/SagDEG.html http://hubblesite.org/videos/video_details/17-galaxy-collisions-simulation-vs-observations

Milky Way Galaxy: our home 100,000 light years across 15,000 light years thick Named by Greeks – whitish strips looked like split milk On a very clear dark night it looks like a wide hazy strip stretching across the sky. This is a side way look at another spiral arm stretching thousands of light years across the sky

Milky way facts Home to about 400 billion stars including our Sun and solar system The Milky Way does not sit still, but is constantly rotating. The Sun and the solar system moves with the arms as they rotate. The solar system travels at an average speed of 515,000 miles per hour It will take our Solar System about 230 million years to travel all the way around the Milky Way.