•Systems containing millions, or even billions, of stars

•Observable universe contains as many as 50 billion to 100 billion galaxies; millions of light years apart

•No two galaxies are alike, but they can be grouped

Galaxies

•Sun is one star of hundreds of billions; we are 26,000 light years from the center

•Spiral galaxy; thin disk with a central bulge

•diameter is about 100,000 light years

Our Galaxy: The Milky Way

•Part of Local Group: more than 30 galaxies

•Neighbors: 2 large Magellanic clouds (seen in southern hemisphere only) and Andromeda (seen in northern hemisphere)

Our Galaxy: The Milky Way

Spiral Galaxy

Elliptical Galaxy

Irregular Galaxy

• Galaxies that emit more energy than given off by their stars—lots of light

• Milky Way is not an active galaxy

• Powered by a supermassive black hole at its center

• Quasars: very distant objects that are highly luminous

Active Galaxies

Active Galaxy

http://www.spacetelescope.org/images/opo9814d/

Big Bang (artist view)http://nothingoutofnothing.files.wordpress.com/2010/05/bigbang.jpg

http://www.youtube.com/watch?v=gs-yWMuBNr4

• explains the history from a tiny fraction of a second after it came into being up to the present time

• 13.7 billion years ago the universe began: superhot & superdense

• expanded from a single point very rapidly and continues to expand today

The Big Bang Model

• became cooler as it expanded, allowing atoms to form

• atoms clumped together, mostly hydrogen

• hydrogen gathered into clouds, clouds gathered into stars, stars into galaxies over billions of years

The Big Bang Model

Big Banghttp://3.bp.blogspot.com/-PFXq_QZOZbE/T5GokOaONTI/AAAAAAAADMY/Kl32j-Jpcqs/s1600/Big+bang+expansion+diagram+2.PNG

• distance is increasing between galaxies (redshift)

• presence of cosmic background radiation (big bang leftovers)

Big Bang Evidence