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

Big Bang (artist view)

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 Bang

distance is increasing between galaxies (redshift) presence of cosmic background radiation (big bang leftovers) Big Bang Evidence