Galaxies NGC4013NGC4013, a large, nearby, edge-on spiral galaxy. Taken at the WIYN Telescope

Galaxies  Universe- all of space and everything in it  More than 100 billion galaxies  Galaxy- giant structure that contains hundreds of billions of stars, dust, and gas held together by gravity  Classified into three categories by shape

Galaxies  Galaxies range in diameter from a few thousand to a half-million light-years.  Small galaxies have fewer than a billion stars.  Large galaxies have more than a trillion.

1. Spiral Galaxies  Looks like a pinwheel  Most galaxies are spiral  Stars are forming  Has a center of bright stars stars  Has 2 or more spiral arms spiral arms  Largest

Milky way Galaxy  Our sun is 25,000 light years from the center of the galaxy  Can’t see the center because it is hidden by huge clouds of stars, gas, and dust Sagittarius and the Central Milky Way

Milky way

2.Elliptical Galaxies  Look like flattened balls  Billions of stars  Little gas and dust between stars  No new stars forming  Only old stars  Smaller than spiral galaxies

elliptical

3. Irregular Galaxies  Fewer stars  No definite shape  Smallest galaxies  Least common galaxies

Leo A: Nearby Dwarf Irregular Galaxy

Quasar An enormously bright, distant galaxy with a giant black hole at its center.

Identify the shape of each galaxy. Elliptical Spiral about 350 million light-years from Earth. Cataloged as AM , the pair is located in the southern constellation Doradofrom Earthas AM constellation Dorado

 Only three galaxies outside the Milky Way are visible with the unaided eye.  People in the Northern Hemisphere can see the Andromeda Galaxy, which is about 2 million light-years away.  People in the Southern Hemisphere can see the Large Magellanic Cloud, which is about 160,000 light-years from Earth, and the Small Magellanic Cloud, which is about 180,000 light-years away. Visible Galaxies

Andromeda Galaxy

Large Magellanic Cloud Small Magellanic Cloud

 All big bang theories of galaxy formation agree that no new galaxies -- or very few -- have formed since the earliest times.

Local Group  Small galaxy cluster- 30 galaxies  Caused by gravity  Most galaxies are orbiting the Milky Way or Andromeda galaxies  3 million light years across  Mostly spiral galaxies

Virgo Supercluster  Local Group, Virgo Cluster  Caused by gravity  200 million light years across