A galaxy is a giant, spinning object made of gas, dust, and stars held together by gravity. Galaxies come in different shapes and sizes and can contain billions of stars. Most occur in groups that are also held together by gravity. We now know that the Universe has billions of galaxies, and the Milky Way is only one of them.

Shapes: spiral, elliptical, and irregular
Number: about 100 billion
Composition: a huge collection of stars, gas,
and dust held together by the force of gravity
Breakthrough: astronomer Edwin Hubble in the 1920s determined that our galaxy was only one
of many in the Universe
Significance: the basic building blocks in the Universe

Image credits: main image:The Southern Pinwheel Galaxy from VLT, FORS Team 8.2-meter VLT ESO Hubble Deep Field ,R. Williams and the HDF Team (STScI) and NASA M31: The Andromeda Galaxy, © and Courtesy of Jason Ware; Charles Liu: courtesy of AMNH.