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A light-year is 9.5 trillion kilometers and is the distance light travels in one year. Galaxies range from a few thousand to a million light-years in diameter.

A galaxy is a cluster of stars, dust, and gas which is held together by gravity. Galaxies are scattered throughout the universe and they vary greatly in size. A galaxy may be alone or it may be in a large group of galaxies called a "supercluster". Galaxies are classified by scientists according to their shape and appearance. An irregular galaxy has an undefined shape and is full of young stars, dust, and gas. A spiral galaxy is shaped like a disk. The disk tends to resemble a pinwheel with arms which spiral outward as it rotates. Spiral galaxies tend to contain more middle-aged stars along with clouds of gas and dust. The next galaxy classification is an elliptical galaxy. The elliptical galaxies contain older stars and very little gas and dust. Elliptical galaxies vary in their shape from round to flattened, elongated spheres.

M100, a typical spiral galaxy
By studying the Doppler shift of different galaxies, scientists have concluded that all of the galaxies are moving away from each other. Galaxies that are the farthest from Earth appear to be traveling the fastest (relative to the Earth) of any galaxies.

A starburst galaxy has an exceptionally high star birth rate. The Hubble Space Telescope's high resolution has allowed astronomers to see dense star clusters, dust lanes with tiny regions of dense gas, and filaments of glowing gas in the core of a starburst galaxy.

A Question

What evidence do scientists have that galaxies are continuing to move away from each other?

Boy holding satellite

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The Answer
The Answer


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The StarChild site is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA/GSFC.

StarChild Authors: The StarChild Team
StarChild Graphics & Music: Acknowledgments
StarChild Project Leader: Dr. Laura A. Whitlock
Responsible NASA Official: Phil Newman

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