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085

Galaxies

OLogy Series
astronomy
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085

Galaxies

OLogy Series
astronomy

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.

The Great Debate
In 1920, two prominent astronomers were invited by the National Academy of Sciences to discuss the size and shape of the Universe. One of them, Harlow Shapley, said that the entire Universe was contained within our galaxy, the Milky Way. The other, Heber Curtis, argued that the Milky Way was just one "island universe" among many. Three years later, using the largest telescope in the world at California's Mt. Wilson observatory, Edwin Hubble settled the matter. He showed that a fuzzy object in the sky was too far away to be part of our own galaxy and must be another, separate galaxy. This was the Andromeda galaxy, one of our closest galactic neighbors. Once the existence of another galaxy outside our own was proven, we had to accept that our galaxy was not the only one
-- we were just one of many that make up the Universe.

Ninety percent of the mass of a galaxy is made up of:

stars

dark matter

gas

Are you right?

Correct!

This mysterious stuff is called "dark matter." The stars and gas we see make up less than 10 percent of the mass of a galaxy. We know something else is there because we can observe the effects of its gravity, but we can't see what it is.

Charles Liu, astrophysicist

Galaxies are to the Universe as cells are to the body. Just as we study cells to understand humans, we study galaxies to understand the Universe.

Large galaxies grow by colliding and merging with smaller ones.

Fact
or
Fiction
?

Fact

Large galaxies, especially elliptical ones, probably grow by "eating" smaller galaxies. They are called cannibal galaxies.

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: 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.