What is a Galaxy?

by Fraser Cain on May 6, 2009

Hickson Galaxy Group.

Hickson Galaxy Group.


You’ve probably heard that our Sun is located in the Milky Way galaxy. But what is a galaxy anyway? The simple answer is that a galaxy is a collection of stars held together by their mutual galaxy. In other words, all the stars in a galaxy are kept together by the gravity of all the other stars (as well as the invisible, mysterious dark matter).

We know the Milky Way pretty well, so let’s consider it as a good example of a galaxy. The Milky Way is a spiral galaxy. It has a bright central core with a high density of stars, and then a flattened disk surrounding it – like a spinning record. Two spiral arms start just outside the core, and then spiral outward like a pinwheel to the outer edges of the galaxy. The Milky Way measures about 100,000 light-years across, and is thought to contain 200-400 billion stars.

But the stars we can see are just a tiny fraction of the complete galaxy. It’s also surrounded by a vast halo of dark matter. This material is invisible, and doesn’t interact with regular matter or give off any kind of radiation that we can detect. But astronomers can measure its effects because it does exert a gravitational force on other matter. In fact, the Milky Way is made up of mostly dark matter. The stars account for about 580 billion solar masses, and the dark matter could be another 6 trillion solar masses.

Our Milky Way is just an example of a galaxy, though. There is another type of galaxy called elliptical, and they’re even more common. The smallest galaxies in the Universe, the ultra-compact dwarf galaxies are only a little larger than a globular star cluster. But then the largest galaxies in the Universe also have this elliptical (egg-like) shape. A good example is the galaxy M87. It’s thought to have 2.7 trillion stars.

Stars are collected together into galaxies. Galaxies are collected together into groups of galaxies, and these groups are collected into clusters. The largest structures in the Universe are galaxy superclusters, which contain millions of galaxies and can measure hundreds of millions of light-years across.

We have written many articles about galaxies for Universe Today. Here’s an article about new research about the Milky Way.

If you’d like more info on galaxies, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.

We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.

References:
NASA Imagine the Universe
NOAA.org
SEDS.org

About 

Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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