Galaxies

Galaxies are large systems of stars, gas, dust and dark matter that orbits a center and is bound by gravity. Learn about the characteristics of galaxies.

Scientists have known for some time that the Milky Way Galaxy is not alone in the Universe.

During the early 1900's, which is not very long ago, astronomers were unaware that there were other galaxies outside our own Milky Way Galaxy. When they saw a small fuzzy patch in the sky through their telescopes, they called it a nebula. When examined closely, some of the nebulae had a spiral shape. So astronomers at first called these "spiral nebulae". These nebulae were all believed to be part of our Galaxy, our community of stars.

Pictures of galaxies from the Hubble space craft.

Everyone has seen pictures of galaxies in their elementary school science books, but what you might not know is that not all galaxies are created equal. In fact, some of them are downright bizarre-looking. While the Milky Way may look pristine and almost flawless, the following galaxies are the poor, snaggle-toothed children of the cosmos.

There are about 50 galaxies in the Local Group (see list of nearest galaxies for a complete list), on the order of 100,000 in our Local Supercluster and an estimated number of about 100 to 200 billion in all of the observable universe.

You've probably heard that our Sun is located in the Milky Way galaxy. But what is a galaxy anyway?

Get information, facts, photos, news, videos, and more about galaxies from National Geographic.

A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek galaxias, literally "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few thousand stars to giants with one hundred trillion stars, each orbiting its galaxy's center of mass. Galaxies are categorized according to their visual morphology as elliptical, spiral and irregular.

Our galaxy, the Milky Way, is typical: it has hundreds of billions of stars, enough gas and dust to make billions more stars, and about six times as much dark matter as all the stars and gas put together. And it's all held together by gravity. Like more than two-thirds of the known galaxies, the Milky Way has a spiral shape. At the center of the spiral, a lot of energy and, occasionally, vivid flares. are being generated. Based on the immense gravity that would be required explain the movement of stars and the energy expelled, the astronomers conclude that at the center of the Milky Way is a supermassive black hole.