By Katherine Anderson, 8th Grade, 2nd Period

General Information

What Are Galaxies?

In short, a galaxy is a large collection of millions stars, gas, cosmic rays, dark matter, and dust. They can range in both size and and diameter, and while some contain over a trillion stars and range several hundred thousand light years across, others have only several million stars and are just a few thousand light years across. Galaxies are held together, as most everything is, by gravity. In most galaxies, stars orbit around the center of the galaxy. Most scientists think black holes are the cause of this massive gravitational pull on stars, and that they are at the center. Black holes are stars that, while dying, collapsed under their own mass into one almost infinitely dense point that nothing, not even light, can escape. Astronomers did not recognize galaxies as separate from the Milky Way until the early twentieth century. The Andromeda Galaxy and the Large and Small Magellanic Clouds are visible to the naked eye, but were not seen as different galaxies. After the invention of the telescope, astronomers were able to better see and understand galaxies and their components.

What Are They Made Of?

Galaxies are made of stars, gas, cosmic rays, dark matter, and dust. Also included is anything that revolves around the stars, such as planets. Stars are large balls of gas that burn hydrogen at their core, cosmic rays are electrons and nuclei of mainly hydrogen atoms that fly through the universe at nearly the speed of light, and dark matter is not well defined but is, in a general sense, something in space that has gravitational force but does not emit any detectable light.

Approximately How Many Do We Think Are In the Universe?

Astronomers guess that there are about one hundred of billion galaxies in the universe, based on what we can see with telescopes, but as technology advances we may be able to see even more.

Types of Galaxies

Our Galaxy

The Milky Way

Our galaxy, the Milky Way, is a spiral galaxy. The Milky Way is 100,000 light years in diameter, has around 300 billion stars in it, and is between 12,000,000,000 and 14,000,000,000 years old. The Milky Way is also part of a cluster of around 30 galaxies called the Local Group. Like most other galaxies, the major components of our galaxy include stars, gas, and dust.