A galaxy is a system of stars, clouds of gas, and dust particles that move together through the universe. There are billions of galaxies in the universe. All of them probably formed about 15 billion years ago, soon after the universe began. Astronomers first recognized the existence of galaxies in the 1900s.Galaxies are divided into three general categories based on their shape. They are described as spiral, elliptical, and irregular. Spiral galaxies look like pinwheels. They have a center consisting of many stars and two or more curved arms winding outward. The arms of some spiral galaxies began as straight bars that appear to pass through the galaxy's central region. These are called barred spiral galaxies.
In elliptical galaxies the stars are spread out smoothly in an oval pattern, though they are packed more densely in the center than on the edges. Some elliptical galaxies are nearly round like balls, while others are more stretched out. Elliptical galaxies range from giants to dwarfs.A small percentage of galaxies are called irregular galaxies because they are neither spiral nor elliptical. Many have no obvious center point. Some are enormous and appear to be the result of a collision between several galaxies. Even small galaxies are made up of hundreds of millions of stars and may be 5,000 light-years across. (A light-year is the distance light travels in a year—about 5.8 trillion miles, or 9.5 trillion kilometers.) The largest have more than a trillion stars, and may stretch out over a distance of more than 500,000 light-years. Certain galaxies called giant radio galaxies send out radio waves from an even larger area.Since ancient times people have noticed a milky band of light in the night sky. From this formation of far-away stars came the word galaxy, which is taken from the word for milk in the Greek language. The Milky Way is the galaxy to which the Earth belongs.The Milky Way is a spiral galaxy with a diameter of about 100,000 light-years. It includes many billions of stars. Most of the stars of the Milky Way are in the center or the flat spiral arms, but some occur in clusters above and below these areas.The Earth and the rest of the solar system are located on one of the spiral arms, about two-thirds of the way from the center of the galaxy. As the Earth travels around the sun, the entire solar system is traveling around the galaxy. It takes about 200 million years to complete one rotation around the center of the galaxy. At the center of the Milky Way is probably a massive black hole.Some galaxies that are very distant from the Earth have small yet spectacularly bright centers. These centers are quasi-stellar radio sources, or quasars. They consist of a combination of hot gases and dust that gives off energy. A quasar as small as the Earth's solar system can be as bright as 100 entire galaxies. At the center of other galaxies are black holes, objects so dense that not even light can escape their gravity.Galaxies usually move through the universe as part of groups, or clusters. The Milky Way is at one end of a cluster called the Local Group. It includes more than 20 galaxies, most of which are much smaller than the Milky Way. The closest large galaxy within the Local Group is the Andromeda galaxy, a spiral galaxy that is large enough to be seen from the Earth without a telescope. It is located about 2 million light-years from the Earth.
