A galaxy is a group of stars in the universe.
Stars are not scattered about haphazardly in space. They gather together into great spinning star islands, or galaxies. All the stars we see in the sky belong to to our home galaxies, which we call the Milky Way, or just the Galaxy. It is one of perhaps 100 billions of galaxies in the Universe. Each one contains billions of stars. Most galaxies lead relatively peaceful lives, giving out a steady output of light. Some, however, are noticeably more active. They pour out up to a million times more energy than normal, particularly as radio waves.
There are some galaxies that do not fit neatly into any class. Many are spiral galaxies with unusual features. Some may be colliding or have collided at some time in the past. For example, astronomers reckon that the Cartwheel galaxy was a spiral whose center was knocked out in a collision hundreds of millions of years ago.
Spirals, elliptical and irregularsEdit
Galaxies are enormous families of stars, which lie scattered across the never-ending space of the universe. Each galaxy contains many millions of stars—a mixture of giant and dwarfs stars, old and young stars, and clusters of stars. Some galaxies are spiral in shape, while other are elliptical (like a flattened circle). Those that do not seem to have much of a shape at all are called irregular galaxies. There are countless numbers of galaxies, and they are grouped together in clusters. Our solar system, for example, is part of the Milky Way Galaxy. This belongs to a collection of galaxies called the Local Group, which contains about 25 galaxies, such as the Large and Small Magellanic Clouds. The Andromeda Galaxy, the largest member of our cluster, is so huge we can see it in a very dark sky without a telescope. It lies more than 2 million light years away from Earth. Light reaching us now from the Andromeda Galaxy began its journey across space long ago when the earliest humans lived on Earth.
- Spiral: The arms of a normal spiral galaxy are filled with stars and gas clouds. Spiral galaxies have a central bulge or nucleus, from which a number of arms curve out. They are classed as a, b or c depending on how far open the arms happen to be.
- Barred spiral: A barred spiral galaxy has a bar of stars across its center. The spiral arms begin at the ends of the bar. Barred-spiral galaxies, on the other hand have spiral arms that come out of the ends of a line of stars (bar) through the nucleus. They are classed as a, b or c.
- Elliptical galaxy: Giant elliptical galaxies are massive. This galaxy has five trillion stars. Elliptical galaxies are described by a number from 0 to 7 which indicates how flattened they are.
- Irregular galaxy: Irregular galaxies have random shapes and they are smaller than the Milky Way.
The most intriguing of all the active galaxy-type objects in the Universe are quasars. Quasars are extraordinary powerful beacons, scattered deep in the universe. The word "quasar" stands for "quasi-stellar" (resembling a star), but quasars have far more energy than stars. The first two to be discovered were not given names, but coded as 3C-48 (in 1960) and 3C-273 (in 1962). They were strong radio sources whose positions in the sky matched those of two faint blue stars. The spectra of the stars, however, were quite unlike any seen before.
Astronomers now know the reason for this. The lines in the spectrum of these star-like objects are shifted to the red by an enormous amount. In other words the sources must be very far away. 3C-273 proves to be more than 2,000 million light-years away. No star can be seen from this distance. Therefore it cannot be an ordinary star, even though it looks like one. And for it to be visible from such a distance, it must be hundreds of times brighter than an ordinary galaxy.
Astronomers have since discovered more than 1,500 other quasars, or quasi-stellar radio sources.
Quasars do not shine steadily like ordinary galaxies. They vary in brightness over periods of days or years. For this reason they cannot possibly be as big as an ordinary galaxy. For if a quasar changes in brightness in a year, say, it cannot be more than one light-year across. And if it changes in brightness in a day, it cannot be more than a light-day across. From its variation in brightness, 3C-273 works out to be less than one-hundredth of a light-year across, which makes it only one ten-millionth the size of a typical galaxy.
However, it seems that quasars are not separate bodies. They appear to be eruptions at the center of massive galaxies are too faint to be visible at the distances involved. Astronomers believe that a quasar is a black hole at the center of a distant galaxy, which consumes all the matter around it. The whirling matter being sucked into the hole creates an amazing source of energy and powerful "jets" of material (top), which are projected out of the galaxy's glowing core.