The Astrophysics Spectator
The Astrophysics Spectator
When we look at the immense expanse outside our own Galaxy, we see numerous other galaxies. Like stars and planets, galaxies are objects that are held together by gravity, but while pressure counters gravity in stars and planets, angular momentum counters gravity in a galaxy.
Galaxies are generally divided into three types: elliptical galaxies, spiral galaxies, and irregular galaxies. Elliptical galaxies can range in shape from spheres to highly-elongated objects. Their three-dimensional structure may be more complex, but we cannot see this, because we only see the two-dimensional projection of a galaxy on the sky. Elliptical galaxies do not appear to have a single axis of rotation, but instead contain stars moving in all different directions. They are generally composed of old stars, and they are generally free of gas. Elliptical galaxies vary greatly in size, with the small elliptical galaxies much more common than the large elliptical galaxies. The small elliptical galaxies are called dwarf elliptical galaxies.
Spiral galaxies are what we generally think of when we hear the word galaxy. The gas and the brightest, and therefore youngest, stars in a spiral galaxy rotate in a disk around a definite center. The disk of a spiral galaxy therefore defines an axis of rotation for the galaxy. The gas and youngest stars in the disk of a spiral galaxy often lie on great, twisting arms. At the center of a spiral galaxy is a bulge of stars. Small spiral galaxies do not exist. Compared to the elliptical galaxies, the spiral galaxies have a narrow range of sizes, with a definite upper and lower bounds on their size.
The spiral galaxies are divided into two broad classes: normal and barred. In a barred spiral galaxy, a single straight cylinder of stars and gas is at the center of the galaxy. A spiral arm may twist away from each end of the cylinder, a ring of stars may enclose the bar, or some combination of ring and spiral arms may surround the bar. The normal spiral galaxy has sets of arms spiraling way from the center of the galaxy. There can be just two prominent arms twisting symmetrically about the galactic center, or there can be many ill-defined arms twisting around the center in a somewhat chaotic jumble of stars and gas.
Anything that does not fall into either the elliptical or the spiral categories is tossed into the irregular galaxy category. The galaxies in this category are not well-ordered, like the spiral and the elliptical galaxies. They are also smaller in size than the spiral galaxies. One feature, however, aligns them with the spiral galaxies: they have an abundance of gas and young stars.
Some galaxies produce tremendous amounts of radiation from their cores. These brilliant cores are called active galactic nuclei. Most of these objects are very distant, and the activity producing this radiation appears to be a stage in the early evolution of a galaxy. The theory is that a massive black hole at the center of the galaxy is releasing tremendous power as it pulls gas and stars onto itself. Besides being brilliant, active galactic nuclei can drive jets of material far into extragalactic space. These jets of matter, which move at close to the speed of light, are seen through the radio waves they radiate.
The three most prominent galaxies in our sky are visible to the naked eye: the Large and Small Magellanic Clouds of the Southern Hemisphere, and the Andromeda galaxy of the Northern Hemisphere. Our ability to see a galaxy depends on both its size and its distance. The Magellanic Clouds are very prominent in the sky because they are the closest galaxies to our own. The closer of the two, the Large Magellanic Cloud, extends about 8 degrees on the sky, while the more distant Small Magellanic Cloud extends about 3 degrees on the sky. These galaxies are small, irregular galaxies. In contrast, the Andromeda galaxy, which is also known as M31 and as NGC 224, is much small on the sky, less than three degrees in length, which is larger than the Moon or the Sun. It can only be seen when the sky is very clear, and generally only the inner bulge of the galaxy is visible. This galaxy is visible because it is a very large spiral galaxy, similar in structure to our own Galaxy. These three galaxies are members of the Local Group of galaxies, which also contains the Triangulum Galaxy, which is another large spiral galaxy, several large elliptical galaxies, and numerous dwarf elliptical and irregular galaxies.
