Radio telescopes can be used to study the universe and celestial objects.
A galaxy is a cluster of stars, nebulae, dark matter, and other astronomical objects. Most galaxies are tens of thousands of light years across and contain billions of stars. Galaxies have three primary forms; spiral galaxies are thin disks with spiral arms around a central hub; elliptical galaxies are uniform, oval-shaped clusters; and irregular galaxies have little or no defined structure.
Spiral galaxies are the most abundant in the universe.
A spiral galaxy revolves around a center, which is roughly spherical and protrudes from the disk. The axis is the gravitational center of the galaxy and normally contains a supermassive black hole. Surrounding the center are spiral arms, which are grouped waves of stars and gas that orbit around the center. The spiral arms tend to have more gas and dust and contain most of the newly formed stars, causing them to appear blue in color images. A spiral galaxy may have a horizontal bar running through its center; Our own galaxy, the Milky Way, is thought to contain a slash.
The center of a spiral galaxy may contain a supermassive black hole.
Elliptical galaxies have a large central core but do not have the arms of a spiral galaxy; they generally appear more yellowish in color images than old, reddish stars. Irregular galaxies usually have no discernible structure, but can be recognized by having a distorted spiral or elliptical shape. Elliptical and irregular galaxies are often the result of collisions, which trigger massive bursts of star formation and distort the structure of galaxies due to complex gravitational interactions. Eventually, the two galaxies usually merge to form one large galaxy, which has released most of its gas and dust clouds.
Our galaxy, the Milky Way, is believed to have up to 400 billion stars.
There are about a hundred billion galaxies in the visible universe; most of them occur in large clusters called superclusters of galaxies. Between these superclusters are voids with few or no galaxies, usually a hundred million light years or more. By studying the rotation of spiral galaxies, astronomers have discovered that most of the matter in the universe is not stars or gas, but invisible “dark matter” that cannot be seen but still exerts gravity. Dark matter is thought to be responsible for these large clusters, as the gravitational pull of dark matter pulled galaxies together.