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A galaxy is a massive ensemble of stars, dust, and gas, all gravitationally interacting, and orbiting about a common center. The Sun is one of the billions of stars in our Galaxy, one of billions of galaxies in the Universe. All the stars visible to the unaided eye from earth belong to the earth's galaxy, the Milky Way. The sun with its associated planets is just one star in this galaxy.
If you look up at the sky on a clear night, you see that the sky is full of stars. During the summer months in the Northern Hemisphere, a faint band of light stretches from horizon to horizon, a swath of pale white cutting across a background of deepest black.
This band is composed of countless stars in a flattened disk seen edge on. The stars are so close to one another that the unaided eye has difficulty discerning the individual members. Astronomers find myriads of like systems sprinkled throughout the depths of space. They call such vast collections of stars galaxies, after the Greek word for milk, and call the local galaxy to which the Sun belongs the Milky Way Galaxyor simply the Galaxy.
The Milky Way Galaxy is a flattened system of stars and nebulae. During the early 1920s Edwin Powell Hubble determined conclusively that galaxies exist beyond the Milky Way system. Furthermore, Hubble's discovery (in 1929) that these external galaxies are apparently receding at speeds increasing with distance, and Albert Einstein's general theory of relativity together established modern cosmology.
A galaxy contains gas, dust, and billions of stars. It is held together by the gravitational attraction between its parts, and its rotational motion prevents it from collapsing on itself. Galaxies also contain atomic hydrogen gas; molecular hydrogen; complex molecules composed of hydrogen, nitrogen, carbon, and silicon, among others; and cosmic rays.
They are typically elliptical (oval), irregular, or spiral.
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- Most galaxies are elliptical, ranging from egg shapes to ball shapes, and show a decrease in brightness from the center outwards. An elliptical galaxy, lacking spiral arms entirely and containing little or no gas and dust, resembles the nucleus of a spiral galaxy.
- Spiral galaxies are flattened disk-shaped systems, about 100,000 light-years in diameter, with a central bulge, or nucleus, containing old stars; young stars are concentrated in spiral arms of dust, gas, and young stars. Our Milky Way galaxy is spiral. Barred spiral galaxies are distinguished by a bright central bar from which the spiral arms emerge.
- A small minority of galaxies are classified as irregular, i.e., showing no definite symmetry or nucleus. One theory suggests that irregulars evolve into spirals or ellipticals, depending on the initial amount of rotational motion.
Current theories suggest that all the galaxies were formed from immense clouds of gas soon after the Big Bang.
Some galaxies radiate a large fraction of their energy in forms other than visible light, such as radio waves, X rays, and infrared and ultraviolet radiation; their optical counterparts may be faint or undetectable.
Gravitation also holds clusters of galaxies together; the Local Group cluster includes the Milky Way (containing the sun and solar system near the edge of one of the spiral arms, about 30,000 light years from the center) and the Andromeda Galaxy, both spirals, and the irregular Magellanic Clouds. The stars of the spiral arm form the Milky Way.
If you look through a powerful telescope you can see galaxies millions of light-years away (a light-year is the distance travelled by light in one year). You are not seeing them as they are now, but as they were millions of years ago when their light first set out towards us.
An astronomer studies objects remote in time as well as in space. Light travels a distance of 300,000 kilometers in one second, or ten thousand billion kilometers in a year. The nearest star, Alpha Centauri, is 3 light years from us: we see it as it was three years ago. The nearest galaxy to our own Milky Way is two million light years distance: the Andromeda galaxy, a naked eye object in a dark sky, as it was when homo sapiens had not yet evolved. A large telescope is a time-machine that can take us part way to creation, to examine regions from which light emanated more than five billion years ago, before our sun had ever formed.