Galaxies are groups of billions of stars that are held together by the force of gravity. Most
galaxies are either spiral or elliptical, but some are irregular in shape. The best known galaxy is the Milky Way. The word galaxy itself comes from the Greek word of Milk. This is because before telescopes were powerful enough to prove that they were made up of individual stars, galaxies looked like milky or cloudy areas in the sky. Our relatively small solar system is only one of 100-200 billion stars in the Milky Way Galaxy, which is 100,000 light years in diameter. The sun and all the planets take about 200,000,000 years to complete one orbit around its center. The large Magellanic Cloud, which is visible only in the southern
hemisphere, is 170,000 light years from Earth and 39,000 light years in diameter. The central galaxy of the Abell 2029 galaxy cluster was discovered in 1990. It is 1,070 million light years distant and has a diameter of 5.6 million light years, 80 times the diameter of our own galaxy.
                            A typical spiral galaxy is shaped like a flat disk, about 100,000 light-years in diameter, with a central bulge, or nucleus, containing old stars; winding through the disk are the characteristic spiral arms of dust, gas, and young stars (see stellar populations). This type of galaxy is further classified as being either a normal or a barred spiral. In the normal spiral, the arms, at least two in number, join smoothly with the nucleus; in the barred spiral, the arms project from a bank of stars that runs through the nucleus. The elliptical galaxies, lacking spiral arms entirely and containing little or no gas and dust, resemble the nuclei of spiral
galaxies. Their shapes vary from nearly spherical to highly flattened ellipsoids. Elliptical galaxies have a much greater variation in size, mass, and luminosity than do spiral galaxies; their sizes range from the largest known
galaxies of all, with luminosities about 10 times that of the Andromeda Galaxy, to the small dwarf elliptical, which can contain as few as a million stars. Irregular galaxies appear structure less and without any nucleus or rotational symmetry; their light comes mostly from young stars.

Our Galaxy


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