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Galaxies
A Galaxy is a gravitational bound system composed by stars, stellar remnants, an interstellar medium of gas and dust, and a component called dark matter. Galaxies are categorized according to their shape, some of them are elliptical galaxy which has an ellipse-shaped light profile, spiral galaxies are disked shaped with curvy arms, smaller galaxies are referred as irregular galaxies. Galaxies don’t float through space, they are together in groups known as clusters, some clusters are large containing thousands of galaxies, and some are smaller, our galaxy the milky way it’s in a cluster known as the local group containing only fifty galaxies. Some time galaxies collide together mixing their stars and dust together this is an important step in the evolution and growth of galaxies. Probably there are more than 170 billion galaxies in the observable universe. Astronomers suggest that galaxies were formed shortly after the big bang that began the universe. Galaxies emit radiation wavelengths that give astronomers information about the content of galaxies such as stars, gas, and presence of black holes. The outer parts of galaxies don’t produce much radiation, but if there are sources like quasars in the background they can be studied in great detail. Quasars are a very energetic active galactic nucleus, they are very luminous, and they were first indentified as being redshift sources of electromagnetic sources. Most quasars are known to be three billion light years away.

When stars die they leave behind compact objects of some sort like white dwarf or neutron stars. White dwarf is a stellar remnant composed of electron degenerate matter, white dwarfs are the final evolution of a star, and they are very hot when formed but since they have no more energy they would eventually cool down. Dark matter composes the majority of the universe, Dark matter is not on the form of star or planets that we see, we cannot se dark matter but we can detect it by its...
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