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Characteristics to classify stars include color, temperature, size, composition, and brightness. The brightness of a star depends upon both its size and temperature. Scientists use light years to measure distances between stars. All stars begin their lives as parts of nebulas, large clouds of gas and dust spread out in a large volume. Gravity pulls the gas and dust together eventually forming a protostar. A star is born when the contracting gas and dust become so dense and hot that nuclear fusion starts. A star is born when nuclear fission starts. The longevity of a star depends on its mass. After a star runs out of fuel, it can become a white dwarf, neutron start, or black hole. A white dwarf is only about the size of the Earth but as much as mass as the sun. A neutron star is the remains of high-mass stars. A black hole is an object with gravity so strong that not even light can escape. These are usually formed from the death of the most massive stars. Most stars belong to star systems. Galaxies can be classified as spiral, elliptical, and irregular. Our solar system is the Milky Way Galaxy and it is a spiral galaxy. A spiral galaxy have a bugle in the middle and arms that spiral outwards. Elliptical galaxies look round or like flatted balls. Irregular galaxies do not have regular shapes. Obtained from: http://www.sciencephot o.com/media/332837/ enlarge According to the big band theory, the universe formed instantly in an explosion over a billion years ago. Our solar system was formed from a giant cloud of gas and dust that collapsed. Astronomers believe the universe will likely expand forever. Our solar system began as a gaseous cloud called a nebula. Collapse of nebula formed planetesimals. They collided and grew larger sticking together to form planets. Most gases escaped the gravity of the planets that were forming and the inner planets, Mercury, Venus, Earth, and Mars are relatively small and rocky. Farther from the sun it was much cooler. Gravity increased in this region and it was able to capture much of the hydrogen and helium gas. This resulted in the outer planets, Jupiter, Saturn, Uranus, and Neptune. Hubble discovered that galaxies are moving away from us and from each other. Evidence from this is the doppler effect and cosmic background radiation. Cosmic background radiation is the leftover thermal energy from the big bang. The doppler effect is red light left over from the expansion.