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A giant sphere of hot plasma inside which thermonuclear fusion is occurring. The star emits light and heat as a result. Stars are typical celestial objects, estimated to number 100—400 billion in our Milky Way galaxy alone. One of the stars is the Sun, around which the planet Earth orbits. The predominant element in which stars are composed of is hydrogen. Their mass ranges from about 1 × 1029 kg to 6 × 1032 kg, their surface temperature from 2,000 K (red) to 50,000 K (blue-white). Stars form from a molecular cloud of hydrogen gas and undergo fusion over billions of years. At the end of their lives, they often end in an explosion that scatters a portion of the star into the surroundings, enriching it with many of the elements created in the final fusion reactions. These elements can form planets or other stars.