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A star is a hot glowing ball of gas. It gives off light and other forms of electromagnetic radiation whose source is nuclear energy. Stars differ widely in mass or weight, size, temperature, and brightness.
The brightest stars are about a million times more powerful than the sun, while the least bright are only one hundredth as powerful red giants. The biggest stars are hundreds of times greater in size than the sun. Among the smallest stars, white dwarfs are no larger than a planet. Neutron stars are only a mile or so. .
The stars are divided into six groups according to brightness. The brightest are said to be of the first magnitude and the least bright are sixth magnitude. The stars differ in brightness for two reasons - they are actually brighter or not as bright and also because of their distance from the earth.
Variable stars do not shine in a regular manner. The supernova is a type of variable star.
Light from a star is made up of a spectrum of wavelengths. The hotter the star, the shorter the wavelength at which the light is most intense. The color of a star depends on its surface temperature.
The mass and substances that make it up are what characterizes a star. Most stars are more than 90% hydrogen which makes them chemically similar. Differences in mass is the most important factor. By doubling a star's mass increases the brightness ten times. For a star to be stable, the force of gravitation must be exactly balanced by the tendency of gases to expand or get bigger.
Despite the tremendous pressure created by the massive layers above it, the core or center, of a star stays a gas because the core is so very hot. Because the temperature is so hot, , nuclear energy is let off by the fusion or joining together of hydrogen atoms to form helium. By the time nuclear energy reaches the surface of the star, it has been changed to visible light. Scientists think that a star must change as it uses its hydrogen in the nuclear reactions that power it. Each star must die. Once in a while a supernova explodes and some of the matter that is thrown from the explosion gradually over time comes together to form new stars.
The stars we can see without instruments are all in the Milky Way Galaxy. Stars are not spread evenly through a galaxy. They are often grouped together in star clusters of thousands of stars. Many stars that appear as single are really systems of two or more stars.