Star

Star

A star is a massive, luminous sphere of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, the brightest of which gain proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations.

Formation

Stars are formed within extended regions of higher density in the interstellar medium, although the density is still lower than the inside of an earthly vacuum chamber. These regions are called molecular clouds and consist mostly of hydrogen, with about 23–28% helium and a few percent heavier elements. One example of such a star-forming region is the Orion Nebula.

Structure

Stars are composed of hydrogen, helium, and trace amounts of heavier elements. The star's core is a hot, dense mix of these elements, which fuels nuclear fusion reactions. These reactions release energy, which pushes against the star's gravity to maintain its structure.

Evolution

Stars spend most of their lives in the main sequence phase, during which they undergo nuclear fusion to convert hydrogen into helium. Once the hydrogen fuel is exhausted, the star expands to become a red giant or contracts to become a white dwarf.

Classification

Stars are classified by their spectral class, with each class representing a range of temperatures and intrinsic brightnesses. The spectral classes, from hottest to coolest, are O, B, A, F, G, K, and M.

See also

• Binary star
• Supernova
• Neutron star
• Black hole

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