Star
A star is a massive, self-luminous sphere of plasma held together by its own gravity. It shines by converting hydrogen into helium through nuclear fusion in its core, releasing energy across the electromagnetic spectrum.
Stars form in large clouds of gas and dust—called nebulae—where regions collapse under gravity, creating protostars. When core temperatures reach millions of degrees, fusion ignites, marking the birth of a star. Observatories like Hubble and missions such as NASA’s Infrared telescopes have imaged this process in action.
The majority (~90%) of stars are main-sequence stars, fusing hydrogen into helium. These include a broad range of masses—from red dwarfs (small, long-lived, faint) to blue giants and supergiants (massive, hot, and short-lived). Our Sun is a middle-aged G-type main sequence star.
As stars exhaust their hydrogen fuel, their evolution depends on mass. Lower-mass stars become red giants then white dwarfs. More massive stars undergo successive fusion stages, end in supernova explosions, and leave behind neutron stars or black holes.
Stars vary in brightness, size, and color. They are classified using spectral types (O, B, A, F, G, K, M) based on surface temperature and absorption lines. For example, O- and B-type stars are hot and blue; M-type are cool and red.
Stellar remnants include white dwarfs (Earth-sized cores of former stars), neutron stars (city-sized remnants of supernovae), and black holes (extreme-density objects from the most massive stars).
Stars are not static—many rotate, exhibit magnetic activity (like sunspots and flares), and broadcast stellar winds. Their lifecycle enriches the interstellar medium with heavier elements, seeding future generations of stars and planets.
Stars often exist in groups—binary or systems within star clusters and galaxies. Their properties are studied via brightness, spectra, parallax, variability, and statistical surveys by missions like Gaia and Kepler.
Sources
APODs including "Star"
Palomar at Night
7 August 2003
What's wrong with this picture? The summer night sky is clear, and moonlight illuminates the dome of the Hale 200-inch Telescope at Palomar Observatory, northeast of San Diego, California, USA. The familiar stars of the Teapot asterism in the constellation Sagittarius shine above the dome and to the right. In fact, the only thing wrong with the picture is that the observatory dome's two massive shutters are closed tight ... on a clear night. This extremely unusual situation is a precaution prompted by the presence of airborne ash and smoke from wildfires in the area on July 17 which could have damaged the historic mirror's aluminum coating. Amateur astronomer Greg Redfern notes that this year has been a particularly bad one for observatories and wildfires, with the Mt. Stromlo Australian Observatory disaster and fires also threatening telescopes on Mount Lemmon near Tucson, Arizona.