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"
Comet, Meteor, Nebula, Star
22 December 2004
Several wonders of the late-year northern sky appeared together for a few fleeting moments on December 13. On the bottom left, just above the hill, is blue Sirius, the brightest star in the sky. Above Sirius and slightly to the right of the belt of Orion is the red Orion Nebula, one of the most famous nebulas on the sky. Below and to the right of the Orion Nebula streaks a yellow meteor, although moving in the wrong direction to be from the Geminids meteor shower that peaked the night. Finally, above and to the right of the meteor is Comet Machholz, whose coma appears here relatively green. Since the time since this image was taken over a Californian hill, the Geminid meteor has long since evaporated. Comet Machholz has brightened and moved to the north. Sirius, however, will remain in the constellation of Canis Major indefinitely.