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"
A Slow Explosion
25 March 2003
Why would a gamma ray burst fade so slowly? This behavior, recorded last October, is considered a new clue into the cause of gamma-ray bursts, the most powerful explosions known in the universe. The burst, first detected by the orbiting HETE satellite and later tracked by numerous ground-based telescopes, showed an unusually slow and tumultuous decay in visible light. Speculations on the cause of the unusual light curve include a blast wave striking a windy circumburst medium, a blast wave energetically refreshed by a faster outgoing shock, and non-uniformity in a fast moving jet. Pictured above is the massive Wolf-Rayet star WR124, a star itself undergoing a slow explosion by producing a very powerful but tumultuous wind. Popular candidate progenitor sources for GRBs include supernova or hypernova explosions from massive stars, possibly ones with similarities to Wolf-Rayet stars.