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"
The GRB 110328A Symphony
19 April 2011
A symphony of planet-wide observations began abruptly on March 28 when the Earth-orbiting Swift satellite detected a burst of high-frequency gamma-rays from GRB 110328A. When the same source flared again after a 45 minute pause it was clear this event was not a typical gamma-ray burst. Twelve hours after the initial fanfare astronomers using the 2.5-meter Nordic Optical Telescope chimed in with a mid-range observation of the optical counterpart. Early the next day the explosion was picked up in baritone low-frequencies of radio waves by the EVLA radio dishes in the USA. Later many optical telescopes, including the 8-meter Gemini North telescope in Hawaii, began playing along by tracking the optical counterpart. The unusual source was spotted at a higher register in X-rays by the Chandra X-ray Observatory and was intermittently followed in the even more soprano-like gamma-ray range for a week. Joining the chorus, Hubble Space Telescope recorded this image in optical and infrared light, confirming that the flash was located along the path of a galaxy at redshift 0.351. If associated with the galaxy, this explosion occurred when the universe was about two thirds of its present age. There is much speculation that the unusual gamma-ray burst was a star being ripped apart by a supermassive black hole in the center of a galaxy and the puzzling features of the distant detonation are still being explored.