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"
LP 944-20: A Failed Star Flares
13 July 2000
The tiny spot circled on the right actually represents a big astronomical discovery -- the first detected flare from a failed star. Failed stars, termed brown dwarfs in astronomers' parlance, are too low in mass to ignite nuclear hydrogen burning in their cores, yet still shine feebly as the energy from their gravitational collapse is converted to heat and light. In fact, the dim brown dwarf cataloged as LP944-20 is estimated to have only 6 percent the mass of the Sun (60 times the mass of Jupiter) and one-tenth the Sun's diameter. A mere 16 light-years distant in the southern constellation Fornax it is well studied, but this failed star recently startled astronomers by producing a flare visible at x-ray energies. The above Chandra X-ray Observatory images of the LP944-20 star field were recorded in December 1999. Showing nothing (left) for the first nine hours, the brown dwarf generated a significant x-ray flare during the final hours of the observation. How did a failed star produced such a high-energy flare? Magnetic fields twisted and broken by turbulent motions near the surface of the brown dwarf may be the culprit. Difficult to detect because they are otherwise faint, brown dwarf stars are believed to be common throughout the galaxy.