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.
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APODs including "Star"
Water Ice At The Lunar Poles
6 March 1998
After seven weeks in lunar orbit, instruments on board NASA's Lunar Prospector spacecraft have produced strong evidence for water ice on the Moon. While not in extensive sheets, this ice could be in the form of crystals, mixed in low concentrations with material in craters around the frigid North and South lunar poles. How much ice? So far, the estimates are from 10 to 300 million metric tons (about 3 to 90 billion gallons). The water ice is likely a result of meteoritic and cometary bombardment. Using very different instruments, the Star-Wars-technology Clementine spacecraft also detected telltale signs of ice as it orbited the Moon in 1994. Although the lunar samples returned by the Apollo missions suggested that the Moon was bone dry, the Clementine result sparked the controversy and hope that water ice might still be found - holding out the tantalizing possibility that a substantial reservoir of lunar water could be used to support human exploration. Above is a mosaic of Clementine images of the cratered terrain surrounding the Moon's North pole. Surprisingly, Lunar Prospector's data indicates that the North polar region contains about twice as much ice as the South. As Lunar Prospector's mission continues, further data will help determine more precisely the amount and distribution of the water ice.