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"
How Big Is 2001 KX76?
30 August 2001
Newly discovered minor planet 2001 KX76 is circled in the top panel above, a recent composite image from the European Southern Observatory's 2.2 meter telescope at La Silla, Chile. Though 2001 KX76 appears here as single point of light in an unremarkable star field, its orbit has been accurately measured by Astrovirtel, a newly operational "virtual telescope" capable of mining many years of archival data for previously unrecognized images of 2001 KX76. The results show this minor planet to be very distant, now orbiting just beyond Pluto and Charon in the realm of the Kuiper Belt. At its distance, apparent brightness, and assuming a reasonable surface reflectivity, 2001 KX76 would be 1,200 kilometers or more across -- larger than the largest main-belt asteroid, Ceres. In fact, the illustration in the bottom panel graphically compares this size estimate to Pluto, Charon, and the largest previously known Kuiper Belt objects, indicating the newfound minor planet is second only to Pluto in diameter. Along with other evidence, the comparison suggests that Pluto and Charon are closely related to Kuiper Belt worlds like 2001 KX76.