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"
Gliese 581 and the Habitable Zone
26 April 2007
The unremarkable star centered in this skyview is Gliese 581, a mere 20 light-years away toward the constellation Libra. But astronomers are now reporting the discovery of a remarkable system of three planets orbiting Gliese 581, including the most earth-like planet found beyond our solar system. Gliese 581 itself is not a sun-like star, though. Classified as a red dwarf, the star is much smaller and colder than the Sun. Still, the smallest planet known to orbit the star is estimated to be five times as massive as Earth with about 1.5 times Earth's diameter. That super-earth orbits once every 13 days, about 14 times closer to its parent star than the Earth-Sun distance. The close-in orbit around the cool star implies a mean surface temperature of between 0 and 40 degrees C - a range over which water would be liquid - and places the planet in the red dwarf's habitable zone.