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"
Alborz Mountain Milky Way
3 May 2008
Snow-capped stratovolcano Mt. Damavand climbs to 5,670 meters (18,598 feet) near the left edge in this panoramic view of the world at night. In the sky to the left of Damavand's peak are the stars of the Big Dipper in Ursa Major. Pan to the right and your gaze will sweep across the arch of our Milky Way Galaxy above the Alborz Mountain Range bordering the Caspian Sea. Near the center of the panorama, recorded in the predawn hours of April 4th, bright stars Deneb and Altair lie close to the curve of the Milky Way, above the glow of the Haraz valley. Farther right, brilliant Jupiter dominates the sky near the stars, nebulae, and dark dust clouds toward the bulging galactic center. Finally, the horizon glow at the right edge, below bright yellowish giant star Antares, is from the city of Damavand, named for the legendary mountain peak.