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"
The Milky Way Near the Southern Cross
17 May 2007
The glow of the southern Milky Way and the well-known Southern Cross are featured in this colorful skyscape recorded in April over La Frontera, Chile. The Southern Cross (Crux) itself is at the right of the 20 degree wide field of view, topped by bright, yellowish star Gamma Crucis. A line from Gamma Crucis through the blue star at the bottom of the cross, Alpha Crucis, points toward the south celestial pole. Against faint Milky Way starlight, the dark expanse of the Coal Sack Nebula lies just left of the cross, while farther left along the Milky Way are the bright stars Hadar and Rigil Kentaurus, also known as Beta and Alpha Centauri. Blazing in the lower left, Alpha Cen is the closest star to the Sun, a mere 4.3 light-years distant. In fact, yellowish Alpha Cen is actually a triple star system that includes a sun-like star. Seen from Alpha Cen, our own Sun would be a bright yellowish star in the otherwise recognizable constellation Cassiopeia.