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"
Dark River, Wide Field
19 July 2010
A Dark River of dust seems to run from our Galactic Center, then pool into a starfield containing photogenic sky wonders. Scrolling right will reveal many of these objects including (can you find?) the bright orange star Antares, a blue(-eyed) horsehead nebula, the white globular star cluster M4, the bright blue star system Rho Ophiuchi, the dark brown Pipe nebula, the red Lagoon nebula, the red and blue Trifid nebula, the red Cat's Paw Nebula, and the multicolored but still important center of our Galaxy. This wide view captures in exquisite detail about 50 degrees of the nighttime sky, 100 times the size of the full Moon, covering constellations from the Archer (Sagittarius) through the Snake Holder (Ophiuchus), to the Scorpion (Scorpius). The Dark River itself can be identified as the brown dust lane connected to Antares, and spans about 100 light years. Since the Dark River dust lane lies only about 500 light years away, it only appears as a bridge to the much more distant Galactic Center, that actually lies about 25,000 light years farther away.