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"
Recurrent Nova RS Ophiuci
24 February 2006
This pretty star field in the constellation Ophiucus is centered on a star not often seen - RS Ophiuci. In fact, early last week RS Oph suddenly became visible to the naked eye for the first time since 1985. A type of cataclysmic variable star classified as a recurrent nova, RS Oph dramatically increased in brightness from 11th magnitude, too faint to appear on some star charts. Historically, RS Oph was seen to go through only four similar outbursts since 1898. Such stars are now modeled as interacting binary star systems, composed of a compact white dwarf star co-orbiting with a swollen red giant. As material falls away from the red giant it collects in a rotating accretion disk before ultimately falling on to the white dwarf. Disk instabilities, or a build up of material on the compact star result in the occasional but rapid release of energy through nuclear burning. At an estimated distance of 3,000 light-years, RS Ophiuci is now reported to be fading rapidly. This telescopic view spans about 2 degrees (4 full moons) and was captured on the morning of February 16 from the RAS Observatory under New Mexico skies.