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"
X-Rays and the Circinus Pulsar
13 September 2001
A bizarre stellar corpse 19,000 light-years from Earth, pulsar PSR B1509-58 beckons from the small southern constellation of Circinus. Like its cousin at the heart of the Crab nebula, the Circinus pulsar is a rapidly spinning, magnetized neutron star. Seen in this false-color Chandra Observatory image, the environment surrounding this cosmic powerhouse glows in high energy x-rays. The Circinus pulsar itself, thought to generate more than 7 quadrillion volts (7 followed by 15 zeros), lies within the knot of bright emission near the center of the picture. Stretching toward the bottom left, x-ray emission traces a jet of particles almost 20 light-years long that seems to arise from the pulsar's south pole, while the arc of bright emission above the central knot is likely a shockwave produced by particles driven from the pulsar's equator. Near the top of the picture, lower energy x-ray emission shown in green is from gas shock-heated to millions of degrees Celsius. The shocked gas was produced by debris blasted out from the stellar explosion that created the Circinus pulsar.