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"
3C58: Pulsar Power
23 December 2004
Light from a star that exploded some ten thousand light-years away first reached our fair planet in the year 1181. Now known as supernova remnant 3C58, the region seen in this false-color image glows in x-rays, powered by a rapidly spinning neutron star or pulsar - the dense remains of the collapsed stellar core. A cosmic dynamo with more mass than the sun, the pulsar's electromagnetic fields seem to accelerate particles to enormous energies, creating the jets, rings, and loop structures visible in this stunning x-ray view from the orbiting Chandra Observatory. While adding 3C58 to the list of pulsar powered nebulae explored with Chandra, astronomers have deduced that the pulsar itself is much too cool for its tender years, citing 3C58 as a show case of extreme physics not well understood. The close-up inset above spans about six light-years.