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"
Explosions Discovered Near Galactic Center
28 February 1996
Tremendous explosions near the center of our Galaxy were discovered just this past December and are being announced today by a paper in Nature and a press conference at NASA. Bursts like these have never been seen before, and so the exact cause is unknown and will likely be the source of astronomical speculations and observations for years to come. Much more powerful than any explosions we humans can create, these eruptions likely involve the extreme conditions found only on the surface of a neutron star in a binary system, possibly similar to the X-ray binary system depicted in the above drawing. The new source, dubbed GRO J1744-28 for its discovery spacecraft and position, currently produces multiple pulsed bursts of energy per day, each of which last several seconds. The bursts are quite prominent in X-ray light. Discovery team leaders include Chryssa Kouveliotou (USRA) and Gerald Fishman (NASA /MSFC).