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"
Centaurus A: Young Blue Star Stream
17 October 2002
Almost lost in this cosmic jumble of stars, gas and dust is a faint but definite blue arc -- a stream of young stars whose formation was probably triggered as a small dwarf galaxy was torn apart approaching the giant elliptical galaxy Centaurus A. The 2,000 light-year long arc is revealed in the upper right corner of this processed color digital image, while the dense central region of Centaurus A is near the bottom. Star clusters that make up the blue arc are likely strung out along the incoming trajectory of the small galaxy and are estimated to be only 200-400 million years old. The remarkable result suggests that astronomers have identified a spectacular example of a kind of galactic cannibalism in progress, a process which is believed to contribute to the formation and evolution of large galaxies, including our own Milky Way. Over time, stars and star clusters in this stream should eventually disperse and merge with tumultuous Centaurus A. The image data was recorded with the four meter Blanco telescope at Cerro Tololo Inter-American Observatory.