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"
A Light and Dusty Night
2 October 2021
Posing as a brilliant evening star, Venus lies near the western horizon in this southern hemisphere, early spring, night skyscape. To create the composite view exposures tracking the sky and fixed for the foreground were taken on September 25 from Cascavel in southern Brazil. In view after sunset, Venus appears immersed in a cone of zodiacal light, sunlight scattered from dust along the Solar System's ecliptic plane. In fact from either hemisphere of planet Earth, zodiacal light is most visible after sunset near a spring equinox, (or before sunrise near an autumn equinox) when its luminous arc lies at steep angles to the horizon. Extending above the sunset on this night, the zodiacal light reaches toward rich starfields and immense interstellar dust clouds in the bulge of the central Milky Way. Follow along the Milky Way from the central bulge back toward the horizon and you'll spot the closest star system to the Sun, Alpha Centauri, a mere 4.37 light-years away.