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"
North American Nightscape
29 January 2021
On January 21, light from the Moon near first quarter illuminated the foreground in this snowy mountain and night scene. Known as The Lions, the striking pair of mountain peaks are north of Vancouver, British Colombia, Canada, North America, planet Earth. Poised above the twin summits, left of Deneb alpha star of the constellation Cygnus, are emission regions NGC 7000 and IC 5070. Part of a large star forming complex about 1,500 light-years from Vancouver, they shine with the characteristic red glow of atomic hydrogen gas. Outlines of the bright emission regions suggest their popular names, The North America Nebula and The Pelican Nebula. The well-planned, deep nightscape is a composite of consecutive exposures made with a modified digital camera and telephoto lens. Foreground exposures were made with camera fixed to a tripod, background exposures were made tracking the sky. The result preserves sharp natural detail and reveals a range of brightness and color that your eye can't quite see on its own.