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.
21 February 2017
The night sky is always changing. Featured here are changes that occurred over a six hour period in late 2014 June behind the dual 6.5-meter Magellan Telescopes at Las Campanas Observatory in Chile. The initial red glow on the horizon is airglow, a slight cooling of high air by the emission of specific colors of light. Bands of airglow are also visible throughout the time-lapse video. Early in the night, car headlights flash on the far left. Satellites quickly shoot past as they circle the Earth and reflect sunlight. A long and thin cloud passes slowly overhead. The Small Magellanic Cloud rises on the left, while the expansive central band of our Milky Way Galaxy arches and pivots as the Earth rotates. As the night progresses, the Magellan telescopes swivel and stare as they explore pre-determined patches of the night sky. Every night, every sky changes differently, even though the phenomena at play are usually the same. Explore the Universe: Random APOD Generator