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.
1 February 2017
Does life exist outside our Solar System? To help find out, NASA has created the Nexus for Exoplanet System Science (NExSS) to better locate and study distant star systems that hold hope of harboring living inhabitants. A new observational result from a NExSS collaboration is the featured time-lapse video of recently discovered planets orbiting the star HR 8799. The images for the video were taken over seven years from the Keck Observatory in Hawaii. Four exoplanets appear as white dots partially circling their parent star, purposefully occluded in the center. The central star HR 8799 is slightly larger and more massive than our Sun, while each of the planets is thought to be a few times the mass of Jupiter. The HR 8799 system lies about 130 light years away toward the constellation of the Flying Horse (Pegasus). Research will now continue on whether any known or potential planets -- or even moons of these planets -- in the HR 8799 star system could harbor life. Open Science: Browse 1,400+ codes in the Astrophysics Source Code Library