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.
5 September 2011
Stars remain where they are. Nebulas appear the same. Day after day. Year after year. Given the vast distances in astronomy, even fast moving objects will not appear to change their appearance in a human lifetime. Typically. A recent spectacular exception to this, however, is the supersonic jet in the star forming Herbig Haro 47. HH 47 is so close -- and the jets are moving so fast -- that images from the Hubble Space Telescope from 1994 to 2008 have been combined into a time-lapse movie that actually shows a powerful jet expanding. Visible above, jets of plasma extending over 10,000 times the Earth-Sun distance shoot out from a forming star at speeds in excess of 150 kilometers per second. Studying how these jets evolve gives clues not only to how the star in HH 47 is forming, but how stars like our Sun formed billions of years ago. HH 47 is located about 1,500 light years away toward the constellation of Sails of a Ship (Vela).