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"
Two New Hubble-Quality Telescopes Gifted to NASA
10 June 2012
What if you were given a new Hubble telescope for free? How about two? The astronomical community is abuzz with just this opportunity as the US National Reconnaissance Office has unexpectedly transferred ownership of two space-qualified Hubble-quality telescopes to NASA. The usefulness of these telescopes in addressing existing science priorities has begun, but preliminary indications hold that even one of these telescope could be extremely useful in searching for extrasolar planets as well as distant galaxies and supernovas that could better explore the nature of dark energy. Although they start out as free, making even one telescope operational and fitting it with useful cameras would be quite expensive, so NASA is being decidedly careful about how to fit these new telescopes into its existing budget. Pictured above, the original Hubble Space Telescope floats high above the Earth during a servicing mission in 2002. Prefer Spanish?: Follow APOD's new Facebook feed "Universo"