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"
Watch the Sky Rotate
10 January 2001
If you could watch the sky for an entire night, what would you see? The above time-lapse sequence from the CONtinuous CAMera (CONCAM) project shows the answer for the skies above Kitt Peak National Observatory on 2000 December 23. First and foremost stars appear to orbit about Polaris, a star near the top of the image. Actually, the Earth is spinning under the sky, and the camera is affixed to the Earth. The diffuse band of light that moves across the image is actually the central disk of our Milky Way Galaxy. Identifiable objects rotating across the frame include the constellation of Orion, stars such as Sirius and Betelgeuse, and planets such as Jupiter and Saturn. The CONCAM project is deploying astronomical quality web-cameras to major observatories with goals of starting a continuous record of the sky and helping astronomers using large telescopes monitor weather conditions remotely.