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"
The Tail of the Small Magellanic Cloud
7 January 2010
A satellite galaxy of our Milky Way, the Small Magellanic Cloud is wonder of the southern sky, named for 16th century Portuguese circumnavigator Ferdinand Magellan. Some 200,000 light-years distant in the constellation Tucana, the small irregular galaxy's stars, gas, and dust that lie along a bar and extended "wing", are familiar in images from optical telescopes. But the galaxy also has a tail. Explored in this false-color, infrared mosaic from the Spitzer Space Telescope, the tail extends to the right of the more familiar bar and wing. Likely stripped from the galaxy by gravitational tides, the tail contains mostly gas, dust, and newly formed stars. Two clusters of newly formed stars, warming their surrounding natal dust clouds, are seen in the tail as red spots. Note: An APOD editor will review astronomy images of 2009, hosted by the Amateur Astronomers Association of New York on Friday, January 8 at the American Museum of Natural History, NYC.