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"
Sun 360: STEREO Captures Views of the Entire Sun
7 February 2011
For the first time, the entire Sun is being imaged all at once. This has become possible because the two STEREO satellites orbiting and monitoring the Sun are now on opposite sides of the Sun. The two satellites have been drifting apart, as expected, since their launch in 2006, since one satellite orbits slightly closer to the Sun than the other. The above image shows nearly the entire Sun as it appeared one day last week, a few days before maximum exposure. Yesterday, the dark gap in the center closed completely, and STEREO was able to beam back to Earth full 360 degree images of the closest star. Full solar images are useful scientifically for a number of reasons, including catching rapidly evolving flares, coronal mass ejections, tsunamis, and filaments, no matter where they occur on the Sun, as well as monitoring days-long sunspots and active regions without losing them as they rotate out of view. Even though the STEREO satellites will continue to drift apart at about 44 degrees per year, Sun-staring instruments on or near the Earth will augment them to provide a full view of the Sun for the next several years. Students (young and old): See free lectures on astronomy