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"
SOHO Comet 367: Sungrazer
9 November 2001
The most prolific comet discovering instrument in history rides aboard the sun-staring SOHO spacecraft, 1.5 million kilometers sunward of planet Earth. Of course, most of these SOHO comets have been sungrazers - like the one illustrated in the dramatic montage above. Three frames taken hours apart on October 23rd, show bright SOHO comet number 367 plunging toward the fiery solar surface, its tail streaming away from the Sun located just beyond the left hand border. Each panel spans about one million kilometers at the distance of the Sun. From bottom to top, the comet's tail grows as the intensifying solar radiation heats the frozen comet material and increases the outflow of gas and dust. Because of their orbits, sungrazers are believed to belong to a family of comets produced by the breakup of a single much larger comet. Coincidentally, this sungrazer was discovered shortly after solar active regions blasted out clouds of energetic particles, like those that triggered the recent spectacular auroral storms. And like all SOHO sungrazers so far, comet number 367 was not seen to survive its close solar encounter.