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"
On The Trail Of A Fireball
19 February 1999
This exceptionally bright fireball meteor trail was photographed with a fish-eye camera at a Czech Republic station of the European Fireball Network on January 21, 1999. Of the star trails visible in this night-long exposure, the bright short arc in the upper left is due to Polaris, the north star. The breaks seen near the beginning of the fireball trail itself were produced by a shutter rotating 15 times a second. In all, three stations recorded the dazzling streak and their combined tracking information has revealed details of the meteor's brief atmospheric flight and previous interplanetary voyage. For example, the luminous trail is measured to begin at an altitude of 81.9 kilometers and covered 71.1 kilometers in 6.7 seconds. The projected prior orbit for the meteoroid corresponds to one typical for Apollo class asteroids which can cross Earth's orbit. In forty years of operations the European Network has multistation recordings of less than 10 or so fireballs as bright as this one. It is thought likely that a small (a few hundred grams) meteorite survived this fiery fall to Earth and landed near the Czech-Poland border.