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"
Fireball!
26 February 1996
On rare but spectacular occasions, fireballs, meteors brighter than the brightest stars, flash through the heavens - sometimes making audible sounds and occasionally surviving to strike the Earth's surface. The path of one such fireball, recorded last year in January skies over Hannover Germany is shown above, the brilliant meteor easily outshining the stars of the constellation Orion (visible at the far right). In fact, for this brief moment Sirius, the brightest star in the night sky, is overwhelmed as the image of the fireball grazes it. This dramatic event was recorded by a video camera observation technique known as MOVIE. Interplanetary space is littered with meteoroids, "rocks" tens of yards in diameter and less. Striking the Earth's atmosphere at high relative speeds, they leave visible trails known as meteors or, more poetically, "shooting stars". The trails are created when the intense heat caused by friction vaporizes them. Fireballs can be caused by meteroids weighing small fractions of an ounce which do not survive to reach the ground. Remnants of larger ones, which do reach the ground after running this fiery gauntlet, are called meteorites. Though fireballs are rare, meteors are visible on any clear night of the year. Even outside the predicted regular meteor shower events, patient observers in dark sky areas can see several an hour - by just looking up! Sightings of fireballs should be reported.