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"
Moon Struck
8 December 1999
Craters produced by ancient impacts on the airless Moon have long been a familiar sight. But now observers have seen elusive optical flashes on the lunar surface - likely the fleeting result of impacting meteoroids. Orchestrated by David Dunham, president of the International Occultation Timing Association (IOTA), video recordings made with modest equipment and visual telescopic observations have, for the first time, detected and confirmed a total of six flashes on the Moon's dark side. The flashes, some initially as bright as a third magnitude star, were all seen within hours of the peak of this year's Leonid meteor shower. Their locations are indicated by the red Xs on this projection of the Moon as it appeared on the night of November 18. Similar flashes would have been difficult to see if viewed against the Moon's brightly lit portion. It has been estimated that the brightest flashes were made by meteoroids weighing around a tenth of a kilogram, resulting in lunar craters about one meter across. And ... the next chance to observe lunar impact flashes is coming up! Enterprising astronomers interested in long distance lunar prospecting should be monitoring the dark side of a nearly first quarter Moon during the Geminids meteor shower which will peak around December 13.