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"
Geminid Meteor over Monument Valley
12 December 2009
The Geminids are expected to put on a good show this year. Created as planet Earth sweeps through dusty debris from extinct comet Phaethon, the annual Geminid meteor shower is predicted to peak on December 14th, around 0510 UT (12:10am EST). With better viewing for northern hemisphere observers, pictures of Geminids streaking through the night could include wintery landscapes, like this snow-tinged image of a 2007 Geminid meteor over buttes of the Monument Valley region in the southwestern US. The meteor streak points back to the constellation Gemini and the shower's radiant point, just off the upper left edge of the scene. Along with Rigel, the sword and belt stars of Orion are at the upper right. Near the eastern horizon are bright stars Procyon (left) and Sirius. The two buttes at the far left are known as The Mittens - clearly a reminder that if you want to watch a meteor shower on a cold December night, wearing mittens would be a good idea.