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"
Swift Launches
22 November 2004
Where do gamma ray bursts occur? To help find out, NASA launched the Swift satellite on Saturday, as pictured above. What Swift is designed to do better than any previous satellite is to quickly locate these enigmatic explosions in both sky position and distance. Once a gamma ray burst (GRB) is located, Swift itself will rotate to face it head-on and determine its distance directly. Swift locations will be immediately beamed down to Earth-based telescopes operating around the world and across the electromagnetic spectrum that are just waiting for a place on the sky to point. Subsequently collected GRB and afterglow information will help astronomers not only determine the nature of the initial explosions, but also the nature of the expanding shockwaves. Some Swift-located GRBs could turn out to be the most distant transient events ever detected, holding unique clues to the nature and rate of the very first wave of star formation in the early universe.