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"
HETE-2 Rides Pegasus
12 October 2000
The Stargazer, a modified Lockheed L-1011 aircraft, soared into the skies above Kwajalein Atoll in the pacific on October 9th. A small satellite observatory known as the High Energy Transient Explorer - 2 (HETE-2) was tucked into Stargazer's winged Pegasus rocket, slung beneath the large trimotor jet's fuselage. Dropped from its mother ship, the Pegasus then successfully flew HETE-2 into orbit. HETE-2's mission is to hunt gamma-ray bursts, brief, random flashes of high energy photons from the distant cosmos. Gamma-ray bursts are impressive, believed to be the most powerful explosions in the Universe, but so few have been well located and studied that the nature of the bursters themselves is still shrouded in mystery. HETE-2's x-ray and gamma-ray instruments will be able to rapidly alert ground-based observatories to point toward ongoing, bright gamma-ray bursts. Communications antennae and solar panels neatly folded, HETE-2 is seen here being carefully enclosed in the Pegasus nose fairing.