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"
Aerogel For STARDUST
18 February 1999
On February 7th, this honey comb of aluminum cells filled with aerogel was launched on the STARDUST mission to interplanetary space. STARDUST's goal is to capture dust from a comet's tail and return to planet Earth - the first sample return mission to a comet! This structure represents about 1,000 square centimeters of area for collecting dust trailing within 150 kilometers of the nucleus of P/Wild-2. Comet P/Wild-2 is new to the inner Solar System. Having spent its life in orbit between Jupiter and Uranus, this comet was deflected in 1974 by a close encounter with Jupiter and now orbits between Jupiter and Earth. Dust from P/Wild-2 should impact the aerogel at high speeds and come to rest leaving carrot-shaped tracks in this amazingly tough, transparent, ultra-low density material. Returning to Earth by parachute in 2006, the cometary dust sample will be analyzed for clues to the formation and primordial composition of our Solar System.