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"
Full Throttle For Deep Space 1
31 August 2000
At full throttle the Deep Space 1 spacecraft's innovative ion drive produces about 1/50th of a pound of thrust ... a force so great that it would just about hold up a piece of paper on planet Earth! Still, powered by solar arrays ion propulsion systems can run continuously. For long duration space missions they ultimately win out over the powerful but brief blasts of less efficient chemical rockets. Deep Space 1 is seen here suspended in an assembly room, a folded solar array resting above the circular ion propulsion module. Already a successful technology demonstrator with experimental autonomous software, the spacecraft flew by asteroid 9969 Braille in July of 1999 but later that year, in November, the robot probe was nearly lost due to the failure of its wide-field star tracker camera. Now, the adventures of Deep Space 1 can continue. Engineers were able to reprogram the navigation system to utilize another on-board camera and on 28 June 2000 the ion drive was throttled up. Once again steering by the stars, Deep Space 1 is presently bound for a September 2001 rendezvous with periodic Comet Borrelly.