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"
X-Ray Stars and Winds in the Rosette Nebula
19 October 2001
This mosaic of x-ray images cuts a swath across the photogenic Rosette Nebula, a stellar nursery 5,000 light-years from Earth in the constellation Monoceros, the Unicorn. Constructed from data recorded by the orbiting Chandra X-ray Observatory, the mosaic spans less than 100 light-years and is color coded to show low energies in red and high energy x-rays in blue. At the upper right is the young star cluster NGC 2244, central to the Rosette Nebula itself. The hot outer layers of the massive stars are seen to be copious sources of x-rays, but a diffuse x-ray glow also pervades this cluster of newborn stars. Since these stars are so young (less than few million years old!) the diffuse x-ray emission is thought to be powered by energetic, colliding stellar winds rather than remnants of supernovae explosions, a final act in the life cycle of a massive star. Moving away from the center, south and east across the nebula (upper right to lower left), the hot, blustery environment gives way to dense molecular gas, absorbing low energy x-rays while revealing the penetrating high energy x-rays from embedded stars.