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-Rays from Stephan's Quintet
12 August 2003
Stephan's Quintet is a picturesque but clearly troubled grouping of galaxies about 300 million light-years away toward the high-flying constellation Pegasus. Spanning over 200,000 light-years at that distance, this composite false-color image illustrates the powerful nature of this multiple galaxy collision, showing x-ray data from the Chandra Observatory in blue superposed on optical data in yellow. The x-rays from the central blue cloud running vertically through the image are produced by gas heated to millions of degrees by an energetic shock on a cosmic scale. The shock was likely the result of the interstellar gas in the large spiral galaxy, seen immediately to the right of the cloud, colliding with the quintet's tenuous intergalactic gas as this galaxy plunged through group's central regions. In fact, over billions of years, repeated passages of the group galaxies through the hot intergalactic gas should progressively strip them of their own star forming material. In this view, the large spiral galaxy just seen peeking above the bottom edge is an unrelated foreground galaxy a mere 35 million light-years distant.