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"
Clouds in Cygnus
30 November 2012
Cosmic clouds of gas and dust drift across this magnificent mosaic covering a 12x12 degree field within the high flying constellation Cygnus. The collaborative skyscape, a combination of broad and narrow band image data presented in the Hubble palette, is anchored by bright, hot, supergiant star Deneb, below center near the left edge. Alpha star of Cygnus, Deneb, is the top of the Northern Cross asterism and is seen here next to the dark void known as the Northern Coal Sack. Below Deneb are the recognizable North America and Pelican nebulae (NGC 7000 and IC 5070). Another supergiant star, Sadr (Gamma Cygni) is near the center of the field just above the bright wings of the Butterfly Nebula. A line continuing up and right will encounter the more compact Crescent Nebula and finally the Tulip Nebula near the top of the frame. Most of these complex nebulosities are located about 2,000 light-years away. Along with the Sun, they lie in the Orion spiral arm of our Milky Way galaxy.