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"
Airglow Over Germany
5 September 2012
Does air glow? It does, but it is usually hard to see. When conditions are right, however, a faint glow about 90 kilometers up can be observed, most easily with a wide-angle long-duration camera exposure. The same airglow can also frequently be seen looking down -- in pictures taken from Earth orbit -- as a faint arc hovering above the surface. Pictured above between the beige clouds, above the curving Earth, behind the streaking airplane, and in front of the sparkling stars are some green bands of airglow. The glow is predominantly created by the excitation of atoms by ultraviolet light from the Sun, with the bands resulting from density fluctuations caused by upward moving atmospheric gravity waves. The above image was taken in mid-July above Weikersheim, Germany. Lightning and aurorae can also cause air to glow, but result from particle collisions and are more fleeting. Astrophysicists: Now over 500 codes in the Astrophysics Source Code Library