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"
Dim World, Dark Nebula
8 July 2010
Dim, distant, dwarf planet Pluto can be hard to spot, especially in recent months as it wanders through the crowded starfields of Sagittarius and the central Milky Way. But fortunately for backyard Pluto hunters, it crossed in front of a dark nebula in early July. The diminutive world is marked with two short lines near the center of this skyscape recorded from New Mexico Skies on July 5. Pluto stands out only because obscuring dark nebula Barnard 92 (B92) blocks the background of the Milky Way's congeries of faint, innumerable stars. Another of astronomer E. E. Barnard's cataloged dark markings on the sky, B93, is easy to pick out just left of B92. Prominent at the lower left is open star cluster NGC 6603. In fact, Pluto, dark nebulae, and star cluster all lie within a portion of M24, also known as the Sagittarius Star Cloud, filling most of the frame.