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"
The Tail of a Wonderful Star
17 August 2007
To seventeenth century astronomers, Omicron Ceti or Mira was known as a wonderful star, a star whose brightness could change dramatically in the course of about 11 months. Mira is now seen as the archetype of an entire class of long-period variable stars. Surprisingly, modern astronomers have only recently discovered another striking characteristic of Mira -- an enormous comet-like tail nearly 13 light-years long. The discovery was made using ultraviolet image data from the Galaxy Evolution Explorer (GALEX) satellite. Billions of years ago Mira was likely similar to our Sun, but has now become a swollen red giant star, its outer layers of material blowing off into interstellar space. Fluorescing in ultraviolet light, the cast off material trails behind the giant star as it plows through the surrounding interstellar medium at 130 kilometers per second. The amount of material in Mira's tail is estimated to be equivalent to 3,000 times the mass of planet Earth. About 400 light-years away toward the constellation Cetus, Mira is presently too faint to be seen by the unaided eye, but will become visible again in mid-November.