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"
Protoplanetary Survivors in Orion
4 May 2001
The Orion Nebula is a nuturing stellar nursery filled with hot young stars and their natal clouds of gas and dust. But for planetary systems, the active star-forming region can present a hazardous and inhospitable birthplace. While the formation of dusty protoplanetary disks seems common in Orion, these Hubble Space Telescope close-up images dramatically reveal the torturous conditions they must face while trying to grow into full-fledged planetary systems. In each case, a central young star is surrounded by a disk substantially wider than our solar system. The disks likely contain material in the process of planet formation. However, withering ultraviolet radiation from one of Orion's nearby hot stars is rapidly destroying the disks -- ultimately creating the comet-shaped clouds of glowing gas seen engulfing the protoplanetary systems. Planet formation must occur quickly here, if at all. Researchers estimate that about 90 percent of Orion's youngest protoplanetary disks will not survive the next 100,000 years.