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"
AM1054: Stars Form as Galaxies Collide
20 February 2024
When galaxies collide, how many stars are born? For AM1054-325, featured here in a recently released image by the Hubble Space Telescope, the answer is millions. Instead of stars being destroyed as galaxy AM1054-325 and a nearby galaxy circle each other, their gravity and motion has ignited stellar creation. Star formation occurs rapidly in the gaseous debris stretching from AM1054-325’s yellowish body due to the other galaxy’s gravitational pull. Hydrogen gas surrounding newborn stars glows pink. Bright infant stars shine blue and cluster together in compact nurseries of thousands to millions of stars. AM1054-325 possesses over 100 of these intense-blue, dot-like star clusters, some appearing like a string of pearls. Analyzing ultraviolet light helped determine that most of these stars are less than 10 million years old: stellar babies. Many of these nurseries may grow up to be globular star clusters, while the bundle of young stars at the bottom tip may even detach and form a small galaxy.