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 Pleiades: Seven Dusty Sisters
29 January 2024
The well-known Pleiades star cluster is slowly destroying part of a passing cloud of gas and dust. The Pleiades is the brightest open cluster of stars on Earth's sky and can be seen from almost any northerly location with the unaided eye. Over the past 100,000 years, a field of gas and dust is moving by chance right through the Pleiades star cluster and is causing a strong reaction between the stars and dust. The passing cloud might be part of the Radcliffe wave, a newly discovered structure of gas and dust connecting several regions of star formation in the nearby part of our Milky Way galaxy. Pressure from the stars' light significantly repels the dust in the surrounding blue reflection nebula, with smaller dust particles being repelled more strongly. A short-term result is that parts of the dust cloud have become filamentary and stratified. The featured deep image incorporates nearly 9 hours of exposure and was captured from Utah Desert Remote Observatory in Utah, USA, last year.