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"
Dunhuang Star Atlas
19 June 2009
This ancient Chinese map of planet Earth's northern sky is part of the Dunhuang Star Atlas, one of the most impressive documents in the history of astronomy. The oldest complete star atlas known, it dates to the years 649 to 684, discovered at the Silk Road town of Dunhuang in 1907. A recent analysis that examines the accuracy and projections used to make it notes the atlas marks positions of over 1,300 stars and outlines 257 Chinese star groups or asterisms. The star positions in the hand drawn atlas were found to be accurate to within a few degrees. In this example showing the north polar region, a very recognizable Big Dipper, part of the modern constellation Ursa Major, lies along the bottom of the chart. An additional 12 charts depicting equatorial regions in 30 degree sections also includes a group resembling the modern constellation Orion. The atlas is on display at the British Library in London to celebrate the International Year of Astronomy.