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"
A Hotspot Map of Neutron Star J0030's Surface
18 December 2019
What do neutron stars look like? Previously these city-sized stars were too small and too far away to resolve. Recently, however, the first maps of the locations and sizes of hotspots on a neutron star's surface have been made by carefully modeling how the rapid spin makes the star's X-ray brightness rise and fall. Based on a leading model, an illustrative map of pulsar J0030+0451's hotspots is pictured, with the rest of the star's surface filled in with a false patchy blue. J0030 spins once every 0.0049 seconds and is located about 1000 light years away. The map was computed from data taken by NASA's Neutron star Interior Composition ExploreR (NICER) X-ray telescope attached to the International Space Station. The computed locations of these hotspots is surprising and not well understood. Because the gravitational lensing effect of neutron stars is so strong, J0300 displays more than half of its surface toward the Earth. Studying the appearance of pulsars like J0030 allows accurate estimates of the neutron star's mass, radius, and the internal physics that keeps the star from imploding into a black hole.