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 Protostar within L1527
18 November 2022
The protostar within dark cloud L1527 is a mere 100,000 years old, still embedded in the cloud of gas and dust that feeds its growth. In this NIRCam image from the James Webb Space Telescope, the dark band at the neck of the infrared nebula is a thick disk that surrounds the young stellar object. Viewed nearly edge-on and a little larger than our Solar System, the disk ultimately supplies material to the protostar while hiding it from Webb's direct infrared view. The nebula itself is seen in stunning detail though. Illuminated by infrared light from the protostar, the hourglass-shaped nebula's cavities are created as material ejected in the star-forming process plows through the surrounding medium. As the protostar gains mass it will eventually become a full-fledged star, collapsing and igniting nuclear fusion in its core. A likely analog to our own Sun and Solar System in their early infancy, the protostar within dark cloud L1527 lies some 460 light-years distant in the Taurus star-forming region. Webb's NIRCam image spans about 0.3 light-years.