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 Furious Sky over Mount Shasta
28 September 2022
Is the sky angry with Mount Shasta? According to some ancient legends, the spirits of above and below worlds fight there, sometimes quite actively during eruptions of this enormous volcano in California, USA. Such drama can well be imagined in this deep sky image taken in late June. Evident above the snow-covered peak is the central band of our Milky Way Galaxy, on the left, and a picturesque sky toward the modern constellations of Scorpius and Ophiuchus, above and to the right. The bright orange star Antares and the colorful rho Ophiuchi cloud complex are visible just to the right of Mount Shasta, while the red emission nebula surrounding the star zeta Ophiuchi appears on the top right. The static earth image in the featured composite was taken during the blue hour, while a two-panel panorama tracking the background sky was taken later that night with the same camera and from the same location. Within a few million years, Antares, some stars in the rho Ophiuchi system, and zeta Ophiuchi will all likely explode as supernovas.