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 Cotton Candy Clouds of Rho Ophiuchi
1 July 2026
Although they look like cotton candy, you cannot eat these clouds! Taken in Cádiz, Spain, today's image features the Rho Ophiuchi complex, a rich tapestry of young and old astronomical phenomena. This colorful cloud complex is a nearby star-forming region containing hundreds of young stellar objects, including protostars and T Tauri stars. Light from the triple star system at its center reflects off of small dust grains to create the blue reflection nebula. Ultraviolet light from hot stars ionizes the surrounding hydrogen gas, creating the red emission nebula. Antares, a red supergiant big enough to engulf the Solar System’s asteroid belt, lights up the yellow region. Dark interstellar dust blocks some of the complex’s color. Recent JWST observations exhibit shadows cast by hidden circumstellar disks, the beginning stages of planet formation. Messier 4, a globular cluster almost as old as the universe, sits in the bottom right and witnesses yet another chaotic burst of youth in the Milky Way.