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 X-Ray Sky from eROSITA
23 June 2020
What if you could see X-rays? The night sky would seem a strange and unfamiliar place. X-rays are about 1,000 times more energetic than visible light photons and are produced by violent explosions and high temperature astronomical environments. Instead of the familiar steady stars, the sky would seem to be filled with exotic stars, active galaxies, and hot supernova remnants. The featured X-ray image captures in unprecedented detail the entire sky in X-rays as seen by the eROSITA telescope onboard Spektr-RG satellite, orbiting around the L2 point of the Sun-Earth system, launched last year. The image shows the plane of our Milky Way galaxy across the center, a diffuse and pervasive X-ray background, the hot interstellar bubble known as the North Polar Spur, sizzling supernova remnants such as Vela, the Cygnus Loop and Cas A, energetic binary stars including Cyg X-1 and Cyg X-2, the LMC galaxy, and the Coma, Virgo, and Fornax clusters of galaxies. This first sky scan by eROSITA located over one million X-ray sources, some of which are not understood and will surely be topics for future research.