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"
X-Rays From The Galactic Center
20 January 2000
ring quasars and active galaxies in the distant universe, astronomers have come to believe that most galaxies have massive black holes at their centers. Swirling stars and a strong, variable radio source offer convincing evidence that even our own Milky Way galaxy's center harbors such a bizarre object, a mere 30,000 light-years away. Still, it has long been realized that if a massive black hole lurks there it should produce X-rays - which have not previously been identified. Now, though relatively faint, the missing X-ray source may have been found. Taking advantage of the sensitive Chandra Observatory astronomers have recorded this false-color X-ray image of the Galactic Center. Embedded in a diffuse cloud of X-ray hot gas, the white dot at the center corresponds to an X-ray source at exactly the position of the strong radio source and suspected black hole. Other individual X-ray sources are also present in the picture which spans about 10 light-years at the distance of the galactic center. With radio and X-ray emission generated by infalling material, the Milky Way's central black hole is thought to have a mass of over 2 million suns. News: Lunar Eclipse Tonight!