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"
M82's Middle Mass Black Hole
14 September 2000
Black holes are probably the most bizarre creatures in the modern astronomical zoo. And after years of pondering black holes as either stellar mass objects seen in binary star systems or enormous supermassive black holes at the centers of galaxies, astronomers now have strong evidence for another exotic species -- middle mass black holes. The leading candidate for the ultradense middle ground is indicated in this false-color detail of a sharp x-ray picture from the space-based Chandra Observatory. A close-up of x-ray sources near the center of starburst galaxy M82, the cropped Chandra image spans about 4,000 light-years. M82 itself is around 11 million light-years distant. The arrowed source has recently been convincingly demonstrated to exhibit x-ray characteristics of an object whose gravitational field holds more than 500 times the mass of the sun within a volume the size of the moon! Astronomers also note that unlike the supermassive variety which are thought to lie at the centers of galaxies, this middle mass black hole is about 600 light-years from the center of M82. Theories for the formation of a middle mass black hole include the collapse of a "hyperstar" formed by the coalescence of many normal stars, or the direct merger of stellar mass black holes.