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"
Gliese 876 System Includes Large Terrestrial Planet
14 June 2005
Is our Earth unique? In continuing efforts to answer this question, astronomers have now discovered an Earth-like planet orbiting a distant normal star. Previously over 150 gas-giant planets like Jupiter had been so discovered. Slight, fast, but regular wobbles of nearby small M-dwarf star Gliese 876 showed evidence for a planet with a likely mass slightly higher than a minimum six times the mass of Earth. The planet's small mass indicates that it is likely terrestrial in nature, similar in composition to the inner planets of our Solar System. If indeed made predominantly of rock, the planet's surface gravity would not even be able to contain the gasses of a Jupiter-like planet. The newly discovered planet would not make a good vacation spot for humans, however, as it orbits so close that the surface temperature probably tops a searing 200 degrees Celsius. The system is illustrated in the above drawing as seen from a hypothetical moon orbiting one of the two Jupiter-like planets already known. The newly discovered terrestrial-like planet is depicted in the insert. Gliese 876 lies only 15 light-years away and is visible with binoculars toward the constellation of Aquarius.