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 832c: The Closest Potentially Habitable Exoplanet
9 July 2014
This planet is only 16 light years away -- could it harbor life? Recently discovered exoplanet Gliese 832c has been found in a close orbit around a star that is less bright than our Sun. An interesting coincidence, however, is that Gliese 832c receives just about the same average flux from its parent star as does the Earth. Since the planet was discovered only by a slight wobble in its parent star's motion, the above illustration is just an artistic guess of the planet's appearance -- much remains unknown about Gliese 832c's true mass, size, and atmosphere. If Gliese 832c has an atmosphere like Earth, it may be a super-Earth undergoing strong seasons but capable of supporting life. Alternatively, if Gliese 832c has a thick atmosphere like Venus, it may be a super-Venus and so unlikely to support life as we know it. The close 16-light year distance makes the Gliese 832 planetary system currently the nearest to Earth that could potentially support life. The proximity of the Gliese 832 system therefore lends itself to more detailed future examination and, in the most spectacularly optimistic scenario, actual communication -- were intelligent life found there.