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"
Ghosts and Star Trails
31 October 2015
Don't be scared. Stars won't fall from the sky and ghosts won't really haunt your neighborhood tonight. But it looks like they might be doing just that in this eerie picture of an eccentric old abandoned house in moonlight. A treat for the eye the image is a trick of stacked multiple exposures, 60 frames exposed for 25 seconds each. While the digital frames were recorded with a camera fixed to a tripod, stars traced concentric arcs about the north celestial pole. But that's only a reflection of planet Earth's rotation on its axis. Conveniently marked by bright star Polaris, the pole could be positioned above the peaks of the deserted dwelling. Wrapped in a blanket to stay warm, the photographer's own movements during the exposures were blended into the ghostly apparitions. Of course, the grinning Jack-o-Lantern is there to wish you a safe and Happy Halloween!