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"
Atlantis over Rhodes
28 May 2010
A moonlit chapel stands in the foreground of this night-scape from the historic Greek island of Rhodes. The tantalizing sky above features a colorful lunar corona, where bright moonlight is diffracted by water droplets in the thin clouds drifting in front of the lunar disk. Captured in the early evening on May 17, the image is a composite of 9 exposures in sequence, each 20 seconds long. It shows star trails too, including the very bright trail of planet Venus setting, below the Moon, near the right edge of the frame. Arcing from the horizon on the right to the picture's left edge is a surprisingly colorful trail produced by space shuttle orbiter Atlantis docked with the International Space Station. Some 350 kilometers above Earth's surface, the orbiter and station pair are still bathed in sunlight. Though it might seem more appropriate when seen in skies over Rhodes, the shuttle orbiter Atlantis wasn't directly named for the legendary island of Atlantis. Instead, it was named for 1930s vintage sailing ship RV Atlantis, the first research vessel operated by the Woods Hole Oceanographic Institute.