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"
Star Trails and Lightning over the Pyrenees
19 September 2022
The beauty in this image comes in layers. On the bottom layer is the picturesque village of Manlleu in Barcelona, Spain. The six-minute exposure makes car lights into streaks. The next layer is a mountain -- Serra de Bellmunt -- of Europe's famous Pyrenees. Next up is a tremendous lightning storm emanating from a classically-shaped anvil cloud. The long exposure allowed for the capture of many intricate lightning bolts. Finally, at the top and furthest in the distance are stars. Here, the multi-minute exposure made stars into trails. The trailing effect is caused by the rotation of the Earth, and the curvature of the trails indicates their distance from the north spin pole of the Earth above. Taken after sunset in early June, the lightning storm soon moved off. The stars, though, will continue to circle the pole for as long as the Earth spins -- surely billions of years into the future.