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"
Andromeda in Southern Skies
21 October 2022
Looking north from southern New Zealand, the Andromeda Galaxy never gets more than about five degrees above the horizon. As spring comes to the southern hemisphere, in late September Andromeda is highest in the sky around midnight though. In a single 30 second exposure this telephoto image tracked the stars to capture the closest large spiral galaxy from Mount John Observatory as it climbed just over the rugged peaks of the south island's Southern Alps. In the foreground, stars are reflected in the still waters of Lake Alexandrina. Also known as M31, the Andromeda Galaxy is one of the brightest objects in the Messier catalog, usually visible to the unaided eye as a small, faint, fuzzy patch. But this clear, dark sky and long exposure reveal the galaxy's greater extent in planet Earth's night, spanning nearly 6 full moons.