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"
Southern Craters and Galaxies
26 December 2015
The Henbury craters in the Northern Territory, Australia, planet Earth, are the scars of an impact over 4,000 years old. When an ancient meteorite fragmented into dozens of pieces, the largest made the 180 meter diameter crater whose weathered walls and floor are lit in the foreground of this southern hemisphere nightscape. The vertical panoramic view follows our magnificent Milky Way galaxy stretching above horizon, its rich central starfields cut by obscuring dust clouds. A glance along the galactic plane also reveals Alpha and Beta Centauri and the stars of the Southern Cross. Captured in the region's spectacular, dark skies, the Small Magellanic Cloud, satellite of the Milky Way, is the bright galaxy to the left. Not the lights of a nearby town, the visible glow on the horizon below it is the Large Magellanic Cloud rising.