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 Cross Star Colors
8 July 2004
Fix your camera to a tripod, lock the shutter open, and you can easily record an image of star trails, the graceful concentric arcs traced by the stars as planet Earth rotates on its axis. Gradually change the focus of the camera lens during the exposure, and you could end up with a dramatic picture like this one where the out-of-focus portion of the trail shows off the star's color. In this case, the subject is one of the most famous constellations in the night sky, Crux, the Southern Cross. Gacrux or gamma Crucis is the bright red giant star only 88 light-years distant that forms the top of the Cross seen here near top center. Acrux, the hot blue star at the bottom of the Cross is about 320 light-years distant. Actually a binary star system, Acrux is the alpha star of the compact Southern Cross and lies along a line pointing from Gacrux to the South Celestial Pole, off the lower right edge of the picture. Adding a separate short exposure to the end of the step-focussed trails to better show the positions of the stars themselves, astronomer Stefan Seip recorded this remarkable image last May in the dark night skies above Namibia.