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"
A Southern Sky View
10 March 2002
On 1996 March 22, a Galaxy and a comet shared the southern sky. They were captured together, from horizon to horizon, in the night sky above Loomberah, New South Wales, Australia by astronomer Gordon Garradd. Garradd used a home made all-sky camera with a fisheye lens, resulting in a circular 200 degree field of view. This gorgeous sky view was dominated by the luminous band of our Milky Way Galaxy cut by dramatic, dark interstellar dust clouds. Along with the bright stars of our Galaxy, the Large Magellanic Cloud is visible at the lower left. That night sky was also graced by the long, lovely, bluish tail of Comet Hyakutake, which can be seen toward the top of the image, near the bright star Arcturus. Bright city lights from nearby Tamworth glow along the Northwestern horizon.