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"
Milky Way Over Piton de l'Eau
25 June 2012
Sometimes, if you wait long enough for a clear and moonless night, the stars will come out with a vengeance. One such occasion occurred earlier this month at the Piton de l'Eau on Reunion Island. In the foreground, surrounded by bushes and trees, lies a water filled volcanic crater serenely reflecting starlight. A careful inspection near the image center will locate Piton des Neiges, the highest peak on the island, situated several kilometers away. In the background, high above the lake, shines the light of hundreds of stars, most of which are within 100 light years, right in our stellar neighborhood. Far in the distance, arching majestically overhead, is the central band of our home Milky Way Galaxy, shining by the light of millions of stars each located typically thousands of light years away. The astrophotographer reports waiting for nearly two years for the sky and clouds to be just right to get the above shot. Help Evaluate APOD: Has viewing APOD increased your interest in NASA?