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"
Little Planet Lovejoy
11 January 2012
Once a bright apparition in the southern hemisphere dawn Comet Lovejoy is fading, but its long tail still stretches across skies near the south celestial pole. Captured on the morning of December 30th, the comet appears near edge of this little planet as well. Of course, the little planet is actually planet Earth and the image was created from a 12 frame mosaic used to construct a spherical panorama. The type of stereographic projection used to map the image pixels is centered directly below the camera and is known as the little planet projection. Stars surrounding this little planet were above the photographer's cloudy horizon near the Bay of Islands on the Great Ocean Road in southern Victoria, Australia. Running alongside the Milky Way the comet can be identified, with other celestial highlights, by putting your cursor over the picture. Very bright stars Canopus and Sirius are right of the little planet.