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"
The Four Suns of HD 98800
30 July 2007
How would it look to have four suns in the sky? Planets of the HD 98800 system, if they exist, would experience such a view. HD 98800 is a multiple star system about 150 light years from Earth -- right in our section of the Milky Way Galaxy. For years it has been known that HD 98800 consists of two pairs of double stars, with one pair surrounded by a disk of dust. The star pairs are located about 50 AU from each other -- in comparison just outside the orbit of Pluto. Recent data from the Earth-trailing Spitzer Space Telescope in infrared light, however, indicate that the dust disk has gaps that appear consistent with being cleared by planets orbiting in the disk. If so, one planet appears to be orbiting at a distance similar to Mars of our own Solar System. Pictured above is an artist's drawing of how the HD 98800 system might appear to a nearby observer.