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"
Where Your Shadow Has Company
20 February 2016
Want to take a relaxing interstellar vacation? Consider visiting Kepler-16b, a world in a binary star system. In fact Kepler-16b is the first discovered circumbinary planet. It was detected in a wide 229 day orbit around a close pair of cool, low-mass stars some 200 light-years away. The parent stars eclipse one another in their orbits, observed as a periodic dimming of starlight. But Kepler-16b itself was discovered by following the additional very slight dimming produced during its transits. Like sci-fi planet Tatooine of Star Wars fame, two suns would set over its horizon. Still, Kepler-16b is probably not a Tatooine-like terrestrial desert world. Instead, Kepler-16b is thought to be a cold, uninhabitable planet with about the mass of Saturn and a gaseous surface ... so plan to dress accordingly. Or, choose another Visions of the Future vacation destination.