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.
9 April 2014
Asteroids can have rings. In a surprising discovery announced two weeks ago, the distant asteroid 10199 Chariklo was found to have at least two orbiting rings. Chariklo's diameter of about 250 kilometers makes it the largest of the measured centaur asteroids, but now the smallest known object to have rings. The centaur-class minor planet orbits the Sun between Saturn and Uranus. The above video gives an artist's illustration of how the rings were discovered. As Chariklo passed in 2013 in front of a faint star, unexpected but symmetric dips in the brightness of the star revealed the rings. Planetary astronomers are now running computer simulations designed to investigate how Chariklo's unexpected ring system might have formed, how it survives, and given the asteroid's low mass and close passes of other small asteroids and the planet Uranus, how long it may last.