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.
1 November 2018
Will spacecraft Hayabusa2 be able to land safely on asteroid Ryugu? Since arriving in June, pictures show that the surface of kilometer-sized Ryugu is covered with boulders, so that finding a flat enough area for the bus-sized spacecraft to touch down is proving a challenge. In the featured video, the shadow of Japan's robotic Hayabusa2 can be seen on the rugged face of Ryugu while ascending last week from a touchdown rehearsal only 20 meters over the surface. Previously, small frisbee-sized landers detached from Hayabusa2, made contact with the diamond-shaped asteroid's surface, and started hopping around. Studying Ryugu could tell humanity not only about the minor planet's surface and interior, but about what materials were available in the early Solar System for the development of life. The touchdown of the Hayabusa2 mother ship is slated for early next year, hopefully followed by a soil sample collection for return to Earth.