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.
29 March 2016
Point or tilt to see a spectacular view of Mars visible to the Curiosity rover last December. In the foreground, part of Curiosity itself is visible, including its dusty sundial. Starting about seven meters back, the robotic rover is seen posing in front of a 5-meter tall dark sand dune named Namib, one of many dunes that span Bagnold field. Further in the distance is the summit of Mt. Sharp, the 5.5-kilometer peak at the center of 150-km wide Gale crater, the crater where Curiosity landed a few years ago. The featured composite spans a full 360-degrees around by combining several images taken on the same day, while the result has been color adjusted to mimic Earth lighting. Most recently, Curiosity is crossing the rocky and uneven Naukluft Plateau as it continues to make its way around and up Mt. Sharp. Note: If your browser does not support YouTube 360 panoramas, see a cool static version of the image here.