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.
21 August 2026
What does it feel like to zoom around a spinning supermassive black hole? The featured video is an animation showing a time-lapse of 4 years of observations of the star S301 orbiting Sagittarius A*, the 4-million-solar-mass black hole in the center of our Galaxy. S301 was discovered in 2023 with the Very Large Telescope Interferometer of the European Southern Observatory, located in the Atacama Desert in Chile. Astronomers recently found that S301 takes approximately 8.7 years to go around the black hole and reaches speeds of 25,000 km/s. It comes closer to the black hole than any other stars detected before, at a distance similar to that from Saturn to the Sun. Because it comes so close to Sagittarius A*, S301 could be used to directly measure how fast the black hole spins and test Einstein's theory of general relativity. According to the theory, a spinning black hole drags the fabric of spacetime around itself, affecting the orbits of close-by objects.