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.
31 August 2026
A new telescope has been launched into space to study the universe. The Nancy Grace Roman Space Telescope (RST) has the same size main mirror as the Hubble Space Telescope (HST) but sees 100 times more sky during each snapshot. This is possible because when compared to HST, RST's main mirror is more curved, its secondary mirror is closer, and its main camera is larger. The result is that RST can inspect more of the sky more quickly, likely allowing, among other capabilities, the discovery of many more supernovas which tell us more about the expansion rate and composition of our universe, and many more planets orbiting other stars that tell us more about the possibilities for life elsewhere in the universe. RST will orbit the Sun, not the Earth, like the James Webb Space Telescope. The featured video shows Roman being launched yesterday from Kennedy Space Center, Florida, USA aboard a SpaceX Falcon Heavy rocket. APOD's main NASA site is moving: From apod.nasa.gov to science.nasa.gov/apod