Star
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.
Sources
APODs including "Star"
M41: The Little Beehive Star Cluster
25 February 2025
Why are there so many bright blue stars? Stars are usually born in clusters, and the brightest and most massive of these stars typically glow blue. Less-bright, non-blue stars like our Sun surely also exist in this M41 star cluster but are harder to see. A few bright orange-appearing red giant stars are visible. The red-light filaments are emitted by diffuse hydrogen gas, a color that was specifically filtered and enhanced in this image. In a hundred million years or so, the bright blue stars will have exploded in supernovas and disappeared, while the slightly different trajectories of the fainter stars will cause this picturesque open cluster to disperse. Similarly, billions of years ago, our own Sun was likely born into a star cluster like M41, but it has long since drifted apart from its sister stars. The featured image was captured over four hours with Chilescope T2 in Chile.