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.
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APODs including "Star"
GRB 221009A
15 October 2022
Gamma-ray burst GRB 221009A likely signals the birth of a new black hole, formed at the core of a collapsing star long ago in the distant universe. The extremely powerful blast is depicted in this animated gif constructed using data from the Fermi Gamma Ray Space Telescope. Fermi captured the data at gamma-ray energies, detecting photons with over 100 million electron volts. In comparison visible light photons have energies of about 2 electron volts. A steady, high energy gamma-ray glow from the plane of our Milky Way galaxy runs diagonally through the 20 degree wide frame at the left, while the transient gamma-ray flash from GRB 221009A appears at center and then fades. One of the brightest gamma-ray bursts ever detected GRB 221009A is also close as far as gamma-ray bursts go, but still lies about 2 billion light-years away. In low Earth orbit Fermi’s Large Area Telescope recorded gamma-ray photons from the burst for more than 10 hours as high-energy radiation from GRB 221009A swept over planet Earth last Sunday, October 9.