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"
Leo Triplet Spiral Galaxy M65
12 August 1996
Spiral galaxy M65 is a normal spiral galaxy not unlike our own Milky Way. In fact, M65 is a typical spiral galaxy of a type that could be found anywhere in the local universe. Given a morphological type of "Sa", M65 shows tightly wrapped spiral arms and a large nuclear central bulge. The central bulge stars are older and redder than disk stars, which appear more blue. Stars in the bulge of the our own Milky Way Galaxy are also typically older and redder than stars in the disk where our Sun resides. M65 is a member of the Leo Triplet of galaxies, along with its neighbors M66 and NGC 3628. Although it appears that M65's gravity has distorted M66's symmetry, M65's symmetry seems unaffected by M66. M65 is located roughly 35 million light years away, so that light recorded today left after the fall of the dinosaurs but when many land mammals were just evolving on Earth.