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"
Fermi's Gamma-ray Pulsars
9 July 2009
Born in supernovae, pulsars are spinning neutron stars, collapsed stellar cores left from the death explosions of massive stars. Traditionally identified and studied by observing their regular radio pulsations, two dozen pulsars have now been detected at extreme gamma-ray energies by the Fermi Gamma-ray Space Telescope. The detections include 16 pulsars identified by their pulsed gamma-ray emission alone. This gamma-ray all-sky map, aligned with the plane of our Milky Way Galaxy, shows the pulsar positions, with the 16 new Fermi pulsars circled in yellow (8 previously known radio pulsars are in magenta). Bizarre stellar corpses, the Vela, Crab, and Geminga pulsars on the right are the brightest ones in the gamma-ray sky. Pulsars Taz, Eel, and Rabbit are named for the nebulae they are now known to power. The Gamma Cygni and CTA 1 pulsars at the left also reside within expanding supernova remnants of the same name.