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"
Stephan's Quintet from Hubble
3 June 2019
When did these big galaxies first begin to dance? Really only four of the five of Stephan's Quintet are locked in a cosmic tango of repeated close encounters taking place some 300 million light-years away. The odd galaxy out is easy to spot in this recently reprocessed image by the Hubble Space Telescope -- the interacting galaxies, NGC 7319, 7318B, 7318A, and 7317 (left to right), have a more dominant yellowish cast. They also tend to have distorted loops and tails, grown under the influence of disruptive gravitational tides. The mostly bluish galaxy, large NGC 7320 on the lower left, is in the foreground at about 40 million light-years distant, and so is not part of the interacting group. Data and modeling indicate that NGC 7318B is a relatively new intruder. A recently-discovered halo of old red stars surrounding Stephan's Quintet indicate that at least some of these galaxies started tangling over a billion years. Stephan's Quintet is visible with a moderate sized-telescope toward the constellation of Winged Horse (Pegasus).