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"
NGC 4651: The Umbrella Galaxy
2 July 2014
Spiral galaxy NGC 4651 is a mere 62 million light-years distant, toward the well-groomed northern constellation Coma Berenices. About the size of our Milky Way, this island universe is seen to have a faint umbrella-shaped structure that seems to extend (left) some 100 thousand light-years beyond the bright galactic disk. The giant cosmic umbrella is now known to be composed of tidal star streams - extensive trails of stars gravitationally stripped from a smaller satellite galaxy. The small galaxy was eventually torn apart in repeated encounters as it swept back and forth on eccentric orbits through NGC 4651. In fact, the picture insert zooms in on the smaller galaxy's remnant core, identified in an extensive exploration of the system, using data from the large Subaru and Keck telescopes on Mauna Kea. Work begun by a remarkable collaboration of amateur and professional astronomers to image faint structures around bright galaxies suggests that even in nearby galaxies, tidal star streams are common markers of such galactic mergers. The result is explained by models of galaxy formation that also apply to our own Milky Way.