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"
The Magellanic Stream
25 January 2010
Spanning the sky toward the majestic Clouds of Magellan is an unusual stream of gas: the Magellanic Stream. The origin of this gas remains unknown but likely holds a clue to the origin and fate of our Milky Way's most famous satellite galaxies: the LMC and the SMC. Until recently, two leading genesis hypotheses have been considered: that the stream was created by gas stripped off these galaxies as they passed through the halo of our Milky Way, or that the stream was created by the differential gravitational tug of the Milky Way. Recently, however, wide angle radio images -- including those from the Byrd Green Bank Telescope -- have shown that the Magellanic Stream is longer and older than previously thought, perhaps as old as 2.5 billion years. These observations bolster a third possible origin for the stream -- that the Large and Small Magellanic Cloud galaxies once passed so close to each other that gravitational tides triggered a burst of star formation that left the stream. Pictured above digitally superposed on a recently-completed all-sky image in visible light, the radio emission of Magellanic stream is shown in false color pink extending across the sky and ending at the two Magellanic galaxies on the lower right. New Mirror: APOD now available in Hebrew from Israel