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"
Galaxy Zoo Catalogs the Universe
26 October 2009
You, too, can Zoo. The Galaxy Zoo project has been enabling citizen scientists -- inquisitive people like yourself armed with only a web browser-- to sort through the universe. Specifically, after a brief training session, volunteers are asked to use the superior image-processing power of their minds to classify and measure properties of galaxies in the vast Sloan Digital Sky Survey. In its two short years of existence, millions of galaxies have already been inspected by thousands of enthusiastic volunteers. Using Galaxy Zoo data, for example, the universe has been discovered to create no preferred spin direction, an unusual and unclassified object was found that is still being investigated, and a whole class of small galaxies dubbed Green Peas were uncovered where star formation occurs at an extraordinarily high rate. Further, the Galaxy Zoo may be setting a precedent for a new type of scientific inquiry where the web helps collect, focus and coordinate human and machine intelligence. Pictured above, a group of vibrant mergers found by Zooites demonstrates the diverse zoo-like nature of many interacting galaxies in the universe.