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"
Space Shuttle Tribute Poster: Endeavour
7 September 2010
They are some of the most complex machines ever built. From a standing start they can launch a school- bus sized object up so high and moving so fast that it won't fall back down. They have launched numerous revolutionary satellites that enable humans to communicate across the globe, to better understand Earth's atmosphere, and to peer into the distance universe. They are NASA's Space Shuttles, and NASA has recently released large digital posters to honor them. While the inaugural flight was in 1981, the shuttle fleet is aging and is now nearing retirement. Pictured above, the space shuttle Endeavour is shown rising to orbit, with patches for each of its missions shown in a spiral. Endeavour was named for the HMS Endeavour, a British research ship that explored the south Pacific Ocean in the 1700s, depicted on the lower right. On the upper left are panoramic windows delivered by Endeavour to the International Space Station earlier this year. In the background near the top is the NGC 602 nebula as imaged by the Hubble Space Telescope, which was serviced by Endeavour in 1993. Posters for all of the shuttles, including Atlantis, Challenger, Columbia, Discovery, Endeavour are available.