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"
Far Side of the Sun
3 May 2001
You may think it's impossible to see through the Sun, but maps of the Sun's far side are now made routinely by instruments on board the sun-staring SOHO spacecraft. This is one such map from April 12. At right, is a map projection of calculated magnetic field strengths on the Earth-facing solar hemisphere with yellow and red indicating high magnetic fields characteristic of solar active regions. At left is a similar map of the solar hemisphere opposite planet Earth, which shows the large active region AR9393 as the 27-day solar rotation carried it across the far side. The largest sunspot group in a decade, AR9393 was easily seen as it tracked across the Sun's Earth-facing hemisphere in late March. When AR9393 swung around to the Sun's far side, SOHO's Michelson Doppler Interferometer (MDI) instrument continued to map its position by measuring changes in motions caused by solar sound waves - transmitted through the Sun and influenced by the active region's strong magnetic fields. Known as helioseismology, analyzing solar sound waves is like using seismological records of earthquakes to probe the interior of the Earth. On the Sun, sound waves are produced by turbulent convection cells seen on the surface as dynamic solar granules.