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"
A Late Leonid from a Sparse Shower
25 November 2003
The 2003 Leonids Meteor Shower contained relatively few meteors. As expected and unlike the last few years, the Earth just did not pass through any dense particle streams left over by the Sun-orbiting Comet Tempel-Tuttle. Preliminary reports had the peak meteor rates only as high as about one relatively faint meteor a minute even from good locations at good times. Pictured above is one of the brighter Leonids of 2003, caught by one of the continuously operating night sky web cameras (CONCAMs) of the global Night Sky Live project. The fisheye image shows the night sky from horizon to horizon above Mauna Kea, Hawaii, USA. The image is annotated with several bright stars and planets. Note that this meteor, as do all Leonids, appears to emanate from the constellation Leo, labeled on the upper left. Although the peak of the Leonids this year was on November 19, this meteor flashed through the sky the next night.