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"
Help Search for Interstellar Dust
25 October 2006
What caused that hole? The hole in question appears as a small dark circle on the far right. If the above image of aerogel seems dull and uninteresting, then welcome to one aspect of real world science. The interesting part is that something created that dark hole, and it might well be one of the first pieces of matter ever captured from outside our Solar System. Whatever created that hole was captured by the aerogel of the robotic Stardust spacecraft that flew across our Solar System for years and then returned a capsule to Earth. Scientists are now poring over the aerogel, looking to see what particles have become trapped. Many particles are surely from local Comet Wild 2, which Stardust flew past in 2004. Just a few particles, though, perhaps 10 or less, are expected to be from outside our Solar System. It is so difficult to find them that the Stardust team has created a downloadable interactive microscope program to allow anyone with a standard computer to help inspect aerogel slices and look for interstellar dust tracks. Good candidate tracks will later be inspected in great detail by members of the Stardust team.