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"
An Extrasolar Planet?
29 May 1998
This infrared Hubble Space Telescope view may contain the first ever direct image of a planet outside our own solar system. The picture shows a very young double star located about 450 light-years away toward the constellation of Taurus. Cataloged as TMR-1 (Taurus Molecular Ring star 1), the binary system is still embedded in the dust cloud that formed it. This double star and dust cloud are the brightest grouping in the picture, glowing strongly at infrared wavelengths. A filament extends from the binary system toward the lower left and points toward the spot of light representing the candidate planet. Astronomers believe this planet is a "runaway" object which was gravitationally ejected, the filament tracing the path to its present location at about 1500 times the Earth-Sun distance from the parent star system. Models suggest that the planet and binary system are a mere 300,000 years old, with the planet having a mass of about 2 to 3 Jupiters. Future observations to look for the planet's continued runaway motion and spectral signatures should be able to confirm the nature of this object. While this and other tantalizing discoveries of extrasolar planetary objects and protoplanetary disks don't seem to offer direct examples of solar systems like our own, they do strongly hint that planet formation is a varied and common process. Update: TMR-1 likely just a background star.