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"
Zooming in on the First Stars
10 June 2003
What became of the first stars? No known stars appear to be composed of truly primordial gas -- all of the stars around us have too many heavy elements. Our own Sun is thought to be a third generation star, with many second-generation stars seen in globular clusters. This year, however, significant progress is being made on solving this perennial astronomical mystery. Analyses of recent WMAP satellite images of the cosmic microwave background indicate that this primordial light was ionized by a first generation of stars that came and went only 200 million years after the Big Bang. Additionally computer codes are now more-accurately tracking the likely creation and evolution of first stars in the early universe. Pictured above at a scale of one light-month, a computer-generated model resolves the scale of the first stars, indicating clean cocoons that condensed into stars always over 30 times the mass of our Sun. Stars like this quickly fused pristine gas into heavier elements and then exploded, seeding the universe with elements that would become part of the stars we know and, ultimately, ourselves.