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 Dark Winter Sky over Monfrag�e National Park in Spain
14 March 2017
You, too, can see a night sky like this. That is because Monfrag�e National Park in Spain, where this composite image was created, has recently had its night sky officially protected from potential future light pollution. Icons of the night sky that should continue to stand out during northern winter -- and are visible on the featured image -- include very bright stars like Sirius, Betelgeuse, and Procyon, bright star clusters like the Pleiades, and, photographically, faint nebulas like the California and Rosette Nebulas. Even 100 years ago, many people were more familiar with a darker night sky than people today, primarily because of the modern light pollution. Other parks that have been similarly protected as dark-sky preserves include Death Valley National Park (USA) and Grasslands National Park (Canada). Areas such as the city of Flagstaff, Arizona and much of the Big Island of Hawaii also have their night skies protected.