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"
Starry Night by Jean-Francois Millet
13 March 2020
A dramatic nocturnal landscape from around 1850, this oil painting is the work of French artist Jean-Francois Millet. In the dark and atmospheric night sky are shooting stars, known too as meteors, above a landscape showing a path through the faintly lit countryside that leads toward trees and a cart in silhouette on the horizon. Millet was raised in a farming family in Normandy and is known for his paintings of rural scenes and peasant life. This Starry Night was painted after the artist moved to Barbizon, about 30 kilometers southeast of any 19th century light pollution from Paris. Millet wrote to his brother at this time, "If only you knew how beautiful the night is ... the calm and grandeur of it are so awesome that I find that I actually feel overwhelmed." Dutch artist Vincent van Gogh was an admirer of Millet's work, and later also painted two dramatic starry nights.