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"
Cold Dust in the Eagle Nebula
14 September 2001
Stars are born in M16's Eagle Nebula, a stellar nursery 7,000 light-years from Earth toward the constellation Serpens. The striking nebula's star forming pillars of gas and dust are familiar to astronomers from images at visible wavelengths, but this false-color picture shows off the nebula in infrared light. Data from ESA's Infrared Space Observatory satellite (ISO) was used to construct the detailed two color image, dominated by infrared emission from clouds of interstellar material at temperatures below -100 degrees Celsius. Blue colors highlight emission thought to indicate the presence of complex carbon molecules, known on planet Earth as PAHs, while red colors trace emission from cold, microscopic dust grains. Hot young stars are formed as this frigid material condenses under the influence of gravity. Once begun, the process takes only tens of thousands of years for truly massive stars and up to tens of millions of years for low mass stars like the Sun.