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"
Big Dipper over Pyramid Mountain
10 December 2023
When did you first learn to identify this group of stars? Although they are familiar to many people around the world, different cultures have associated this asterism with different icons and folklore. Known in the USA as the Big Dipper, the stars are part of a constellation designated by the International Astronomical Union in 1922 as the Great Bear (Ursa Major). The recognized star names of these stars are (left to right) Alkaid, Mizar/Alcor, Alioth, Megrez, Phecda, Merak, and Dubhe. Of course, stars in any given constellation are unlikely to be physically related. But surprisingly, most of the Big Dipper stars do seem to be headed in the same direction as they plough through space, a property they share with other stars spread out over an even larger area across the sky. Their measured common motion suggests that they all belong to a loose, nearby star cluster, thought to be on average only about 75 light-years away and up to 30 light-years across. The cluster is more properly known as the Ursa Major Moving Group. The featured image captured the iconic stars in 2017 above Pyramid Mountain in Alberta, Canada. Night Sky Network webinar: APOD editor to review coolest space images of 2023