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 Path North
7 April 2020
What happens if you keep going north? The direction north on the Earth, the place on your horizon below the northern spin pole of the Earth -- around which other stars appear to slowly swirl, will remain the same. This spin-pole-of-the-north will never move from its fixed location on the sky -- night or day -- and its height will always match your latitude. The further north you go, the higher the north spin pole will appear. Eventually, if you can reach the Earth's North Pole, the stars will circle a point directly over your head. Pictured, a four-hour long stack of images shows stars trailing in circles around this north celestial pole. The bright star near the north celestial pole is Polaris, known as the North Star. The bright path was created by the astrophotographer's headlamp as he zigzagged up a hill just over a week ago in Lower Saxony, Germany. The astrophotographer can be seen, at times, in shadow. Actually, the Earth has two spin poles -- and much the same would happen if you started below the Earth's equator and went south.