Fixed stars

In astronomy, the fixed stars (Latin: stellae fixae) are the luminary points, mainly stars, that appear not to move relative to one another against the darkness of the night sky in the background. This is in contrast to those lights visible to naked eye, namely planets and comets, that appear to move slowly among those "fixed" stars. The fixed stars includes all the stars visible to the naked eye other than the Sun, as well as the faint band of the Milky Way. Due to their star-like appearance when viewed with the naked eye, the few visible individual nebulae and other deep-sky objects also are counted among the fixed stars. Approximately 6,000 stars are visible to the naked eye under optimal conditions.

The term fixed stars is a misnomer because those celestial objects are not actually fixed with respect to one another or to Earth. Due to their immense distance from Earth, these objects appear to move so slowly in the sky that the change in their relative positions is nearly imperceptible on human timescales, except under careful examination with modern instruments, such as telescopes, that can reveal their proper motions. Hence, they can be considered to be "fixed" for many purposes, such as navigation, charting of stars, astrometry, and timekeeping.

Due to the large distances of astronomical objects, human vision is unable to perceive the three-dimensional depth of outer space, giving the impression that all stars and other extrasolar objects are equidistant from the observer. In the astronomical tradition of Aristotelian physics which spanned from ancient Greece to early scientific Europe, the fixed stars were believed to exist attached on a giant celestial sphere, or firmament, which revolves daily around Earth. Hence it was known as the "sphere of fixed stars", which acted as the supposed limit of the whole universe. For many centuries, the term fixed stars was a synonym for that celestial sphere.

Many ancient cultures observed new stars now called novas, which provided some clue the heavens were not completely unchanging, but as novae fade in few weeks or months, the phenomenon was not understood then, as well as of comets. In European scientific astronomy, evidence that disproved the firmament was gathered gradually. The Copernican Revolution of the 1540s fueled the idea held by some philosophers in ancient Greece and the Islamic world that stars were actually other suns, possibly with their own planets. The definitive discovery of proper motion was announced in 1718, and parallax was suspected in the 1670s but shown definitively in the 1830s. Other cultures (such as Chinese astronomy) either never had a belief in a sphere of fixed stars, or constructed it in different ways. (See Cosmology § Historical cosmologies.)

People in many cultures have imagined that the brightest stars form constellations, which are apparent pictures in the sky seeming to be persistent, being deemed also as fixed. That way, constellations have been used for centuries, and still are today, to identify regions of the night sky by both professional and amateur astronomers.