The fixed stars (Latin: stellae fixae) is the background of celestial objects that do not appear to move relative to each other in the night sky compared to the foreground of Solar System objects that do. Generally, the fixed stars are taken to include all stars other than the Sun. Nebulae and other star-like objects may also be counted among the fixed stars.
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Exact delimitation of the term is complicated by the fact that no celestial objects are in fact fixed with respect to each other. Nonetheless, extrasolar objects move so slowly in the sky that the change in their relative positions is nearly imperceptible on typical human timescales, except to careful examination, and thus can be considered "fixed" for many purposes. Furthermore, distant stars and galaxies move even slower in the sky than comparatively closer ones.
People in many cultures have imagined that the stars form pictures in the sky called constellations. In Ancient Greek astronomy, the fixed stars were believed to exist on a giant celestial sphere, or firmament, that revolves daily around Earth.
Origin of name
The phrase originated in classical antiquity, when astronomers and natural philosophers divided the lights in the sky into two groups. One group contained the fixed stars, which appear to rise and set but keep the same relative arrangement over time. The other group contained the naked eye planets, which they called wandering stars. (The Sun and Moon were sometimes called planets as well.) The planets seem to move and change their position over short periods of time (weeks or months). They always seem to move within the band of stars called the zodiac by Westerners. The planets can also be distinguished from fixed stars because stars tend to twinkle, while planets appear to shine with a steady light.
The star catalogue compiled by Claudius Ptolemy in the 2nd century CE lists 1,022 fixed stars visible from Alexandria. This became the standard number of stars in Western culture for hundreds of years. The total number of stars visible to the naked eye is about 6,000; only about half are visible at a given time of night from a given point on the Earth. They are all stars in the Milky Way, and they are actually at different distances from us. Most of the billions of stars in the Milky Way can only be detected with the aid of telescopes, or their existence indirectly inferred, because they are too faint or are obscured by interstellar gas, dust and other foreground stars.
The fixed stars are not fixed
Fixed stars do have parallax, which is a change in apparent position caused by the orbital motion of the Earth. This effect was small enough not to have been noticed until the 19th century. It can be used to find the distance to nearby stars. This motion is only apparent; it is the Earth that moves.
The fixed stars exhibit real motion as well, however. This motion may be viewed as having components that consist in part of motion of the galaxy to which the star belongs, in part of rotation of that galaxy, and in part of motion peculiar to the star itself within its galaxy.
This real motion of a star is divided into radial motion and proper motion, with "proper motion" being the component across the line of sight. In 1718 Edmund Halley announced his discovery that the fixed stars actually have proper motion. Proper motion was not noticed by ancient cultures because it requires precise measurements over long periods of time to notice. In fact, the night sky today looks very much as it did thousands of years ago, so much so that some modern constellations were first named by the Babylonians.
A typical method to determine proper motion is to measure the position of a star relative to a limited, selected set of very distant objects that exhibit no mutual movement, and that, because of their distance, are assumed to have very small proper motion. Another approach is to compare photographs of a star at different times against a large background of more distant objects. The star with the largest known proper motion is Barnard's Star.
The phrase "fixed star" is technically incorrect, but nonetheless it is used in an historical context, and in classical mechanics.
The fixed stars in classical mechanics
In Newton's time the fixed stars were invoked as a reference frame supposedly at rest relative to absolute space. In other reference frames either at rest with respect to the fixed stars or in uniform translation relative to these stars, Newton's laws of motion were supposed to hold. In contrast, in frames accelerating with respect to the fixed stars, in particular frames rotating relative to the fixed stars, the laws of motion did not hold in their simplest form, but had to be supplemented by the addition of fictitious forces, for example, the Coriolis force and the centrifugal force.
As we now know, the fixed stars are not fixed. The concept of inertial frames of reference is no longer tied to either the fixed stars or to absolute space. Rather, the identification of an inertial frame is based upon the simplicity of the laws of physics in the frame, in particular, the absence of fictitious forces. Law of inertia holds for Galilean coordinate system which is a hypothetical system relative to which fixed stars remain fixed.