Fixed star
The fixed stars are celestial objects that do not seem to move in relation to the other stars of the night sky. Hence, a fixed star is any star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

 except for the Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

. A nebula
A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...

 or other starlike object may also be called a fixed star. People in many cultures have imagined that the stars form pictures in the sky called constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....

s. In ancient Greek astronomy
Greek astronomy
Greek astronomy is astronomy written in the Greek language in classical antiquity. Greek astronomy is understood to include the ancient Greek, Hellenistic, Greco-Roman, and Late Antiquity eras. It is not limited geographically to Greece or to ethnic Greeks, as the Greek language had become the...

, the stars were believed to exist on a giant celestial sphere
Celestial spheres
The celestial spheres, or celestial orbs, were the fundamental entities of the cosmological models developed by Plato, Eudoxus, Aristotle, Ptolemy, Copernicus and others...

, or firmament
The firmament is the vault or expanse of the sky. According to Genesis, God created the firmament to separate the oceans from other waters above.-Etymology:...

, that revolved around the Earth daily.

Origination of name

The phrase originated in classical antiquity
Classical antiquity
Classical antiquity is a broad term for a long period of cultural history centered on the Mediterranean Sea, comprising the interlocking civilizations of ancient Greece and ancient Rome, collectively known as the Greco-Roman world...

, 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
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

 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
In astronomy, the zodiac is a circle of twelve 30° divisions of celestial longitude which are centred upon the ecliptic: the apparent path of the Sun across the celestial sphere over the course of the year...

 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
Star catalogue
A star catalogue, or star catalog, is an astronomical catalogue that lists stars. In astronomy, many stars are referred to simply by catalogue numbers. There are a great many different star catalogues which have been produced for different purposes over the years, and this article covers only some...

 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
Naked eye
The naked eye is a figure of speech referring to human visual perception unaided by a magnifying or light-collecting optical device, such as a telescope or microscope. Vision corrected to normal acuity using corrective lenses is considered "naked"...

 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
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

, and they are actually at different distances from us. Most of the millions 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 to not have been noticed until modern times. It can be used to find the distance to nearby stars. This motion is only apparent.

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
Proper motion
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...

, 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
Barnard's star
Barnard's Star, also known occasionally as Barnard's "Runaway" Star, is a very low-mass red dwarf star approximately six light-years away from Earth in the constellation of Ophiuchus . In 1916, the American astronomer E.E...


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
Newton's laws of motion
Newton's laws of motion are three physical laws that form the basis for classical mechanics. They describe the relationship between the forces acting on a body and its motion due to those forces...

 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
Centrifugal force
Centrifugal force can generally be any force directed outward relative to some origin. More particularly, in classical mechanics, the centrifugal force is an outward force which arises when describing the motion of objects in a rotating reference frame...


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.

See also

  • Apparent magnitude
    Apparent magnitude
    The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...

     (Related to apparent brightness)
  • Behenian fixed stars
  • Firmament
    The firmament is the vault or expanse of the sky. According to Genesis, God created the firmament to separate the oceans from other waters above.-Etymology:...

  • Milky Way
    Milky Way
    The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

  • Galaxy rotation curve
    Galaxy rotation curve
    The rotation curve of a galaxy can be represented by a graph that plots the orbital velocity of the stars or gas in the galaxy on the y-axis against the distance from the center of the galaxy on the x-axis....

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