The fixed stars (Latin: stellae fixae) comprise the background of astronomical objects that appear to not 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 deep-sky objects may also be counted among the fixed stars.
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 attempts to explain the universe stem from observations of the objects found in the sky. Different cultures historically have various stories to provide an answer to the questions of what they are seeing. Norse Mythology originates from northern Europe, around the geographical location of modern-day region of Scandinavia and northern Germany. The Norse mythology consists of tales and myths derived from Old Norse, which was a Northern German language from the Middle Ages. There is a series of manuscript texts written in Old Norse which contain a collection of  poems written from oral tradition. Among historians there seems to be speculation of the specific dates of the poems written, however, the estimated record of the texts is around the beginning of the thirteenth century. Although the oral tradition of passing down tales existed long before the advent of text manuscripts and print versions.
Among surviving texts there is mention of the mythological god, Odin. Scholars have recounted the tale of the Αesir Gods creation myth which includes the idea of fixed stars found within the teleology of the tale. Padaric Colum has written a book, The Children of Odin, which in much detail reiterates the story of how the Aesir god's brought the giant named Ymir to his demise and created the world from his body, affixing sparks from the fiery Muspelheim, or the fixed stars, to the dome of the sky, which was the skull of Ymir. The Norse creation myth is one of several cases which treated stars as being fixed to a sphere beyond the earth. Later scientific literature shows astronomical thought which kept a version of this idea until the seventeenth century.
Developing Western Astronomy
Western astronomical knowledge was based on the traditional thoughts from philosophical and observational inquiries of Greek Antiquity. Other cultures contributed to thought about the fixed stars including the Babylonian's, who from the eighteenth to the sixth century BC constructed constellation maps. Maps of the stars and the idea of mythological stories to explain them was largely being acquired all over the world and in several cultures. One similarity between them all was the preliminary understanding that the stars were fixed and immobile in the universe.
This understanding was incorporated into theorized models and mathematical representations of the cosmos by philosophers like Anaximander and Aristotle from the Ancient Greeks. Anaximander wrote a treatise, of which only few excerpts remain. In this work he states his proposed order of the celestial objects, the sun moon and the fixed stars. The stars he mentions are apertures of "wheel-like condensations filled with fire", situated nearest to the earth in this system. The records of Anaximander's work left in fragments only gives a slight insight into reconstructing his intended meaning in understanding his views of the cosmos. Anaximander proposed a differing perspective from other later astronomers in proposing the fixed stars were nearest of the heavenly bodies to the earth. Other models of the planetary system show a celestial sphere containing fixed stars on the outer most part of the universe.
Aristotle and other like Greek thinkers of antiquity, and later the Ptolemaic model of the cosmos demonstrated an Earth centered universe. This Geocentric view was held through the Middle Ages and was later countered by subsequent astronomers and mathematicians alike, such as Nicolaus Copernicus and Johannes Kepler. The tradition of thought which appears in all of these systems of the universe, even with their divergent mechanisms, is the presence of a celestial sphere which contains the fixed stars. Ptolemy was influential with his heavily mathematical work, The Almagest, which attempts to explain the peculiarity of stars that the moved. These "wandering stars", planets, moved across the background of fixed stars which were spread along a sphere surrounding encompassing the universe. Later on, contemporary astronomers and mathematicians, like Copernicus challenged the long-standing view of geocentrism and constructed a sun centered universe, this being known as the heliocentric system. His system still upheld the tradition of a celestial sphere holding the fixed stars. Kepler also provided a model of the cosmos in his 1596 book Mysterium Cosmopgraphicum which pictures an image, labelling one celestial sphere, in Latin, "sphaera stellar fixar," or a sphere of fixed stars.
The fixed stars are not fixed
Astronomers and natural philosophers before 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. However, 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 be accurately measured 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. In the case of star systems or star clusters, the individual components move with respect to each other in a non-linear manner.
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.
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.
- Apparent magnitude (Related to apparent brightness)
- Behenian fixed star
- Galaxy rotation curve
- Milky Way
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