Fixed star (from Latin stellae fixae "fixed stars") is a name going back to antiquity for those stars that do not change their position in the sky and always take the same position in relation to one another, in contrast to the walking stars , the planets .
With the naked eye , around 3,000 to 6,000 fixed stars can be perceived across the sky, but only around half of them can be seen simultaneously from an earthly location. They are all stars of the Milky Way and are at very different distances from us. However, most of the estimated 100 billion stars in the Milky Way are invisible to the naked eye because they are either not bright enough, too far away, or obscured by other astronomical objects .
Meaning and change in meaning
The fixed stars form the constellations and constellations known to us through their mutual positions, which freely appear as unchangeable . The apparent movement of these "fixed stars" in the course of a night (or a year) from east to west across the firmament visible from the earth is caused by the earth's rotation around its axis or the earth's orbit around the sun.
In ancient Greek literature, which deals with celestial phenomena and constellations, only stars ( astra ) were initially mentioned , for example in the Phainomena of Aratos von Soloi in the 3rd century BC. In Latin authors all bright celestial phenomena were then referred to as stars ( stellae ), but the planets were distinguished as roving ( errantia ) from the stars as attached ( adfixa ). This term became generally accepted. Claudius Ptolemy taught in the Almagest that these are forever in the same position to one another (Book 7, Chapter 1). The medieval authors took this over and formed the expression fixer Stern or Fixedstern during the transition to the German language . In his work De revolutionibus, Copernicus differentiates the fixae stellae from the errantes , as does Johannes Kepler in Astronomia Nova, the sphaera fixarum from the planetae . In the middle of the 18th century Immanuel Kant still used the term fixed star to a large extent for stars in his work General Natural History and Theory of the Sky .
The usage of language did not change until the second half of the 19th century, when astrophysics was developed using the methods of spectral analysis , photometry and photography to study celestial bodies . The term fixed star in classical positional astronomy was replaced by star . However, this process was insidious and inconsistent. The physicist and astronomer Karl Friedrich Zöllner as used in its important work Basics of General photometry of the sky in 1861 mainly the concept of Star , in the formulation of underlying price task it but is still: ... the most accurate photometric determination of fixed stars .. .
Currently, the term fixed star is used in historical contexts and when dealing with constellations. In the Kosmos Himmelsjahr published annually by Hans-Ulrich Keller since 1982, the motto Fixed Star Heaven is used for one of the monthly topics .
In fact, contrary to their name, fixed stars also have their own motion , i.e. an apparent motion in the celestial sphere relative to the surrounding stars , as James Bradley recognized in 1728 (discovered in 1725 and correctly interpreted in 1728). Because of their great distances, the changes in position of the fixed stars can hardly be registered with the naked eye even after a few centuries. The star with the largest known proper motion is Barnard's Arrow Star ; it changes its location by 0.3 ° per century, but is not freely visible.
Another effect results from the movement of the earth around the sun. As a result, a fixed star is projected onto the celestial sphere from different points on this earth's orbit over the course of the year and appears to describe an ellipse . This effect is measured by parallax , the angle between two lines of sight directed from different observation locations.
- Jürgen Hamel : Milestones in Astronomy , Stuttgart 2006
- Hans-Ulrich Keller : Dictionary of Astronomy , Stuttgart 2004
- Helmut Zimmermann, Joachim Gürtler: ABC Astronomie , Heidelberg 2008
- Helmut Zimmermann, Joachim Gürtler: ABC Astronomy , Fixed Star
- Hyginus Mythographus : De astronomia 2,41
- Pliny the Elder : Naturalis historia II, 24
- Jürgen Hamel: Milestones of Astronomy , pp. 250-255
- Karl Friedrich Zöller: Fundamentals of the general photometry of the sky , foreword
- Helmut Zimmermann, Joachim Gürtler: ABC astronomy , proper movement
- Hans-Ulrich Keller: Dictionary of astronomy , proper movement
- Helmut Zimmermann, Joachim Gürtler: ABC Astronomy , Parallax