Fundamental star
A fundamental star is a fixed star whose celestial coordinates and their temporal changes ( proper motion ) are known with the greatest possible accuracy and are present in an absolute system.
A larger number of fundamental stars defines the fundamental system of astronomy on the celestial sphere , which also represents the framework for spatially fixed coordinate systems in the geosciences .
The fundamental stars serve as " connecting stars " in determining the locations of all other celestial bodies. Therefore, the determination of their coordinates must take place independently of other stars, i.e. with an absolute method.
Two and three dimensional
In principle, star positions are given in two coordinate angles (α, δ) on the celestial sphere, which are called right ascension and declination . They refer to the celestial equator (extension of the earth's equator) and the vernal equinox , whereby the declination is analogous to the geographical latitude. Due to the slow cone movement of the earth's axis (the 26,000-year precession ) and other effects, this coordinate system is variable over time, but can be modeled with very high accuracy (better than 0.01 ").
Radio astronomy plays an important role in this, in the form of a precise "network" of around 500 quasars , which is connected to the network of fundamental stars . The stars are measured optically ( visually , photographically or with photoelectric sensors ), see the next but one section.
From two to three-dimensional star location (α, δ) is determined by a distance measurement . The only precise means for this is parallax - an apparent displacement of "nearby" stars in the sky caused by the earth's orbit . It was not until 1838 that Friedrich Wilhelm Bessel succeeded in such a measurement on 61 Cygni , a so-called high-speed runner , whose rapid movement suggested a short distance to the earth, but on which Bessel still found only a parallax of 0.31 "(actual value: 0.286" ).
As such, a fundamental system could be content with 2D coordinates if it is limited to "distant" stars. However, bright stars are also required which, statistically speaking, are rather "close" (around 10 fundamental stars up to 50 light years away, where the parallax is still almost 0.1 "). However, a 3D system has the advantage, also the dynamics the Milky Way , and with the Hipparcos astrometry satellite , good measurements of over 100,000 stars can be made anyway.
Fundamental catalogs - today as a database
The fundamental stars are grouped into their own star catalogs and form a coordinate frame in which the movements of the “top of the earth ” can be precisely modeled. The first four " Fundamental Catalogs " were created under German management, which is why they are still abbreviated to FK today. The first was published by Arthur Auwers in 1879 , it contained 539 stars of the northern sky (δ to –10 °). The one from 1907 ( Berliner Astronomisches Jahrbuch ) already had 925 stars and could fall back on over 150 years of precision observations. Such long time series are still decisive for the exact recording of the own movements , since the star locations are to be calculated in advance using these individual speeds in the present and future.
| Short name | Star number |
Official name | publ. | Measurement places |
Measurement of own movements |
|---|---|---|---|---|---|
| Peters J., 1907 |
925 | New fundamental catalog of the Berlin Astronomical Yearbook based on the principles of A. Auwers . (up to Dec. = -89 °) |
1907 | Ø 1880 | 1745-1900 |
| (from here over the whole sky, with epochs 1900, 1950, 2000) | |||||
| FK3 | 873 | Third fundamental catalog | 1937 | 1912-15 | |
| FK3sup | + 662 | ( Supplement stars , Volume II) | 1938 | Ø 1913 | 1845-1930 |
| FK4 | 1535 | Fourth fundamental catalog | 1963 | Ø 1950 | |
| FK5 | 1535 | Fifth Fundamental Catalog | 1988 | Ø 1975 | |
| FK5sup | 3117 | Supplement Stars of FK5 | 1991 | ||
| Hipp. | 118000 | Hipparcos catalog , relative! | 1998 | 1989-93 | 1989-1993 |
| FK6 | 4150 | Sixth Fundamental Catalog | 2000 | Ø 1992 | |
→ Regarding the above table of the fundamental catalogs:
The annual data for the measurement of the places are average values.
Hipparcos is not a fundamental catalog in the strict sense (i.e. not absolute), but has been precisely adapted to the FK5 system and has 'stiffened' it through its higher relative accuracy. The measurements of the astrometric satellite Hipparcos (1989–1993), in particular the precise proper movements of the fundamental stars, contributed to the new system ( FK6 ) .
Absolute declination determination
The declination δ of a star is best measured at its culmination in the north or south branch of the meridian . On the one hand, it is at its highest at this moment and “wanders” horizontally through the field of view of the telescope or sensor, which increases the measurement accuracy. On the other hand, various error influences are eliminated in the azimuth 0 ° or 180 °.
With z as the measured (and corrected due to the refraction ) zenith distance and φ as the geographical latitude , the minimum value of z results in the meridian,
z1 = δ - φ und daher δ = φ + z1
The formula applies to the top culmination of each star. If this takes place south of the zenith, z 1 is to be taken as negative , while north of it is positive. For the lower culmination , where the star passes through the meridian below the celestial pole 12 hours later
z2 = 180° - φ - δ und daher δ = 180° - φ - z2
But because the latitude φ must first be determined precisely and also varies slightly due to the polar movement , circumpolar stars were originally observed in the upper and lower culmination, whereby φ can be eliminated:
δ = 90° + (z1-z2)/2 φ = 90° - (z1+z2)/2
Absolute right ascension determination
Absolute determination of right ascensions is more complicated because they relate to the vernal equinox and therefore require solar observations . However, the solar right ascension can also be determined at other times, for example from absolute declination measurements of the sun.
The right ascension of a star then follows from the sidereal time difference between the meridian passage of the sun and the star, plus the solar right ascension. In order to keep the measured time difference and thus the measurement uncertainty small, daytime observations of stars are necessary, which limits the absolute right ascension determination to bright stars (up to a maximum of 3rd magnitude ).
Influence of the changing earth axis
But now the star locations are not constant because of the somewhat variable earth axis in the inertial space. this means
- on the one hand, that their chronological sequence must be precisely recorded and calculated,
- on the other hand, an opportunity to explore the underlying forces on the earth and its annual orbit around the sun.
The reference planes astronomy subject, as mentioned above, slow shifts by the gravitational influence of the solar system to Earth. Just as every toy spinning top wobbles a bit, so does the earth - only much slower and more regularly. This effect is called precession and its duration of around 25,800 years is called a " Platonic year ". During this time, the earth's axis describes a clearly definable cone with an angle of 22–24 ° ( inclination of the ecliptic ), which can now be calculated with an accuracy of 0.01 "(0.000005%). This also includes a second effect called nutation - a monthly "tremor" caused by the moon that is just as precisely modeled.
These effects are measured by special instruments and methods of astrometry and geodesy ; the most important are the space methods VLBI (direction measurement according to quasars ), space laser and GPS , as well as the earth-bound meridian circle and astrolabe or PZT ; the latter two have lost their importance over the past decade. In addition, a kind of space scanner was added a few years ago , the Hipparcos satellite .
The astronomical-geodetic model of the earth's motion, anchored by fundamental stars, described here represents the best implementation of an inertial system as a fundamental system of astronomy .
Terrestrial fundamental systems are also implemented in the same way - by "bringing them down" onto the rotating earth. It is called ITRS (International Terrestrial Reference System) and the models repeated or refined every 2 to 3 years are called a year. However, they are not represented by stars, but by particularly well and globally determined survey points (see fundamental stations , around 20 in Europe). This global surveying network is being consolidated and permanently marketed by numerous GPS measuring stations .
Web links
- New fundamental system FK6, Astron.Recheninstitut Heidelberg 2000
- Fundamental system of the FK5, ARI / Heidelberg 1988
- Description of important fundamental catalogs