Star Catalogues and Maps

The first actual star catalogue was published by Ptolemy in the second century; this catalogue appeared in the book to be known later as Almagest (which is a Latin corruption of the name of the Arabic translation, Al-mijisti). It had 1025 entries; the positions of these bright stars had been measured by Hipparchos 250 years earlier. Ptolemy's catalogue was the only widely used one prior to the 17th century.

The first catalogues still being used by astronomers were prepared under the direction of Friedrich Wilhelm August Argelander (1799-1875). Argelander worked in Turku and later served as professor of astronomy in Helsinki, but he made his major contributions in Bonn. Using a 72 mm telescope, he and his assistants measured the positions and estimated the magnitudes of 320,000 stars. The catalogue, Bonner Durchmusterung, contains nearly all stars brighter than magnitude 9.5 between the north pole and declination -2°. (Magnitudes are further discussed in Chap. 4.) Argelander's work was later used as a model for two other large catalogues covering the whole sky. The total number of stars in these catalogues is close to one million.

The purpose of these Durchmusterungen or general catalogues was to systematically list a great number of stars. In the zone catalogues, the main goal is to give the positions of stars as exactly as possible. A typical zone catalogue is the German Katalog der Astronomischen Gesellschaft (AGK). Twelve observatories, each measuring a certain region in the sky, contributed to this catalogue. The work was begun in the 1870's and completed at the turn of the century.

General and zone catalogues were based on visual observations with a telescope. The evolution of photography made this kind of work unnecessary at the end of the 19th century. Photographic plates could be stored for future purposes, and measuring the positions of stars became easier and faster, making it possible to measure many more stars.

A great international program was started at the end of the 19th century in order to photograph the entire sky. Eighteen observatories participated in this Carte du Ciel project, all using similar instruments and plates. The positions of stars were first measured with respect to a rectangular grid exposed on each plate (Fig. 2.23a). These coordinates could then be converted into declination and right ascension.

Positions of stars in catalogues are measured with respect to certain comparison stars, the coordinates of which are known with high accuracy. The coordinates of these reference stars are published in fundamental catalogues. The first such catalogue was needed for the AGK catalogue; it was published in Germany in 1879. This Fundamental Katalog (FK1) gives the positions of over 500 stars.

The fundamental catalogues are usually revised every few decades. The fifth edition, the FK 5, appeared in 1984. At the same time, a new system of astronomical constants was adopted. The catalogue contains 1535 fundamental and 3117 additional stars.

A widely used catalogue is the SAO catalogue, published by the Smithsonian Astrophysical Observatory in the 1960's. It contains the exact positions, magnitudes, proper motions, spectral classifications, etc. of 258,997 stars brighter than magnitude 9. The catalogue was accompanied by a star map containing all the stars in the catalogue.

Fig. 2.28. The representations in four atlases of the Hyades ► cluster in the constellation Taurus. (a) Heis: Atlas Coelestis, published in 1872. (b) Bonner Durchmusterung. (c) SAO, (d) Palomar Sky Atlas, red plate. The big blob is the brightest star of Taurus, or a Tauri alias Aldebaran. (e) All the stars in the Tycho Catalog, numbering over one million, are marked on an all-sky chart. The bright lane is the Milky Way. (Picture David Seal, NASA/JPL/Caltech)

In the 1990's a large astrometric catalogue, PPM (Positions and Proper Motions), was published to replace the AGK and SAO catalogues. It contained all stars brighter than 7.5 magnitudes, and was almost complete to magnitude 8.5. Altogether, the four volumes of the catalogue contained information on 378,910 stars.

The PPM was effectively replaced by the Tycho catalogue from Hipparcos satellite. Hipparcos was the first astrometric satellite, and was launched by the European Space Agency (ESA) in 1989. Although Hipparcos didn't reach the planned geosynchronous orbit, it gave exact positions of over a hundred thousand stars. The Hipparcos catalogue, based on the measurements of the satellite, contains astrometric and photometric data of 118,000 stars. The coordinates are precise to a couple of milliarcseconds. The less precise Tycho catalogue contains the data of about one million stars.

In 1999 and 2000, the sixth version of the Fundamental Katalog, the FK6, was published. It combined the Hipparcos data and FK5 for 4150 fundamental stars. The typical mean error in proper motion was 0.35 milliarcseconds per year for the basic stars. With the advance of the Internet, the printed versions of star catalogues were discontinued in the first years of the new millennium, and the catalogues were moved to compact discs and the net.

With the new media, the size of the star catalogues exploded. The first Hubble Guide Star Catalog from the early 1990's contained 18 million stars and the second Guide Star Catalog from the year 2001, nearly 500 million stars. It was surpassed by the U.S. Naval Observatory USN0-B1.0 Catalog, which contains entries for 1,024,618,261 stars and galaxies from digitized images of several photographic sky surveys. The catalogue presents right ascension and declination, proper motion and magnitude estimates.

The next step in the accuracy of astrometry will be achieved in the 2010's with a new European astrometric satellite. The Gaia satellite, planned to be launched in about 2014, is expected to improve the accurary to about 10-5 seconds of arc.

Star maps have been published since ancient times, but the earliest maps were globes showing the celestial sphere as seen from the outside. At the beginning of the 17th century, a German, Johannes Bayer, published the first map showing the stars as seen from inside the celestial sphere, as we see them in the sky. Constellations were usually decorated with drawings of mythological figures. The Uranometria Nova (1843) by Argelander represents a transition towards modern maps: mythological figures are beginning to fade away. The map accompanying the Bonner Durchmusterung carried this evolution to its extreme. The sheets contain nothing but stars and coordinate lines.

Most maps are based on star catalogues. Photography made it possible to produce star maps without the cataloguing stage. The most important of such maps is a photographic atlas the full name of which is The National Geographic Society - Palomar Observatory Sky Atlas. The plates for this atlas were taken with the 1.2 m Schmidt camera on Mount Palomar. The Palomar Sky Atlas was completed in the 1950's. It consists of 935 pairs of photographs: each region has been photographed in red and blue light. The size of each plate is about 35 cm x 35 cm, covering an area of 6.6° x 6.6°. The prints are negatives (black stars on a light background), because in this way, fainter objects are visible. The limiting magnitude is about 19 in blue and 20 in red.

The Palomar atlas covers the sky down to -30°. Work to map the rest of the sky was carried out later at two observatories in the southern hemisphere, at Siding Spring Observatory in Australia, and at the European Southern Observatory (ESO) in Chile. The instruments and the scale on the plates are similar to those used earlier for the Palomar plates, but the atlas is distributed on film transparencies instead of paper prints.

For amateurs there are several star maps of various kinds. Some of them are mentioned in the references.

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