1) Redshift and Distance
The huge distances of the target stations of the Galaxy Trail implied that the distances starting at a few hundred million light years (e.g. Perseus A) were derived from the observed redshifts in the spectral lines of these objects. The first identifications of quasars as highly redshifted objects were proven for a station of our Galaxy Trail (3C 273) already in 1963. Almost all distance values for the targets of the Galaxy Trail have been derived that way. At very high redshift values, relativistic effects and cosmological models must be taken into account. Because of the complex structure of the Universe, there is no distinct distance value any more. For the stations of the Galaxy Trail we have derived “light travel distances" applying standard values in the cosmological model with Ned Wright‘s “Cosmology Calculator“ available in the internet. We define that a galaxy with a light travel time of 10 billion years is in a distance of 10 billion light years. Thus the (observational) edge of the Universe is defined by its age. Pushing the record with higher and higher observed redshift values leads further back in the history of the Universe approaching the moment of its creation approximately 13.8 billion years ago (this value taken from the analysis of the cosmic microwave background with the European Planck satellite).
A concise compilation of methods for cosmic distance calculations is provided in the following chapter in Richard Pogge‘s lecture series at Ohio State University: The Cosmic Distance Problem (Lecture 22).
2) Calibration sources for the 100 m Radio Telescope
The targets of the Effelsberg Galaxy Trail include a number of standard calibration sources in the sky which are used to calibrate radio astronomical observations with the 100 m radio telescope. Calibration observations come in three different flavours: 1) positional accuracy (pointing observations); 2) signal strength (flux calibration); 3) correction of the receiver position in the focus of the parabolic dish because of shape variations (homologous distortion of the dish). The following properties are required for suitable calibration sources: strong compact sources with accurately determined position in the sky, precisely known radio fluxes at different wavelength bands and no variability in their flux. A number of quasars in particular fulfil all these properties. Thus some of the most regularly used calibrators for the 100 m radio telescope include galactic nuclei at distances between two (3C 273) and seven billion light years (3C 286).
3) Target Names and Catalogues
Some of the astronomical targets presented on the Galaxy Trail are simply named by their celestial coordinates. Examples include: J1148+5251 (Right Ascension 11h 48m, Declination +52o 51'), MG J0414+0534, B0218+367 and 0917+62. The other names are based on a number of different catalogues:
Messier Catalogue (M): Famous list of a total of 110 non-stellar objects (nebulae, clusters and galaxies), compiled by the French astronomer Charles Messier in the 18th century. The original intention was a negative list for his hunt for comets. Examples: M 31, M 82, M 87.
New General Catalogue (NGC): New General Catalogue of Nebulae and Clusters. A compilation of nebulae, clusters and galaxies all over the sky with a total of about 8000 entries. Examples: NGC 224 (M 31), NGC 1275, NGC 3034 (M 82), NGC 4486 (M 87).
Third Cambridge Catalogue (3C): Catalogue of radio sources from 1959. Most of the bright radio sources in the northern sky are contained in this catalogue. Examples: 3C 48, 3C 273, 3C 286, 3C 295, 3C 405.
Die stärksten Radioquellen am Himmel (Michael Hamm, 2006). A school work experience project at MPIfR (in German language), describing the strongest radio sources in a number of constellations. The label "Kosmische A-Klasse" (Cosmic Class A) describes optical observations of several of these sources (also in German). Examples: Andromeda A (M 31), Ursa Major A (M 82), Virgo A (M 87), Cygnus A (3C 405), Perseus A (NGC 1275).
Going back to ancient Babylon and Greece, observers of the night sky combined the bright dots of the stars to figures as background of several mythological tales. Orion, the hunter, Ursa Major, the great bear, with the distinctive part of the big dipper, Herkules or the 12 constellations of the zodiac from Aries, the ram, to Pisces, the fishes – some of these go all the way back to Sumerian or Babylonian times. Only in the 1920s, constellations in the sky were determined as well-defined areas in the sky. The complete sky was subdivided into 88 constellations of very different size (88 modern constellations). The coordinate of a celestial object defines the constellation it is referred to. Our neighbouring galaxy Messier 31 for example is found in the direction of Andromeda (thus: Andromeda galaxy or Andromeda nebula) and the most distant object on the Galaxy Trail, J1148+5251, is in the direction of Ursa Major, the great bear.
The story of the names of the constellations is described in the following: "How the Night Sky Constellations Got Their Names" (Joe Rao, space.com).