Constellations

In astronomy the Doppler effect can be seen in stellar spectra, in which the spectral lines are often displaced towards the blue (shorter wavelengths) or red (longer wavelengths) end of the spectrum. A blueshift means that the star is approaching, while a redshift indicates that it is receding.

The displacements due to the Doppler effect are usually very small. In order to measure them, a reference spectrum is exposed on the plate next to the stellar spectrum. Now that CCD-cameras have replaced photographic plates, separate calibration exposures of reference spectra are taken to determine the wavelength scale. The lines in the reference spectrum are produced by a light source at rest in the laboratory. If the reference spectrum contains some lines found also in the stellar spectrum, the displacements can be measured.

Displacements of spectral lines give the radial velocity vr of the star, and the proper motion x can be measured from photographic plates or CCD images. To find the tangential velocity vt, we have to know the distance r, obtainable from e.g. parallax measurements. Tangential velocity and proper motion are related by

At any one time, about 1000-1500 stars can be seen in the sky (above the horizon). Under ideal conditions, the number of stars visible to the naked eye can be as high as 3000 on a hemisphere, or 6000 altogether. Some stars seem to form figures vaguely resembling something; they have been ascribed to various mythological and other animals. This grouping of stars into constellations is a product of human imagination without any physical basis. Different cultures have different constellations, depending on their mythology, history and environment.

About half of the shapes and names of the constellations we are familiar with date back to Mediterranean antiquity. But the names and boundaries were far from unambiguous as late as the 19th century. Therefore the International Astronomical Union (IAU) confirmed fixed boundaries at its 1928 meeting.

The official boundaries of the constellations were established along lines of constant right ascension and declination for the epoch 1875. During the time elapsed since then, precession has noticeably turned the equatorial frame. However, the boundaries remain fixed with respect to the stars. So a star belonging to a constellation will belong to it forever (unless it is moved across the boundary by its proper motion).

The names of the 88 constellations confirmed by the IAU are given in Table C.21 at the end of the book. The table also gives the abbreviation of the Latin name, its genitive (needed for names of stars) and the English name.

In his star atlas Uranometria (1603) Johannes Bayer started the current practice to denote the brightest stars of each constellation by Greek letters. The brightest star is usually a (alpha), e.g. Deneb in the constellation Cygnus is a Cygni, which is abbreviated as a Cyg. The second brightest star is j (beta), the next one y (gamma) and so on. There are, however, several exceptions to this rule; for example, the stars of the Big Dipper are named in the order they appear in the constellation. After the Greek alphabet has been exhausted, Latin letters can be employed. Another method is to use

numbers, which are assigned in the order of increasing right ascension; e.g. 30Tau is a bright binary star in the constellation Taurus. Moreover, variable stars have their special identifiers (Sect. 13.1). About two hundred bright stars have a proper name; e. g. the bright a Aur is called also Capella.

As telescopes evolved, more and more stars were seen and catalogued. It soon became impractical to continue this method of naming. Thus most of the stars are known only by their catalogue index numbers. One star may have many different numbers; e. g. the abovemen-tioned Capella (a Aur) is number BD+45° 1077 in the Bonner Durchmusterung and HD 34029 in the Henry Draper catalogue.

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