manganese appearing in their spectra. Many of these stars were subsequently identified as part of the group of mercury-manganese stars, a class of chemically peculiar stars which have an excess of elements such as mercury, manganese, phosphorus, and gallium in their atmospheres., §3.4. In the case of α Andromedae, the brighter primary star is a mercury-manganese star which, as well as the elements already mentioned, has excess xenon. In 1970, Georges Michaud suggested that such chemically peculiar stars arose from radiative diffusion. According to this theory, in stars with unusually calm atmospheres, some elements sink under the force of gravity, while others are pushed to the surface by radiation pressure., §4. This theory has successfully explained many observed chemical peculiarities, including those of mercury-manganese stars., §4. Variability of primary α Andromedae has been reported to be slightly variable, but observations from 1990 to 1994 found its brightness to be constant to within less than 0.01 magnitude. However, Adelman and his co-workers have discovered, in observations made between 1993 and 1999 and published in 2002, that the mercury line in its spectrum at 398.4 nm varies as the primary rotates. This is because the distribution of mercury in its atmosphere is not uniform. Applying Doppler imaging to the observations allowed Adelman et al. to find that it was concentrated in clouds near the equator. Subsequent Doppler imaging studies, published in 2007, showed that these clouds drift slowly over the star's surface. Observation The location of α Andromedae in the sky is shown on the left. It can be seen by the naked eye and is theoretically visible at all latitudes north of 60° S. During evening from August to October, it will be high in the sky as seen from the northern midlatitudes. Optical companion The binary system described above has an optical visual companion, discovered by William Herschel on July 21, 1781. Designated as ADS 94 B in the Aitken Double Star Catalogue, it is a G-type star with an apparent visual magnitude of approximately 10.8. Although by coincidence it appears near to the other two stars in the sky, it's much more distant from Earth; the parallax observed by Gaia place this star more than 1,300 light years away. Notes and references External links Andromeda (constellation) Andromedae, Alpha Andromedae, 21 B-type subgiants Alpheratz Spectroscopic binaries 000358 000677 G-type stars A-type main-sequence stars Triple stars BD+28 0004 Astronomical objects known