Alpha Andromedae

Alpha Andromedae, officially named Alpheratz, is a blue-giant star located 97 light-years away from the Sun and is the brightest star in Andromeda. It is located northeast of Pegasus, and is the upper left star of the Great Square of Pegasus.

Although it seems like they are a single star, they are actually a binary system consisting of two stars in a close orbit. They are unusual stars as it is a mercury-manganese star but have abnormally high levels of Mercury, Manganese, and other elements such as Gallium and Xenon. It is the brightest mercury-manganese star known.

Name
Alpha Andromedae (abbreviated to α And) is the star's bayer designation, a stellar designation where a greek letter is followed by the genitive form of its constellation name. It also have another bayer designation name; Delta Pegasi (abbreviated to δ Peg). When the modern constellation boundaries were fixed in 1930, the designation was dropped from use.

The official name or common name, Alpheratz, is derived from Arabic word, سرة الفرس sarrat al-fars which means "the mare's navel". This also goes the same for its another common name, Sirrat. The arabic term was incorrect due to Alpheratz being in Andromeda instead of Pegasus, where the word "mare" comes from. Another term for this star used by medieval astronomers writing in Arabic was راس المراة المسلسلة rasal-mar'at al-musalsala "the head of the woman in chains", the chained woman here refers to Andromeda. Other Arabic names include al-kaff al-khaḍīb and kaffunnaṣīr.

In the Hindu lunar zodiac, Alpheratz along with the other stars in the Great Square of Pegasus (α, β, and γ Pegasi), makes up the nakshatras for Pūrva Bhādrapadā and Uttara Bhādrapadā. In Chinese, 壁宿 Bì Sù, meaning "wall", refers to an asterism consisting of α Andromedae and γ Pegasi. Consequently, the Chinese name for α Andromedae itself is 壁宿二 Bì Sù èr, which means "the second star of the wall".

Binary System
The radial velocity of a star away from or towards the observer can be determined by measuring the red shift or blue shift of its spectrum. The American astronomer Vesto Slipher made a series of such measurements from 1902 to 1904 and discovered that the radial velocity of Alpheratz varied periodically. He concluded that it was in orbit in a spectroscopic binary star system with a period of about 100 days.

The fainter star in the system was first resolved interferometrically by Xiaopei Pan and his coworkers during 1988 and 1989, using the Mark III Stellar Interferometer at the Mount Wilson Observatory, California, United States. This work was published in 1992. Because of the difference in luminosity between the two stars, its spectral lines were not observed until the early 1990s, in observations made by Jocelyn Tomkin, Xiaopei Pan, and James K. McCarthy between 1991 and 1994 and published in 1995.

The two stars are now known to orbit each other with a period of 96.7 days. The larger, brighter star, called the primary, has a spectral type of B8IVpMnHg, a mass of approximately 3.6 solar masses, a surface temperature of about 13,800 K, and, measured over all wavelengths, a luminosity of about 200 times the Sun's.

Variability
Alpheratz 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.

Chemical Peculiarities
In 1906, Norman Lockyer and F. E. Baxandall reported that Alpheratz had a number of unusual lines in its spectrum. In 1914, Baxandall pointed out that most of the unusual lines came from manganese, and that similar lines were present in the spectrum of Mu Leporis. In 1931, W. Morgan identified 12 additional stars with lines from 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. In the case of Alpheratz, the brighter primary star is a mercury-manganese star which, as well as the elements already mentioned, has excess xenon.

Position


The location of Alpheratz 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.

It is part of the Great Square of Pegasus, although it is not even the part of the constellation. Due to this, Alpheratz received a bayer designation for the constellation Pegasus, δ Peg.

Its coordinate is around 00h 08m 23.2586s, 29° 05′ 25.555″.