Alpheratz (Alpha Andromedae)
The brightest star in the constellation Andromeda and the northeastern star of the Square
of Pegasus. Its proper name is generally taken to mean "the horse's
shoulder" or "navel", indicating that the star originally
belonged more to Pegasus (in which it served as Delta Peg), though now it
is formally within the boundaries of Andromeda and marks the head of the
royal daughter. Its alternative proper name is Sirrah.
|Credit: 2005 Torsten Bronger
Alpheratz is the brightest mercury-manganese
star (see manganese star) but, unusually for stars in this category, lacks a detectable magnetic
field. It is also a spectroscopic binary with a period of 96.7 days. The dimmer companion seems to be about one tenth
as bright as the primary.
||97.0 light-years (29.8 pc)
||R.A. 00h 08m 23.3s, Dec. +29° 05' 26"
||Sirrah, Sirah, d Pegasi,
21 Andromedae, HR 15,
BD +28°4, HD 358,
SAO 73765, FK5 1, GC 127,
CCDM 00083+2905, ADS 94,
Weather on Alpheratz
In 2007, researchers led by Oleg Kochukhov of Uppsala University in Sweden
announced that Alpheratz was the first star observed to have weather. This
took the form of shifting clouds of mercury. All previously known structures
on stars, such as sunspots, are due to magnetic
fields but the absence of such a field on Alpheratz means that some other
mechanism must be at work. Kochukhov and his colleagues found that the mercury
concentration varies across the surface of Alpheratz by up to a factor of
10,000, and the pattern of clouds changes over time. Kochukhov believes
these changes in the mercury cloud cover may be caused by tides raised on
Alpheratz by its close companion.
|Darker regions show greater concentrations of mercury on the surface of Alpheratz. The mercury clouds tend to concentrate
along the equator, probably due to the star's rotation. Credit: Kochukhov
• NOTABLE STARS