Alpheratz (Alpha Andromedae)
Alpheratz (Alpha Andromedae) is a star in the constellation Andromeda, magnitude 2.1.
The star two constellations shared — the horse’s navel that became the princess’s head.
For centuries this star belonged to two figures at once. Ptolemy treated it as shared by Andromeda and Pegasus, and Johann Bayer gave it two labels, Alpha Andromedae and Delta Pegasi. Its name comes from the Arabic surrat al-faras, “navel of the horse”, yet when the official constellation boundaries were published in 1930 it went to Andromeda alone, marking the chained princess’s head.
To the eye it is one star, but it is two, circling each other every 97 days. Vesto Slipher caught the motion in its spectrum between 1902 and 1904; the fainter partner was only resolved in 1988–89 with an interferometer at Mount Wilson.
The main star, a hot blue-white star of about 3.6 solar masses, has a strange atmosphere. It is the brightest known mercury-manganese star, laced with far too much mercury, manganese, gallium and xenon. In its unusually calm outer layers some elements sink while starlight pushes others up, and the mercury gathers into clouds near the equator that slowly drift across the surface.
Alpheratz (Alpha Andromedae) is 97 light-years away: the light we see from it set off in 1929, around the time Pluto was discovered.
Did you know?
- One of the Three Guides. With Caph in Cassiopeia and Algenib in Pegasus, Alpheratz is one of the “Three Guides” that mark the prime meridian of the heavens.
- Herschel’s false companion. William Herschel recorded a faint companion on 21 July 1781. It is a chance alignment: that 11th-magnitude star lies far behind Alpheratz.
- A lucky star. Old astrologers believed Alpheratz blessed those born under its influence with honour and riches.
A blue-white 2nd-magnitude star at the north-east corner of the Great Square of Pegasus, where the Square joins the chain of Andromeda’s stars. Compare the Square’s four corners: Alpheratz is the brightest.
When and where to see it
It is best placed in the evening sky around October, when it crosses the meridian at about 10 pm local time (around midnight in September). It can be seen from everywhere north of 61°S and never sets north of 61°N.
At its highest it stands 69° up from 50°N, 84° up from 35°N, 61° up from the equator, 26° up from 35°S.
Facts
- Type
- Star
- Constellation
- Andromeda
- Designations
- Alpha Andromedae, Sirrah, α And, 21 And, HR 15, HD 358, HIP 677
- Magnitude
- 2.1
- Position
- RA 0h 8.4m, Dec +29° 5′ (J2000)
- Spectral type
- B8IV-VHgMn
- Colour
- blue-white (B−V -0.11)
- Distance
- 97 light-years
- Discovered
- Known since antiquity
Open AstroPilot to see when Alpheratz (Alpha Andromedae) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view.