Star Facts: Alpheratz

Alpheratz star

Alpheratz (Alpha Andromedae) is the brightest star in the constellation Andromeda with an apparent visual magnitude of +2.06, which also makes it the 54th brightest star in the night sky. While this binary system properly belongs to the constellation Andromeda, it also forms part of the Great Square of Pegasus, marking out the northeastern corner of the asterism.

Quick Facts

• Constellation: Andromeda
• Coordinates: RA: 00h 08m 23.25988s |Dec: +29° 05′ 25.5520″
• Distance to Earth: 97 light years
• Star Type: Primary: B8IVpMnHg
• Mass: Primary 3.8 sol |Secondary 1.85 sol
• Radius: Primary: 2.7 sol | Secondary: 1.65 sol
• Apparent Magnitude: 2.06 (Combined)
• Luminosity: Primary 240 sol |Secondary 13 sol
• Surface Temperature: Primary 13,800K |Secondary: 8,500K
• Rotational Velocity: Primary 52 km/s |Secondary: 110 km/s
• Age: Primary 60 million years |Secondary 70 million years
• Other Designations: Alpheratz, Sirrah, Sirah, Alpha Andromedae, Alpha And, Delta Pegasi, Delta Peg, 21 Andromedae, 21 And, H 5 32A, MKT 11


The Andromeda constellation is visible from June to February in the Northern Hemisphere, although from mid-northern latitudes, it is highest in the sky from August to October. Its brightest star Alpheratz is located on the border between the constellations Pegasus and Andromeda, and can be seen without optical aid from all latitudes north of 60 degrees south.

Physical Properties

While Alpheratz appears to be single star, it is in fact a close binary system with an orbital period of 96.7 days. The primary component of the system has a mass of about 3.6 times that of the Sun, and shines about 200 times brighter if its brightness is measured across all the wavelengths in its spectrum. Meanwhile, its companion is an early A-type star that is about 1.8 times as massive as the Sun, about 10 times as luminous, and has an effective surface temperature of 8,500K.

Chemically Peculiar

The primary’s classification of B8IVpMnHg indicates that it is a chemically peculiar star, with abnormally high abundances of mercury, manganese, and other elements that include gallium and xenon in its atmosphere. It is also the brightest known mercury-manganese star yet discovered.

While it has been known since 1906 that Alpheratz is chemically peculiar and that the manganese lines in its spectrum resemble those of the star µ Leporis, it was only in 1970 that Georges Michaud developed a theory to explain the phenomenon. The theory holds that in stars with unusually calm atmospheres, some elements are pulled under by the force of gravity, while others are pushed towards the star’s surface by radiation pressure. This mechanism is only able to operate in stars in which violent convection currents do not mix all the elements in the star’s upper layers together, and it has successfully explained the chemical peculiarities not only in Alpheratz, but also in many other types of chemically peculiar stars.


Although Alpheratz has been reported to vary slightly in brightness, detailed studies have shown the degree of variability to be less than 0.01 magnitudes. Nonetheless, in a recent study (2002) performed by Aldeman et al, it was found that the mercury lines in the stars’ spectrum varies around the 398.4 nm wavelength because the mercury component in the stars’ atmosphere is concentrated in isolated clouds that drift across the stars’ surface, and especially around the stars’ equatorial region.


Both Ptolemy (AD 100-170) and Johann Bayer (1572-1625) considered Alpheratz to belong to both Pegasus and Andromeda, although the designation Delta Pegasi (d Peg) assigned to Alpheratz by Bayer was dropped when the final borders of the constellations were determined in 1930.

The star’s traditional name of Alpheratz, Alpherat, and sometimes Sirrah derives from the Arabic phrase, “surrat al-faras”, meaning “the navel of the mare”. While the name Sirrah comes from the Arabic word for “alone”, taken together, the horse related names reflect the star’s historical placement in the constellation Pegasus.

Interestingly, another historical Arabic name for this star was “ras al-mar’a al-musalsala”, which translates into “the head of the woman in chains”, in reference to Andromeda, the princess whose father King Cepheus had chained her to rocks by the seashore as a sacrifice to a sea monster sent by Poseidon. As the legend goes, she was then rescued by the hero Perseus flying on the winged horse Pegasus before the creature could devour her.

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