# Alpheratz
**Alpheratz** (α Andromedae, also called Sirrah) is the brightest star of the constellation Andromeda, a blue subgiant of spectral type B8 IV with a visual magnitude of 2.07, lying about 97 light-years from the Sun.[1](https://dictionary.obspm.fr/terms/alpheratz-%CE%B1-andromedae/) It sits at the northeastern corner of the Great Square of [Pegasus](https://www.edgepedia.org/pegasus-constellation), and although the Square is a Pegasus figure, the star itself belongs to Andromeda; it is the only star with a dual [Bayer designation](https://www.edgepedia.org/bayer-designation) of this kind, having been cataloged by Johann Bayer (1572–1625) as both Alpha Andromedae and Delta Pegasi.[2](https://www.star-facts.com/alpheratz/) • [3](https://assets.cambridge.org/97805218/00457/sample/9780521800457ws.pdf) Alpheratz is also a close binary system whose primary is the brightest known mercury–manganese (HgMn) star.[4](https://academic.oup.com/mnras/article/464/1/1095/2268699)
| Key fact | Detail |
|---|---|
| Designations | Alpha Andromedae (α And), formerly also Delta Pegasi; proper name Alpheratz approved by the IAU on June 30, 2016[2](https://www.star-facts.com/alpheratz/) • [5](https://www.iau.org/static/science/scientific_bodies/working_groups/280/WGSN_bulletin1.pdf) |
| Brightness and type | Visual magnitude 2.07; blue subgiant, spectral type B8 IV with strong mercury and manganese lines[1](https://dictionary.obspm.fr/terms/alpheratz-%CE%B1-andromedae/) |
| Distance | Parallax 33.63 ± 0.35 mas, about 97 light-years[6](https://simbad.u-strasbg.fr/simbad/sim-basic?Ident=Alpheratz) |
| Binary nature | Double-lined spectroscopic binary, period about 96.7 days, eccentricity about 0.555[4](https://academic.oup.com/mnras/article/464/1/1095/2268699) • [7](https://www.researchgate.net/publication/234399434_Orbital_elements_and_abundance_analyses_of_the_double-lined_spectroscopic_binary_alpha_Andromedae) |
| Masses | Primary about 3.8 solar masses, secondary about 1.85 solar masses; mass ratio 0.50 ± 0.03[7](https://www.researchgate.net/publication/234399434_Orbital_elements_and_abundance_analyses_of_the_double-lined_spectroscopic_binary_alpha_Andromedae) |
| Sky role | One of the four stars of the Great Square of Pegasus and the starting point for tracing Andromeda and finding the Andromeda Galaxy (M31)[8](https://www.space.com/stargazing/night-sky-october-2026-what-to-look-for-this-month) |
## Name and etymology
The name Alpheratz, along with the variants Alpherat, Sirrah, and Sirah, derives from the Arabic *As-Surrat al-Faras* (السرهالفرس), "The Horse's Navel," from *surrat* (navel) plus *faras* (horse). The name refers to the star's position in the figure of Pegasus, to which it formerly belonged; the star is now considered part of Andromeda.[1](https://dictionary.obspm.fr/terms/alpheratz-%CE%B1-andromedae/) Medieval Latin authors confused the related names and spellings, and the modern application of "Alpheratz" to Alpha Andromedae comes down from late medieval times.[9](https://www.heavensremember.top/en/stars/alpheratz)
A second [Arabic name](https://www.edgepedia.org/arabic-name), *Ra's-ol-Mosalsalé* (رأسالمسلسله), from *Ar-Ra's al-Mar'ah al-Musalsalah*, means "the head of the chained woman," marking the star as the head of Andromeda in the mythological figure.[1](https://dictionary.obspm.fr/terms/alpheratz-%CE%B1-andromedae/) The International Astronomical Union's Working Group on Star Names approved "Alpheratz" for the star on June 30, 2016.[5](https://www.iau.org/static/science/scientific_bodies/working_groups/280/WGSN_bulletin1.pdf)
## Position in the sky
Alpheratz marks one corner of the Great Square of Pegasus, an autumn asterism formed by four stars of roughly equal brightness: Markab, Scheat, Algenib, and Alpheratz. Because the interior of the square contains few bright stars, the shape stands out clearly, and Alpheratz in the top-left corner serves as the starting point for tracing the constellation Andromeda, which extends away from the northeastern corner of the Square.[8](https://www.space.com/stargazing/night-sky-october-2026-what-to-look-for-this-month) • [10](https://www.space.com/stargazing/why-is-orion-lying-down-how-the-hunter-taught-me-how-the-sky-moves) The star is well placed in the evening sky during northern autumn.[10](https://www.space.com/stargazing/why-is-orion-lying-down-how-the-hunter-taught-me-how-the-sky-moves)
The star is also a practical signpost for finding the [Andromeda Galaxy](https://www.edgepedia.org/andromeda-galaxy) (M31), which lies 2.5 million light-years away and is visible to the naked eye as a faint, fuzzy patch under sufficiently dark skies. Observers commonly star-hop from Alpheratz along the chain of Andromeda's stars to Mirach and onward toward Cassiopeia to locate the galaxy; from brighter suburban locations, binoculars make the search considerably easier.[8](https://www.space.com/stargazing/night-sky-october-2026-what-to-look-for-this-month) • [10](https://www.space.com/stargazing/why-is-orion-lying-down-how-the-hunter-taught-me-how-the-sky-moves)
## The dual Bayer designation
When Johann Bayer assigned Greek-letter designations around 1603, Alpheratz received two: Alpha Andromedae and Delta Pegasi, because it lies on the boundary between the two constellations. The Delta Pegasi designation is no longer used.[2](https://www.star-facts.com/alpheratz/) • [11](https://www.xn--meb.pisz.pl/Alfa_Andromedae) The Cambridge Dictionary of Astronomy likewise notes that the star was formerly considered to belong to Pegasus and was designated Delta Pegasi.[3](https://assets.cambridge.org/97805218/00457/sample/9780521800457ws.pdf) In Vedic asterism lists, the nakshatra associated with the Square pairs [Gamma Pegasi](https://www.edgepedia.org/gamma-pegasi) with Alpha Andromedae (Alpheratz), as listed by the maharshi Parasara.[12](https://www.rasc.ca/sites/default/files/World%20Asterisms%20Project%20Handbook%20V%202025.14.pdf)
## A mercury–manganese binary
Slipher (1904) discovered the spectroscopic binary nature of Alpheratz. For decades the system was treated as single-lined, but in 1995 Tomkin, Pan, and McCarthy detected the secondary's spectrum, making it a double-lined spectroscopic binary; in 1992 Pan and colleagues had already resolved the components interferometrically. Alpheratz is therefore both a double-lined spectroscopic binary and an optical binary.[4](https://academic.oup.com/mnras/article/464/1/1095/2268699)
The primary, at visual magnitude 2.22 and spectral type B8IVpMnHg, is the brightest of the mercury–manganese stars, whose atmospheres contain abnormally high levels of mercury, manganese, and other elements; the secondary is an A3 V star of magnitude 4.21.[4](https://academic.oup.com/mnras/article/464/1/1095/2268699) Spectroscopic analysis gives an effective temperature of about 13,800 K for the primary and 8,500 K for the secondary, with the primary showing the large overabundances of manganese, phosphorus, gallium, and mercury typical of hot HgMn stars.[7](https://www.researchgate.net/publication/234399434_Orbital_elements_and_abundance_analyses_of_the_double-lined_spectroscopic_binary_alpha_Andromedae)
The orbit is well determined. A solution combining 42 interferometric observations and 378 radial velocities spanning more than a century of measurements gives a period of about 96.7 days and an eccentricity of about 0.555, with a semi-major axis of 0.72 ± 0.01 au; the dynamical parallax agrees closely with the [Hipparcos](https://www.edgepedia.org/hipparcos) trigonometric parallax of 33.63 ± 0.35 mas.[4](https://academic.oup.com/mnras/article/464/1/1095/2268699) Combining the spectroscopic and visual orbits yields adopted masses of 3.8 ± 0.2 solar masses for the primary and 1.85 ± 0.13 solar masses for the secondary, with a spectroscopic mass ratio of 0.50 ± 0.03; both components lie near the zero-age main sequence, with estimated ages of about 6 × 107 and 7 × 107 years.[7](https://www.researchgate.net/publication/234399434_Orbital_elements_and_abundance_analyses_of_the_double-lined_spectroscopic_binary_alpha_Andromedae)
## Observing notes
At magnitude 2.07, Alpheratz is an easy naked-eye star even from suburban locations, and it anchors two of the most useful autumn navigation patterns: the Great Square of Pegasus, whose right side points down toward the bright star Fomalhaut low in the south, and the star chain of Andromeda leading to M31.[8](https://www.space.com/stargazing/night-sky-october-2026-what-to-look-for-this-month) Because the star sits on the modern constellation boundary, star charts differ in how they attach it, a reminder that constellation boundaries are less tidy than the traditional drawings suggest.[10](https://www.space.com/stargazing/why-is-orion-lying-down-how-the-hunter-taught-me-how-the-sky-moves)
## References
1. [Alpheratz (α Andromedae), An Etymological Dictionary of Astronomy and Astrophysics, Observatoire de Paris](https://dictionary.obspm.fr/terms/alpheratz-%CE%B1-andromedae/)
2. [Alpheratz (Alpha Andromedae), Star Facts](https://www.star-facts.com/alpheratz/)
3. [The Facts on File Dictionary of Astronomy (sample pages), Cambridge University Press](https://assets.cambridge.org/97805218/00457/sample/9780521800457ws.pdf)
4. [Branham, R. L. Jr. (2017). A three-dimensional orbit for the binary star Alpha Andromedae. Monthly Notices of the Royal Astronomical Society 464, 1095](https://academic.oup.com/mnras/article/464/1/1095/2268699)
5. [Bulletin of the IAU Working Group on Star Names, No. 1](https://www.iau.org/static/science/scientific_bodies/working_groups/280/WGSN_bulletin1.pdf)
6. [Alpheratz, SIMBAD Astronomical Database, CDS](https://simbad.u-strasbg.fr/simbad/sim-basic?Ident=Alpheratz)
7. [Ryabchikova, T. A., Malanushenko, V. P., Adelman, S. J. (1999). Orbital elements and abundance analyses of the double-lined spectroscopic binary alpha Andromedae. Astronomy and Astrophysics 351, 963–972](https://www.researchgate.net/publication/234399434_Orbital_elements_and_abundance_analyses_of_the_double-lined_spectroscopic_binary_alpha_Andromedae)
8. [Night sky October 2026 — What to look for in the sky this month, Space.com](https://www.space.com/stargazing/night-sky-october-2026-what-to-look-for-this-month)
9. [Alpheratz — star of Andromeda, Heavens Remember](https://www.heavensremember.top/en/stars/alpheratz)
10. [Why is Orion lying down? How The Hunter taught me how the sky moves, Space.com](https://www.space.com/stargazing/why-is-orion-lying-down-how-the-hunter-taught-me-how-the-sky-moves)
11. [Alfa Andromedae, Encyklopedia internetowa](https://www.xn--meb.pisz.pl/Alfa_Andromedae)
12. [World Asterisms Project Handbook, Royal Astronomical Society of Canada](https://www.rasc.ca/sites/default/files/World%20Asterisms%20Project%20Handbook%20V%202025.14.pdf)
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*Topic: Encyclopedia › Physical world and mathematics › Astronomy › Stars and galaxies › Constellations, star names, and catalogs › Notable stars and star-system lists › Famous individual stars*
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