What kind of star is Alpheratz?
Alpheratz, catalogued as Alpha Andromedae Aa (α And Aa), appears in the official constellation Andromeda. Its apparent visual magnitude is about 2.07 combined, while its estimated distance is about 97 light-years. Astronomers classify the object as B8 IVp mercury-manganese spectroscopic binary. These labels describe temperature, luminosity class or system components; they should not be read as a claim that every naked-eye point is a single isolated star.
Alpheratz Aa is a chemically peculiar late-B star in a short-period spectroscopic binary. Slow mixing and atomic diffusion allow radiation and gravity to sort elements in its stable atmosphere, enhancing mercury, manganese and other species far beyond solar proportions. Surface abundance patches rotate in and out of view, changing spectral-line profiles without implying that the entire star has the same composition everywhere. The companion contributes less light and cannot be routinely separated in a small telescope. Because the system is bright and nearby, it became a benchmark for mapping chemical spots and testing diffusion in non-magnetic or weakly magnetic peculiar stars.
The coordinate pair RA 00h 08m 23.26s, Dec +29° 05′ 25.6″ fixes the object's direction on the celestial sphere for the J2000 reference epoch. Right ascension works like celestial longitude measured in hours, while declination works like latitude north or south of the celestial equator. Proper motion and improved measurements can refine catalogue positions, but these coordinates are sufficiently precise for a finder chart or planetarium search.
Alpheratz data at a glance
| Proper name | Alpheratz | Catalogue designation | Alpha Andromedae Aa (α And Aa) |
|---|---|---|---|
| Constellation | Andromeda | Apparent magnitude | about 2.07 combined |
| Spectral or system type | B8 IVp mercury-manganese spectroscopic binary | Estimated distance | about 97 light-years |
| Right ascension (J2000) | 00h 08m 23.26s | Declination (J2000) | +29° 05′ 25.6″ |
Values are rounded for an observer-friendly overview. Consult the linked SIMBAD record for component-level identifiers, bibliography, measurement provenance and newer catalogue values.
The name Alpheratz
Alpheratz derives from Arabic surrat al-faras, “the horse’s navel.” The name recalls its former dual identity in Pegasus even though modern boundaries place it in Andromeda.
For centuries Alpheratz was shared between Andromeda and Pegasus, carrying both Alpha Andromedae and Delta Pegasi in older usage. The IAU’s fixed constellation boundaries placed it solely in Andromeda, but its Arabic “horse” name and its corner of the Great Square preserve the Pegasus connection. Spectroscopy later established the binary orbit and extraordinary mercury-manganese chemistry. The IAU assigned Alpheratz specifically to component Aa in 2016.
Modern professional catalogues retain identifiers such as the Bayer designation alongside a proper name. The International Astronomical Union standardizes which name belongs to which component—an important distinction for systems whose combined light looks like one star from Earth.
How to find Alpheratz
Alpheratz is the northeastern corner of the Great Square of Pegasus and the start of Andromeda’s chain. It is readily visible in city skies. Compare it with orange Mirach farther along the chain; its close companion and unusual chemistry require spectroscopy rather than visual resolution.
Best on northern autumn evenings; visible from most inhabited latitudes. Visibility on a particular night still depends on latitude, season, time, horizon obstructions, weather and local light pollution. Use the interactive atlas below to check the surrounding field before observing.
Observation checklist
- Find the Great Square; Alpheratz is its corner that leads toward Mirach and Almach.
- Use it as the first step in a star-hop to the Andromeda Galaxy.
- Compare its blue-white appearance with Mirach’s orange colour in binoculars.
- Its chemical spots and close companion require spectroscopy or specialist instruments, not visual magnification.
Interesting facts about Alpheratz
- Alpheratz is officially in Andromeda but forms a corner of the Great Square of Pegasus.
- Older catalogues also called it Delta Pegasi.
- Its primary is a mercury-manganese chemically peculiar star.
- The abundance of elements varies across its stellar surface.
- The proper name belongs to component Aa of a spectroscopic binary.
Continue from Alpheratz into the night sky
Use the Andromeda constellation guide to place this star within its larger pattern. When you are ready to observe, compare dark-sky places, find a nearby observatory, or follow our practical guide to finding a star in the night sky. Symbolically registered names are kept separately in the searchable Star-Register catalogue.
Sources and further reading
Measurements are rounded for readability. Multiple systems, variable stars and distant supergiants can have component-specific or model-dependent values. Survey panels show photographic sky fields, not the physical surface of the star.