Epsilon Persei

Lore & Background

Epsilon Persei is a spectroscopic binary system, with the presence of an orbiting companion revealed by radial velocity variations in the spectrum of the primary. The two components orbit each other with a period of 14 days at a high orbital eccentricity of 0.55. The secondary component has about 6–13% of the primary's mass and may have a stellar classification in the range from A6 V to K1 V. A possible third component may have an orbital period of roughly 9,428 days (25.8 years), though this has not been conclusively demonstrated; if it exists, it would have about 51–139% of the primary's mass, with high uncertainty due to the unknown orbital inclination.

The primary component is a massive star with 12–16 times the Sun's mass and near eight times the radius of the Sun. It was chosen as an MK spectral standard for the class B0.5III, making it a giant star that has evolved away from the main sequence, though it has also been classified as a main sequence star, reflecting its evolutionary state near or slightly beyond the end of core hydrogen fusion. Component A radiates over 28,000 times the Sun's luminosity at an effective temperature of 26,500 K, giving it the blue-white hue typical of B-type stars. It is a Beta Cephei variable star with a primary pulsation period of 0.1603 days (6.24 cycles per day) and may have multiple pulsation frequencies.

Epsilon Persei has several resolved companions: component B is a 9th-magnitude A2 main sequence star considered a common proper motion companion, though they are too far apart for orbital motion to be observed; component C is a 14th-magnitude background star; and HD 275984 is thought to be another common-proper-motion companion.

Reader's Guide

Epsilon Persei holds significance as a bright, multi-component system that serves as an MK spectral standard for the B0.5III class, anchoring the classification of giant B-type stars. Its status as a Beta Cephei variable—with a primary pulsation period of 0.1603 days—makes it a key object for studying stellar pulsations in massive stars. The system's spectroscopic binary nature, with a 14-day orbit and high eccentricity, provides insights into binary evolution and mass ratios. The possible third component, if confirmed, would add complexity to the system's dynamics. In Sámi culture, Epsilon Persei is part of Sarvvis (or Sarvva), the reindeer or moose, a large constellation spanning Cassiopeia, Perseus, and Auriga, pursued by the hunter Fávdna (Arcturus). The star's formal name Áldu, approved by the IAU Working Group on Star Names, reflects the Sámi word for a female reindeer, contrasting with the male reindeer name Sarvvis given to δ Persei A. This cultural connection enriches the star's legacy beyond its astrophysical properties.

Did You Know?

• Epsilon Persei is a spectroscopic binary with an orbital period of 14 days and a high eccentricity of 0.55.
• The primary component is a Beta Cephei variable with a pulsation period of 0.1603 days.
• The star's formal name Áldu was approved by the IAU Working Group on Star Names.
• In Sámi culture, Epsilon Persei is part of the constellation Sarvvis, representing a reindeer or moose.

Orbital Architecture of a Complex System

Epsilon Persei is not a single star but a system whose internal structure has been pieced together through spectroscopic observations. The primary and its close companion trace an elliptical orbit with a period of just 14 days, and the high eccentricity of 0.55 means their separation swings dramatically over each cycle. The secondary carries only 6 to 13 percent of the primary's mass and likely falls somewhere between an A6 and K1 main-sequence classification. Beyond this inner pair, evidence hints at a third body completing a far longer revolution of roughly 9,428 days, or about 25.8 years. If confirmed, that outer companion would possess between 51 and 139 percent of the primary's mass, though the wide range reflects the fact that the orbital inclination remains unknown. The system sits at a distance of approximately 640 light-years, and its combined light reaches Earth at an apparent magnitude of +2.88, bright enough for unaided observation in the northern sky.

A Blue-White Giant at the Edge of Its Life

The dominant member of the system, designated component A, is a massive star weighing between 12 and 16 times that of the Sun while spanning nearly eight solar radii. It has been adopted as the MK spectral standard for the B0.5III class, marking it as a giant that has moved off the main sequence, though some analyses still place it at or just past the end of core hydrogen burning. Its outer envelope radiates more than 28,000 times the Sun's total luminosity at an effective temperature of 26,500 kelvin, producing the characteristic blue-white glow of B-type stars. Beyond its static properties, component A is a Beta Cephei variable whose primary pulsation cycle lasts 0.1603 days, equivalent to 6.24 oscillations per day, and it may exhibit additional pulsation frequencies on top of that fundamental mode.

A Broader Stellar Neighborhood

Looking beyond the spectroscopic pair, several additional stars appear in the vicinity of Epsilon Persei in multiple-star catalogues. Component B is a ninth-magnitude A2 main-sequence star separated by 9 arcseconds; it shares a common proper motion with the primary, suggesting a physical association, yet the angular distance is too great for any orbital interaction to be detected. Further out, a 14th-magnitude object at roughly 80 arcseconds is catalogued as component C, though it is identified as a background star rather than a true companion. At 163 arcseconds, the star HD 275984 is also thought to be a common-proper-motion companion, extending the apparent family of associated objects. Together these resolved neighbors paint a picture of a moderately rich stellar environment around a system that, at about 196 parsecs, remains one of the more accessible B-type giants for ground-based study.

The Sámi Name Áldu and Its Cultural Story

In the traditional sky lore of the Sámi people, Epsilon Persei belongs to a large constellation called Sarvvis, or Sarvva, representing a reindeer or moose that stretches across Cassiopeia, Perseus, and Auriga. This celestial animal is eternally pursued by Fávdna, the hunter identified with the star Arcturus. A curious detail in the mythology is that Sarvvis is described as a male reindeer that retains its antlers through winter, a trait that in reality belongs only to females. The Sámi word for a female reindeer is Áldu, and this is the name the IAU Working Group on Star Names formally approved for component A of Epsilon Persei on 25 December 2025. The name has since been entered into the IAU Catalog of Star Names. Notably, the companion δ Persei A received the proper name Sarvvis, linking the two stars within the same cultural narrative.

Gallery