33 Persei

Alpha Persei ( Latinized from α Persei, abbreviated Alpha Per, α Per), formally named Mirfak (pronounced or ), is the brightest star in the northern constellation of Perseus, outshining the constellation's best-known star, Algol (β Persei). Alpha Persei has an apparent visual magnitude of 1.8, and is a circumpolar star when viewed from mid-northern latitudes.

Alpha Persei lies in the midst of a cluster of stars named as the eponymous Alpha Persei Cluster, or ''Melotte 20'', which is easily visible in binoculars and includes many of the fainter stars in the constellation. Determined distance using the trigonometric parallax, places the star away.

Nomenclature

''α Persei'' is the star's Bayer designation.

The star also bore the traditional names Mirfak and Algenib, which are Arabic in origin. The former, meaning 'Elbow' and also written Mirphak, Marfak or Mirzac, comes from the Arabic ''Mirfaq al-Thurayya'', while Algenib, also spelt Algeneb, Elgenab, Gęnib, Chenib or Alchemb, is derived from الجنب ''al-janb'', or الجانب ''al-jānib'', 'the flank' or 'side'. and was also the traditional name for Gamma Pegasi. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included ''Mirfak'' for this star (Gamma Pegasi was given the name ''Algenib'').

''Hinaliʻi'' is the name of the star in Native Hawaiian astronomy. The name of the star is meant to commemorate a great tsunami and mark the beginning of the migration of Maui. According to some Hawaiian folklore, Hinaliʻi is the point of separation between the Earth and the sky that happened during the creation of the Milky Way.

Assemani alluded to a title on the Borgian globe, ''Mughammid'' (مغمد), or ''Muliammir al Thurayya'' (ملىمرٱلطرى), the Concealer of the Pleiades, which, from its location, may be for this star.

This star, together with γ Persei, δ Persei, η Persei, σ Persei and ψ Persei, has been called ''the Segment of Perseus''.

In Chinese, (), meaning '' Celestial Boat'', refers to an asterism consisting of α Persei, γ Persei, δ Persei, η Persei, μ Persei, ψ Persei, 48 Persei and HD 27084. Consequently, the Chinese name for α Persei itself is (, ).AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 11 日

Physical properties

The spectrum of Alpha Persei matches a stellar classification of F5 Ib, revealing it to be a supergiant star in the latter stages of its evolution. It has a similar spectrum to Procyon A, though the latter star is much less luminous. This difference is highlighted in their spectral designation under the Yerkes spectral classification, published in 1943, where stars are ranked on luminosity as well as spectral typing. Procyon A is thus F5 IV, a subgiant star. Since 1943, the spectrum of Alpha Persei has served as one of the stable anchor points by which other stars are classified.

Alpha Persei has about 7.3 times the Sun's mass and has expanded to roughly 53 times the size of the Sun. It is radiating 4,020 times the luminosity of the Sun from its photosphere at an effective temperature of , which creates the yellow-white glow of an F-type star.

In the Hertzsprung–Russell diagram, Alpha Persei lies inside the region in which Cepheid variables are found. It is thus useful in the study of these stars, which are important standard candles. Despite Mirfak being not a Cepheid, Hipparcos photometry show a 0.006-magnitude variation with a period of 9.5 days.

Search for planets

Alpha Persei has periodic radial velocity variations, with an amplitude of 70.8 ± 1.6 m/s. These variations are of uncertain origin; a 2012 study considered the possibility of them being caused by an orbiting exoplanet, estimated to have a minimum mass of approximately 6.6 times that of Jupiter and an orbital period of 128 days, but the claimed period may not be stable over 20 years so the exoplanet was considered doubtful. The authors suggested rotational modulation due to surface activity such as starspots as a more likely explanation of the radial velocity variations. In previous publications, periodic radial velocity variations of 87.7 or 77.7 days have been reported, but these have not been confirmed.

A further study, published in 2025, analysing radial velocity observations, found that the period of ~130 days weakened and disappeared during a time, while a 66-day period appeared. The hypothesis of a planet around Mirfak can now be discarded, considering the period changes over time. Variations with periods of 9.8 and 77.7 days, reported in previous studies, have not been detected. The most reasonable explanation to the radial velocity variations is stellar oscillations.

References

{{Portal bar, Astronomy, Stars, Outer space

F-type supergiants
Perseus (constellation)
Alpha Persei Cluster
Persei, Alpha
BD+49 0917
Persei, 33
020902
015863
1017
Mirfak