Last modified on 6 April 2019, at 08:18

Gamma Ursae Minoris

Gamma Ursae Minoris
Observational Data
Designation γ Ursae Minoris
Pherkad
Phecda
Right ascension 15h 20m 43.7160s[1][2]
Declination +71° 50′ 02.4596″[1][2]
Constellation Ursa Minor
Type of object Main sequence star
Magnitude Apparent Mag: +2.438[3]
Absolute Mag: +18.32[4]
Astrometry
Distance from Earth 487 ly[5]
Radial velocity -3.9±0.6 km/s[1][6]
Proper motion RA: -17.73 mas/yr[1][2]
Dec.: 17.90 mas/yr[1][2]
Parallax 6.70±0.11 mas[1][2]

Gamma Ursae Minoris (γ Ursae Minoris, Gamma UMi, γ UMi) is a main sequence star in the constellation of Ursa Minor, the little bear.[3] The star is sometimes also called "Pherkad", "Phecda" or "Phad" (from the Arabic fakhð ad-dubb, “the thigh of the bear”). Together with the star Beta Ursae Minoris (Kochab), it is one of the Guardians of the Pole.[7] With an apparent magnitude of roughly +2.4 the star is easily visible to the unaided eye and can be observed in the Northern hemisphere with ease.

The third brightest star in Ursa Minor, it it thought to lie approximately 187 light years away, though other sources place it much lower at 83.2 ly.[5][8][3] It is noticeably larger than the Sun, with around 15 times the solar radius.[5] With a spectral class of A2III C,[1] its surface temperature has been estimated to be 8,280 kelvin.[5] The luminosity of Gamma Ursae Minoris is thought to be 1,050 times greater than that of Sol from this its mass to be 4.8 solar masses. The star rotates rapidly, so that a point on its equator moves at 175 km/s.[5] Its negative radial velocity of -3.9±0.6 km/s means it is travelling towards Earth.[1]

The star is thought to be a delta scuti variable star, though classifying it has proved tricky.[1] It was originally believed to be a member of the "Maia" class (after a star in the Pleiades), but the whole class has since disappeared and is no longer used.[5] Gamma Ursae Minoris has a period of 2.32 hours over which time it varies by around 0.1 magnitudes.[9][10]

References

  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 Gamma Ursae Minoris from the SIMBAD Astronomical Database
  2. 2.0 2.1 2.2 2.3 2.4 van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy & Astrophysics 474 (2): 653-664. doi:10.1051/0004-6361:20078357. BibcodeA&A...474..653V.  arXiv:0708.1752
  3. 3.0 3.1 3.2 Big Dipper. Retrieved on 2019-03-26.
  4. From definition of absolute magnitude, using apparent magnitude (+2.438) and distance (487 ly) given here.
  5. 5.0 5.1 5.2 5.3 5.4 5.5 Pherkad. Retrieved on 2019-03-26.
  6. Gontcharov, G. (2006). "Pulkovo Compilation of Radial Velocities for 35 495 Hipparcos stars in a common system". Astronomy Letters 32 (11): 759-771. doi:10.1134/S1063773706110065. Bibcode2006AstL...32..759G.  arXiv:1606.08053
  7. The Guardians of the Pole. Retrieved on 2019-03-26.
  8. Pherkad (Gamma Ursae Minoris, 13 Ursae Minoris). Retrieved on 2019-03-26.
  9. Hip 75097-GCVS Query forms. Retrieved on 2019-03-26.
  10. Joshi, S.; Gurtu, S.; Joshi, M. (1969). "The light variation of gamma Ursae Minoris". The Observatory 89: 112-114. Bibcode1969Obs....89..112J.