Alpha Persei Cluster


The Alpha Persei Cluster, also known as Melotte 20 or Collinder 39, is an open cluster of stars in the northern constellation of Perseus. To the naked eye, the cluster consists of several blue-hued spectral type B stars. The most luminous member is the ~2nd magnitude white-yellow supergiant Mirfak, also known as Alpha Persei. Bright members also include Delta, Sigma, Psi, 29, 30, 34, and 48 Persei. The Hipparcos satellite and infrared color-magnitude diagram fitting have been used to establish a distance to the cluster of ~560 light-years (172 pc).[6][7] The distance established via the independent analyses agree, thereby making the cluster an important rung on the cosmic distance ladder. As seen from the Earth, the extinction of the cluster due to interstellar dust is around 0.30.[8]

Alpha Persei Cluster
Melotte 20.jpg
Observation data (2000.0 epoch)
Right ascension03h 26m 42.0s[1]
Declination+48° 48′ 00[1]
Distance570 ly[1] (175 pc[1])
Apparent magnitude (V)1.2
Apparent dimensions (V)6.1°[1]
Physical characteristics
Estimated age50–70 Mya[2][3]
Other designationsPer OB3,[4] Cr 39, Mel 20, OCl 392.0[5]
See also: Open cluster, List of open clusters

The cluster is centered to the northeast of Alpha Persei.[8] It has a core radius of 11.4 ± 1.4 ly, a half-mass radius of 18 ly,[8] and a tidal radius of 70.6 ± 8.5 ly,[1] with 517 members being identified within the latter.[8] The cluster shows solid evidence of having undergone mass segregation, with the mean stellar mass decreasing toward the edge.[9] The age of this cluster is about 50–70 million years.[2][3] Cluster member stars show a near-solar metallicity, meaning the abundance of elements with atomic numbers higher than 2 are similar to those in the Sun.[8] The cluster shows evidence of tidal tails, which are most likely of galactic origin.[10]

The cluster field displays evidence of a much larger, background star stream. This feature is quite a bit older than the cluster, with an estimated age of 5 ± 1 Gyr. The center of the stream lies 290 ly from the cluster and it has an overall thickness of 590 ly along the line of sight. This is most likely the remains of an old, massive cluster that now has a combined mass of ~6000 M.[10]


The following prominent stars are considered members of the cluster with high likelihood:

Designation Spectral
α Per (33 Per)[11] F5Ib 1.81
δ Per (39 Per) B5III 3.01
ε Per (45 Per) B1V 2.88
ψ Per[11] B5Ve 4.31
HD 21278[12][13] B5V 4.99
31 Per[11] B5V 5.05
29 Per[11][13] B3V 5.16
30 Per[13] B8V 5.49
34 Per B3V 4.67
48 Per B3Ve 4.03
HD 21071[13] B7V 6.09

See alsoEdit


  1. ^ a b c d e f Kharchenko, N. V.; et al. (2013), "Global survey of star clusters in the Milky Way. II. The catalogue of basic parameters", Astronomy and Astrophysics, 558: 8, arXiv:1308.5822, Bibcode:2013A&A...558A..53K, doi:10.1051/0004-6361/201322302, A53.
  2. ^ a b Prosser, Charles F.; et al. (October 1996). "ROSAT Pointed Observations of the Alpha Persei Cluster". Astronomical Journal. 112: 1570. Bibcode:1996AJ....112.1570P. doi:10.1086/118124.
  3. ^ a b Paunzen, E.; Mermilliod, J.-C. "WEBDA: Alpha Persei". Universität Wien. Retrieved 2020-10-17.
  4. ^ Coyne, G. V.; et al. (March 1979), "Wavelength dependence of polarization. XXXIII - The Alpha Persei star cluster", Astronomical Journal, 84: 356–369, Bibcode:1979AJ.....84..356C, doi:10.1086/112431
  5. ^ "Cl Melotte 20". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2020-10-17.{{cite web}}: CS1 maint: postscript (link)
  6. ^ van Leeuwen, F. (2009). "Parallaxes and proper motions for 20 open clusters as based on the new Hipparcos catalogue". Astronomy & Astrophysics. 497 (1): 209–242. arXiv:0902.1039. Bibcode:2009A&A...497..209V. doi:10.1051/0004-6361/200811382.
  7. ^ Majaess, D.; et al. (2011). "Deep Infrared ZAMS Fits to Benchmark Open Clusters Hosting delta Scuti Stars". Journal of the American Association of Variable Star Observers. 39 (2): 219. arXiv:1102.1705. Bibcode:2011JAVSO..39..219M.
  8. ^ a b c d e Lodieu, N.; et al. (August 2019). "A 5D view of the α Per, Pleiades, and Praesepe clusters". Astronomy & Astrophysics. 628: 26. arXiv:1906.03924. Bibcode:2019A&A...628A..66L. doi:10.1051/0004-6361/201935533. A66.
  9. ^ Sheikhi, Najmeh; et al. (March 2016). "The binary fraction and mass segregation in Alpha Persei open cluster". Monthly Notices of the Royal Astronomical Society. 457 (1): 1028–1036. arXiv:1601.02186. Bibcode:2016MNRAS.457.1028S. doi:10.1093/mnras/stw059.
  10. ^ a b Nikiforova, Victoria V.; et al. (September 2020). "The Relation of the Alpha Persei Star Cluster with the Nearby Stellar Stream". The Astronomical Journal. 160 (3): 11. arXiv:2007.11211. Bibcode:2020AJ....160..142N. doi:10.3847/1538-3881/aba753. 142.
  11. ^ a b c d Zuckerman, B.; et al. (June 2012). "Stellar Membership and Dusty Debris Disks in the α Persei Cluster". The Astrophysical Journal. 752 (1): 12. arXiv:1204.3950. Bibcode:2012ApJ...752...58Z. doi:10.1088/0004-637X/752/1/58. 58.
  12. ^ Morrell, Nidia; Abt, Helmut A. (July 10, 1992), "Spectroscopic binaries in the Alpha Persei cluster", Astrophysical Journal, Part 1, 393 (2): 666–673, Bibcode:1992ApJ...393..666M, doi:10.1086/171534.
  13. ^ a b c d Hoogerwerf, Ronnie (March 2000), "OB association members in the ACT and TRC catalogues", Monthly Notices of the Royal Astronomical Society, 313 (1): 43–65, Bibcode:2000MNRAS.313...43H, doi:10.1046/j.1365-8711.2000.03192.x

External linksEdit

  • Frommert, Hartmut; Kronberg, Christine (March 8, 1998). "Alpha Persei Moving Cluster, Mel 20". The Messier Catalogue. SEDS. Retrieved 2020-10-18.
  • Käld, Janne. "The Alpha Persei Cluster (Mel 20)". Astronomical Photographs. Retrieved 2020-10-18.

Coordinates:   03h 22m 00s, +49° 00′ 00″