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 | |
|---|---|
 | |
| Observation data (2000.0 epoch) | |
| Right ascension | 03h 26m 42.0s[1] |
| Declination | +48° 48′ 00″[1] |
| Distance | 570 ly[1] (175 pc[1]) |
| Apparent magnitude (V) | 1.2 |
| Apparent dimensions (V) | 6.1°[1] |
| Physical characteristics | |
| Estimated age | 50–70 Mya[2][3] |
| Other designations | Per OB3,[4] Cr 39, Mel 20, OCl 392.0[5] |
| Associations | |
| Constellation | Perseus |
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]
Members
The following prominent stars are considered members of the cluster with high likelihood:
| Designation | Spectral type | Visual magnitude |
|---|---|---|
| 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 21699 | B8 III | 5.49 |
| HD 21071[13] | B7V | 6.09 |
See also
References
External links
- 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. Archived from the original on 2017-04-08. Retrieved 2020-10-18.