Perseus Constellation: Algol & Double Cluster
Perseus IAU constellation guide: autumn/winter Milky Way constellation hosting the Double Cluster (NGC 869/884) and the eclipsing binary Algol (Beta Persei).
Sky Navigation & Astrophysical Context
Perseus stretches across 615 square degrees of the northern Milky Way between Cassiopeia and Taurus. Its brightest star, the F5 Ib supergiant Mirfak (Alpha Persei, V = +1.79), sits surrounded by the sparkling Alpha Persei Moving Cluster (Melotte 20), a loose OB association that fills binocular fields with dozens of blue-white stars.
At the northwestern border with Cassiopeia lies the Double Cluster (NGC 869 and NGC 884, h and χ Persei)—two young, supergiant-rich open clusters separated by 30 arcminutes in the Perseus spiral arm at 2.3 kiloparsecs.
In the southwestern branch of the constellation shines Algol (Beta Persei), the prototype semidetached eclipsing binary ('The Demon Star'), which drops by 1.27 visual magnitudes from +2.12 to +3.39 every 2.8673 days during a 10-hour primary eclipse easily tracked with the unaided eye.
Equatorial Center Position & Transit Altitude
Perseus is centered near Right Ascension 03h 30m 00s, Declination +45° 00′ 00″ (Perseus (Hero of Greek Mythology)).
Practical Star-Hopping & Observing Tips
- Sweep binoculars around Mirfak (Alpha Persei) to enjoy the S-shaped star chain of the Alpha Persei Moving Cluster.
- Aim midway between Mirfak and Ruchbah (Delta Cassiopeiae) to frame both cores of the Double Cluster (NGC 869 and NGC 884) at 20×–40×.
- Compare Algol's brightness to nearby Gamma Andromedae (+2.10) and Epsilon Persei (+2.89) during a predicted primary eclipse window.
Notable Celestial Objects in & Near Perseus
| Object | Family | Classification | RA (J2000) | Dec (J2000) | Min. Instrument |
|---|---|---|---|---|---|
| Double Cluster (NGC 869 & NGC 884) | open-clusters | Twin young supergiant open clusters (Perseus OB1 core) | 02h 20m 00.00s | +57° 08′ 00.0″ | Naked eye; best in binoculars and wide-field telescopes |
| Algol (Beta Persei Eclipsing Binary) | variable-stars | Semidetached eclipsing binary prototype in a hierarchical triple system | 03h 08m 10.13s | +40° 57′ 20.3″ | Naked eye |
| Messier 34 (NGC 1039 Spiral Cluster) | open-clusters | Open star cluster | 02h 42m 06.00s | +42° 45′ 42.0″ | Naked eye / 7×50 binoculars |
| Little Dumbbell Nebula (Messier 76) | nebulae | Bipolar planetary nebula | 01h 42m 19.94s | +51° 34′ 31.5″ | 70–100 mm telescope |
| California Nebula (NGC 1499 in Perseus) | nebulae | Diffuse H-alpha / H-beta emission nebula | 04h 03m 24.00s | +36° 25′ 18.0″ | Naked eye + H-beta filter / 60–100 mm rich-field scope + H-beta filter |
| Mirfak (Alpha Persei Yellow Supergiant) | stars | Yellow-white F-type supergiant (MK standard for F5 Ib & Melotte 20 anchor) | 03h 24m 19.37s | +49° 51′ 40.2″ | Naked eye (7×50 or 10×50 binoculars for the spectacular Melotte 20 cluster) |
Scientific Sources Cited
- IAU Official Constellation Boundaries & Designations (IAU 88 Recognized Constellations): Delporte, E. 1930, Délimitation scientifique des constellations (IAU / Cambridge University Press) — International Astronomical Union (IAU)
- SIMBAD Astronomical Database (CDS) (2026-10 release): Wenger, M., et al. 2000, A&AS, 143, 9 (The SIMBAD astronomical database) — Centre de Données astronomiques de Strasbourg
- General Catalogue of Variable Stars (GCVS) & AAVSO VSX (v5.1 (2026 update)): Samus, N. N., et al. 2017, Astronomy Reports, 61, 80 (General Catalogue of Variable Stars: Version GCVS 5.1) — Sternberg Astronomical Institute / AAVSO