Double Cluster Perseus
On the border of Perseus and Cassiopeia sits one of the most spectacular deep‑sky sights — the Double Cluster, also known as h and χ (chi) Persei. To the naked eye it appears as a hazy patch, best seen in autumn/winter between Cassiopeia’s “W” and Perseus. Both clusters lie in the Perseus Arm of our Galaxy.
Key facts
- Distance: ~7,500 light‑years (NGC 869 ~7,460 ly; NGC 884 ~7,640 ly).
- Age: very young, ~12–14 million years (both clusters).
- Separation: only a few hundred light‑years.
- Radial velocity: both clusters are blueshifted, approaching at ~38–39 km/s.
- Stellar content: dominated by hot, young B‑type (B0) stars; notable red supergiants present (e.g., in NGC 884).
- Association: they form the core of Perseus OB1.
Astrophysics: blue and red
The image is dominated by the cool blue of young, hot spectral type B and A stars. However, distinct orange-red points are clearly visible among them.
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Evolution: These are Red Supergiants. Despite the cluster’s young age, the most massive stars have already exhausted their hydrogen, expanded, and entered the helium-burning phase. This is a textbook example of how stellar evolution speed depends on mass.
Kinematics
Blueshift Most deep-sky objects are moving away from us (Redshift). The Double Cluster is an exception. Spectroscopic measurements show that this system is approaching Earth with a radial velocity of approximately 20-22 km/s. This means local gravity within the galactic arm dominates over general expansion here.
Curiosities & context
- In Polish, amateurs sometimes nickname them “hihotki” (wordplay: h + χ). (naming custom)
- For comparison, the Pleiades (M45) are ~75–150 Myr old — several times older.
Parameter table
Parameter | NGC 869 (h Persei) | NGC 884 (χ Persei) |
|---|---|---|
RA / Dec (J2000) | ~02h19m, +57°09′ | ~02h22m, +57°08′ |
Distance | ~7,460 ly | ~7,640 ly |
Age | ~14 Myr (literature: 12–14 Myr) | ~14 Myr (literature: 12–14 Myr) |
Total magnitude (V) | ~3.7 mag | ~3.8 mag |
Apparent size | ~30′ (pair ~60′ combined) | ~30′ (pair ~60′ combined) |
Radial velocity | ~39 km/s (approaching) | ~38 km/s (approaching) |
Association | Perseus OB1 | Perseus OB1 |
Cluster separation | Hundreds of light‑years |
Source:
- https://en.wikipedia.org/wiki/Double_Cluster
- https://www.constellation-guide.com/double-cluster/
- https://apod.nasa.gov/apod/ap970129.html
Technical photo information:
- Composition: Astro Pixel Processor,
- Processing: GIMP v2.10.14 + plug-ins (Linux),
- Lights: 24 x 96[s], ISO 1250,
- Flats, Darks, Bias.
