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.

• 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