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Alpha Persei Cluster (Mel 20)
open cluster in Perseus
The Alpha Persei Cluster is a bright, loose group of young stars around Mirfak in Perseus, spread across about five degrees. It is a naked-eye cluster that looks fine in a wide-field lens image.
Save your telescope or lens on the planner to lay your camera frame over this diagram.
// Key facts
- Catalog
- Mel 20
- Also known as
- Melotte 20, Collinder 39
- Type
- open cluster
- Constellation
- Perseus
- Size
- 5.0° across
- Brightness
- Magnitude 1.2
- Distance
- about 550 light-years
- Peak month (any location)
- Nov
- Typical prime months
- Aug to Jan
- Difficulty
- Beginner
- Filter
- No filter (broadband)
Pick a saved location or share your device location. Your coordinates are used only to calculate tonight and are not stored. To save locations, use the planner.
How to image Alpha Persei Cluster
- Use 50 to 135 mm; it spans about 5 degrees.
- No filter is needed; short exposures keep star colors natural.
- It sits near the California Nebula and the Double Cluster.
Framing: what focal length?
The focal length at which Mel 20 spans about 60% of the long side of the sensor, leaving room around it. Treat it as a starting point; the right framing is a matter of taste.
| Sensor | Focal length |
|---|---|
| 1-inch sensor (13.2 mm) | about 90 mm |
| APS-C (23.5 mm) | about 160 mm |
| Full frame (36 mm) | about 250 mm |
How high does it climb from your latitude?
The highest point Mel 20 reaches in the sky. Above about 60° is excellent, 30° to 60° is workable, and under 30° you are looking through a lot of air.
| Latitude | Highest point | Rating |
|---|---|---|
| 55°N (Scotland, Alaska) | 85° | High in the sky |
| 45°N (Minneapolis, Milan) | 85° | High in the sky |
| 35°N (Los Angeles, Tokyo) | 75° | High in the sky |
| 25°N (Miami, Cairo) | 65° | High in the sky |
| 35°S (Sydney, Santiago) | 5° | Stays low |
Also worth imaging
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