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Whirlpool Galaxy (M51)
spiral galaxy in Canes Venatici
The Whirlpool Galaxy is a face-on spiral caught in the act of colliding with a smaller companion, NGC 5195. The spiral arms are sharply defined, and faint tidal tails stretch away from the pair when you give it enough time. It is a favorite for medium and long focal lengths.
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// Key facts
- Catalog
- M51
- Type
- spiral galaxy
- Constellation
- Canes Venatici
- Size
- 11′ across
- Brightness
- Magnitude 8.4
- Distance
- about 23 million light-years
- Peak month (any location)
- Apr
- Typical prime months
- Jan to Jun
- Difficulty
- Intermediate
- Filter
- No filter (broadband); a light-pollution filter is optional
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How to image Whirlpool Galaxy
- Aim for 1,000 to 2,000 mm. The pair is only about eleven arcminutes across, about a quarter of an APS-C frame at 2,000 mm.
- The outer tidal streams are very faint and need a dark sky and many hours of exposure.
- It sits high in the northern sky in spring, so it is a strong choice for mid-northern latitudes.
Framing: what focal length?
M51 is small, so a "60% of the frame" focal length would be impractical. Here is how much of an APS-C frame it covers at common long focal lengths; expect to crop.
| Focal length | Share of the frame width |
|---|---|
| 1,000 mm | about 14% |
| 2,000 mm | about 27% |
| 3,000 mm | about 41% |
How high does it climb from your latitude?
The highest point M51 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) | 82° | High in the sky |
| 45°N (Minneapolis, Milan) | 88° | High in the sky |
| 35°N (Los Angeles, Tokyo) | 78° | High in the sky |
| 25°N (Miami, Cairo) | 68° | High in the sky |
| 35°S (Sydney, Santiago) | 8° | Stays low |
Also worth imaging
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