M51 · Whirlpool Galaxy
| Chinese name | 涡状星系 |
|---|---|
| Type | Galaxy |
| Constellation | CVn |
| RA | 13h30m |
| Dec | +47° |
| Apparent magnitude | 8.4ᵐ |
| Hemisphere | Northern |
| Best season | Spring |
| Difficulty | Moderate |
| Focal length | 长焦 1000mm+ |
About
The Whirlpool Galaxy lies about 23 million light-years away in Canes Venatici and is the most famous face-on 「grand-design」 spiral. Its bright, orderly arms are gravitationally interacting with the small companion NGC 5195, which is passing behind one arm and leaving a bridge of stars and gas connecting the two—this close encounter is what triggered and amplified M51's magnificent two-arm structure. The arms are studded with pink H II regions and blue young clusters, signatures of active star birth. It was the first galaxy in which spiral structure was recognized, by Lord Rosse in 1845, giving it great historical significance. Deep long-focus imaging reveals abundant arm detail and the tidal bridge, making it an iconic galaxy target.