Pinwheel Galaxy vs Whirlpool Galaxy
Both are face-on spirals high in the northern sky, and the brighter of the two on paper is the harder of the two at the eyepiece. The Pinwheel (M101, NGC 5457) carries magnitude 7.90 against 8.36 for the Whirlpool (M51, NGC 5194), but M101 spreads that light across 23.99 arcminutes to M51's 13.71. The resulting surface brightnesses — about 14.5 and 13.8 magnitudes per square arcminute — put M51 ahead by nearly a magnitude of contrast. M101 has a long reputation among observers for disappearing entirely under skies where M51 still shows structure, and the arithmetic explains why.
Property comparison
| Property | Pinwheel Galaxy | Whirlpool Galaxy |
|---|---|---|
| Object type | Galaxy | Galaxy |
| Catalogue designation | NGC 5457 | NGC 5194 |
| Integrated magnitude | 7.90 | 8.36 |
| Apparent size | 24′ (23.99′) | 14′ (13.71′) |
| Mean surface brightness | 14.5 mag/arcmin² | 13.8 mag/arcmin² |
| Position (J2000) | RA 14h, Dec +54° | RA 13h, Dec +47° |
| Sky region | Far northern sky | Far northern sky |
Mean surface brightness is derived from the integrated magnitude and the apparent diameter; higher values mean a fainter, more diffuse surface.
Full data for each object: Pinwheel Galaxy · Whirlpool Galaxy
Key differences
Key Differences
- ◆ M101 is brighter overall at magnitude 7.90 against 8.36 for M51.
- ◆ M101 spans 23.99 arcminutes against 13.71 for M51, about 1.8 times wider.
- ◆ M51 holds the higher surface brightness, near 13.8 magnitudes per square arcminute against 14.5.
- ◆ Both lie in the far northern sky, M101 at declination +54° and M51 at +47°.
- ◆ M51 pairs with the interacting companion NGC 5195; M101 stands alone in the eyepiece.