I found it interesting to compare images of two Pinwheel galaxies despite the difference in targets and imaging setups.
The Pinwheel Galaxy, M101 in Messier’s catalogue, is a beautiful face-on spiral galaxy that is popular with astrophotographers. Tonight at 11:00 pm from Vancouver, it is at an altitude of 84°- almost straight up – in the constellation Ursa Major. This is a great location for visual observing or imaging to minimize atmospheric disturbance. I collected image data of M101 in 2014 with a small 100 mm refractor.
Another pinwheel from Messier’s catalogue is M83, also known as the Southern Pinwheel. It is visible at the same time but its altitude is just 10° above the southern horizon in the constellation Hydra when viewed from Vancouver. That location makes M83 a difficult target so I decided to use a remote telescope in Chile at latitude 30°S. From Chile, M83 reaches 89° when transiting the meridian at 02:16 am UTC.
M101 – The Pinwheel Galaxy
from Coquitlam, BC, Feb 2014
M83 – The Southern Pinwheel
from Chile, May 2020
Click on the image to open a larger view.
Two very different setups were used to collect the imaging data. The M101 data was collected using my own equipment from Coquitlam, BC:
- Skywatcher 100ED pro refractor
- Nikon D5100 DSLR
- Total Exposure: 30 min with 20 subframes at 90 sec and ISO 1600
- Heavy light pollution (sqm: 18.5, Bortle class 7)
The M83 data was from the Telescope.live remote CH-1 telescope in Chile:
- Planewave CDK24 (60 cm, F6.5)
- FLI ProLine PL900 (3056 x 3056)
- Total Exposure: 90 min with 9 Luminance and 3 each of Red, Green, Blue subframes at 300 sec and 1 x binning.
- Pristine Dark skies at an elevation of 1500 m (sqm: 21.8, Bortle class 1) – same skies as the Gemini South telescope.
So it is not at all surprising that the M83 image is better with 6X more aperture and 3X longer exposure.
The two galaxies are both face-on spirals, appear somewhat similar, and are close to the same visual magnitude but some of their physical properties are quite different as shown in the table below.
| M101 | M83 | |
| Constellation | Ursa Major | Hydra |
| Visual magnitude | 7.86 | 7.54 |
| Angular Size | 28′.8 × 26′.9 | 12′.9 × 11′.5 |
| Distance | 1.8 Mly | 14.7 Mly |
| Diameter | 170,000 ly | 55,000 ly |
| Radial velocity | 241 ± 2 km/s | 508 km/s |
| Number of Supernovae | 4 | 6 |
M83 has spawned a large number of supernova explosions — six in total that we have observed (SN 1923A, SN 1945B, SN 1950B, SN 1957D, SN 1968L, and SN 1983N). Only 4 supernovae have been observed in M101 but SN 2011fe, a Type Ia supernova, reached magnitude 9.9 in 2011 and was visible in binoculars.
M83 is thought to have a double nucleus at its core. The paper “Double nucleus in M83” provides evidence of a second hidden nucleus that is more massive than the visible nucleus.