Messier 101 – The Pinwheel Galaxy in Ursa Major

Messier 101 (NGC 5457), better known as the Pinwheel Galaxy, is a face-on grand-design spiral galaxy in the constellation Ursa Major. It lies at a distance of approximately 21 million light years (≈ 6.4 Mpc) and shines at an apparent magnitude of about 7.9, which puts it within easy reach of a small telescope - provided the sky is dark enough, as its light is spread over an apparent diameter of nearly 29 arc minutes, close to the full Moon's. The galaxy was discovered by Pierre Méchain on 27 March 1781 and reported to Charles Messier, who immediately included it among the very last entries of his famous catalogue (Wikipedia: Pinwheel Galaxy).

Classified as a SAB(rs)cd spiral, M101 is one of the largest disk galaxies in our cosmic neighbourhood: its stellar disk spans roughly 170 000 light years, almost twice the diameter of the Milky Way (NASA/ESA Hubble release on M101). What makes the Pinwheel particularly striking, however, is its pronounced asymmetry. The bright nucleus is offset from the geometric centre of the outer disk, and the spiral arms are noticeably distorted - clear evidence of past gravitational encounters with members of its galaxy group.

M101 is the dominant member of the so-called M101 Group, which also contains the irregular dwarf NGC 5474 (itself heavily perturbed and showing an off-set nucleus pointing toward M101), as well as NGC 5477, NGC 5585, Holmberg IV (UGC 8837) and several other low-surface-brightness companions (of which none is in the field of view) (Karachentsev et al. 2013, AJ 145, 101).

A particularly attractive feature - and the reason I added narrow-band Hα to the LRGB stack - are the giant HII regions that line the spiral arms. Several of them are so luminous that they carry their own NGC numbers, including NGC 5447, NGC 5450, NGC 5451, NGC 5453, NGC 5455, NGC 5458, NGC 5461, NGC 5462 and NGC 5471. NGC 5471 in particular is one of the most luminous HII complexes known in any nearby galaxy (Kennicutt et al. 2003, PASP 115, 928).

M101 also became famous to a wider audience in August 2011, when the Type Ia supernova SN 2011fe was discovered in one of its outer spiral arms. With a peak brightness near magnitude 9.9 and a distance of only ≈ 21 Mly, it was the closest and best-observed Type Ia event in decades and has since become a benchmark object for the cosmological distance ladder (Nugent et al. 2011, Nature 480, 344).

Exposure Data:

• Newton f=1000mm f/4

• Camera Atik 460Exm with Baader LRGB and Hα filters

• Exposure Times

  • 95x300s L

  • 40x300s R

  • 37x300s G

  • 34x300s B

  • 31x600s Hα

• Total: 22h20min

• Date: 2026-04-24 - 2026-04-28

• Location: Fuerfeld / Germany

• Mount Skywatcher EQ8R-pro / Pegasus Astro EQMod

• Guiding and Exposure Control with INDI / PHD2 / CCDCiel running on XUbuntu Linux

• Image Processing PixInsight (RC-Tools) and Darktable

Sources: https://en.wikipedia.org/wiki/Pinwheel_Galaxy https://esahubble.org/images/heic0602a/ https://iopscience.iop.org/article/10.1088/0004-6256/145/4/101 https://iopscience.iop.org/article/10.1086/376941 https://www.nature.com/articles/nature10644