Pinwheel Galaxy

At a glance

Key points

• Almost twice the Milky Way's width
• At least a trillion stars
• One of the closest face-on spirals

Lopsided

Gravitational nudges from companion galaxies have pulled its arms off-centre. They are thin, blue and dotted with bright star-forming regions.

A giant seen face-on

Pierre Méchain found the Pinwheel in 1781 and it joined Messier's catalogue as M101. With a disc about 170,000 light-years across it is nearly twice the Milky Way's width. Hubble's portrait, assembled from 51 exposures taken over almost ten years and measuring about 16,000 by 12,000 pixels, is one of the largest Hubble images of a spiral galaxy, and Webb's infrared data have since been used to study how polycyclic aromatic hydrocarbons, carbon-rich molecules found in interstellar dust, form and evolve there.

Two well-studied supernovae

On 24 August 2011 the Palomar Transient Factory caught SN 2011fe within hours of its explosion, the closest Type Ia supernova in about 25 years. Its early light pointed to an exploding carbon–oxygen white dwarf, a valuable check on the stellar standard candles used to measure cosmic expansion. On 19 May 2023 the Japanese amateur astronomer Koichi Itagaki discovered SN 2023ixf, the collapse of a massive star, which professional observatories then followed in unusual detail.

Observing it

Magnitude 7.9 sounds bright, but M101's light is spread over an area about the size of the full Moon, so its surface brightness is low and it fades quickly under light pollution. From a dark site binoculars show a faint round glow near the end of the Plough's handle (the Big Dipper), and a small telescope shows the core; the patchy arms need a larger aperture. It is a northern-hemisphere target, best placed in June evenings.

Missions

1. 1990–

   Hubble Space Telescope

   NASA / ESAActive

   Ultraviolet to near-infrared imaging from low Earth orbit.

   Mission page (opens in a new tab)

Related entries

• Whirlpool GalaxyM51 · the textbook grand-design spiral

Sources

1. Nugent et al. 2011, Nature 480:344, SN 2011fe from a carbon–oxygen white dwarf (arXiv) (opens in a new tab)
2. NASA Science: Hubble Messier catalog: M101 (opens in a new tab)
3. Transient Name Server: SN 2023ixf (opens in a new tab)

Last reviewed 10 Oct 2026 against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.