Whirlpool Galaxy

At a glance

Key points

• The classic two-armed spiral
• Its companion stirs the arms
• Seen almost exactly face-on

Arms on demand

The companion's gravity compresses gas as it passes, triggering lines of young blue stars and pink hydrogen clouds along the arms.

The first spiral

Messier found M51 on 13 October 1773 and described a very faint nebula without stars; in 1781 Pierre Méchain noticed that it had a second nucleus, the companion NGC 5195. Neither saw any structure. In spring 1845 William Parsons, third Earl of Rosse, turned his 72-inch reflector at Birr Castle in Ireland, then the largest telescope in the world, on M51 and saw its light wound into a spiral. His drawing, presented that June, made M51 the first nebula recognised as spiral. Whether such spirals were clouds inside the Milky Way or separate galaxies was not settled until the 1920s.

What Hubble and Webb showed

Hubble's 2005 mosaic, combined with near-infrared data, mapped M51's dust in detail. Astronomers expected giant dust clouds 100 to 300 light-years wide; instead most of the dust lies in smooth lanes threaded with hundreds of small star clusters about 65 light-years across, perhaps because the encounter with NGC 5195 stopped large clouds forming. In 2023 Webb's FEAST programme imaged M51 with NIRCam and MIRI, tracing warm dust filaments and the ionised gas around newly formed clusters to see how young stars clear away the clouds that made them.

A supernova showcase

M51 has hosted three supernovae observed since 1994: one that year, then SN 2005cs and SN 2011dh, both core-collapse explosions of massive stars. They lie along the spiral arms, which is no coincidence. The most massive stars live only a few million years, too short a time to drift far from the arm regions where they formed.

Observing it

M51 lies about 3.5° from Alkaid, the star at the end of the Plough's handle (the Big Dipper), and rides high in northern spring evenings. From dark skies binoculars show a faint smudge; a small telescope shows both nuclei, and the spiral arms need a larger aperture or a camera. It sits well north of the celestial equator, so it is low or hidden from much of the southern hemisphere.

Missions

1. 1990–

   Hubble Space Telescope

   NASA / ESAActive

   Ultraviolet to near-infrared imaging from low Earth orbit.

   Mission page (opens in a new tab)

2. 2021–

   James Webb Space Telescope

   NASA / ESA / CSAActive

   Infrared observatory at Sun–Earth L2, about 1.5 million km from Earth.

   Mission page (opens in a new tab)

Related entries

• Pinwheel GalaxyM101 · nearly twice the Milky Way's width
• Sombrero GalaxyM104 · a bright bulge ringed by dust

Sources

1. Steinicke 2012, J. Astron. Hist. Heritage 15:19, Rosse and the spiral of M51 (ADS) (opens in a new tab)
2. Astronomy: Oct. 13, 1773, the discovery of the Whirlpool Galaxy (opens in a new tab)
3. ESA: ESA/Hubble heic0506a: Whirlpool Galaxy (opens in a new tab)
4. ESA: ESA/Webb potm2308a: A FEAST for the eyes (opens in a new tab)
5. NASA Science: Hubble Messier catalog: M51 (opens in a new tab)
6. NASA Science: APOD 2011 June 11: Supernovae in the Whirlpool (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.