Triangulum Galaxy

At a glance

Key points

• Third-largest Local Group galaxy
• NGC 604 is 100× the size of the Orion Nebula
• Forms stars 10× faster than Andromeda

A star factory

M33 has little dust and patchy arms studded with pink hydrogen clouds. NGC 604 alone holds more than 200 hot, massive young stars.

Proving spirals are galaxies

Charles Messier catalogued M33 in August 1764. In the 1920s Edwin Hubble found Cepheid variable stars in it and, as with Andromeda, used their pulsation periods to show that it lies far outside the Milky Way. That work helped settle the long argument over whether spiral nebulae were nearby gas clouds or separate galaxies.

A galaxy without a giant black hole

Unlike the Milky Way and Andromeda, M33 seems to lack a supermassive black hole. Hubble spectra of its compact nucleus are best fitted with no black hole at all, setting an upper limit of about 1,500 solar masses. It does host a notable smaller one. M33 X-7, measured with Chandra and Gemini, holds 15.7 solar masses and is eclipsed by a 70-solar-mass companion every 3.5 days; when announced in 2007 it was the heaviest stellar-mass black hole known.

Andromeda's companion

M33 is a suspected gravitational companion of Andromeda and may one day be drawn into the Milky Way–Andromeda encounter. Proper motions from Gaia, published in 2019, suggest it is falling toward Andromeda for the first time rather than circling it on a long orbit. If so, the stretched gas and stars in M33's outskirts are harder to explain as the scars of an earlier close pass, and the question stays open.

Observing it

At magnitude 5.7 M33 sits at the naked-eye limit and needs a very dark site. Its light is spread over an area larger than the full Moon, so it is easy to sweep past in a narrow telescope field; binoculars or a wide, low-power view work best. Larger telescopes show NGC 604 as a separate knot. It is best placed in autumn evenings for northern observers, around November, and rides low in the north from mid-southern latitudes.

What we still don't know

Whether M33 has already had a close encounter with Andromeda, as some stellar streams suggest.

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

• Andromeda GalaxyM31 · the nearest large spiral
• The Milky WayOur barred spiral home galaxy

Sources

1. van der Marel et al. 2019, ApJ 872:24, first Gaia dynamics of the Andromeda system (arXiv) (opens in a new tab)
2. NASA Science: Hubble Messier catalog: M33 (opens in a new tab)
3. ESA: ESA/Hubble heic1901 (opens in a new tab)
4. Gebhardt et al. 2001, AJ 122:2469 (arXiv) (opens in a new tab)
5. NASA Chandra (17 Oct 2007): heaviest stellar black hole in M33 (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.