Messier 33

At half the size of our Milky Way, Messier 33 is the third-largest galaxy in our local group of galaxies.

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Distance

14,000 light-years

Apparent Magnitude

5.7

constellation

Triangulum

object type

Spiral Galaxy

Hubble image of M33
NASA, ESA, and M. Durbin, J. Dalcanton and B. F. Williams (University of Washington)

Spiral galaxy M33 is located in the triangle-shaped constellation Triangulum, earning it the nickname the Triangulum galaxy. About half the size of our Milky Way galaxy, M33 is the third-largest member of our Local Group of galaxies following the Andromeda galaxy (M31) and the Milky Way. Comprised of 54 separate Hubble fields of view, this image is the largest high-resolution mosaic of M33 assembled to date by any observatory. It resolves 25 million individual stars in a 14,000-light-year-wide region spanning the center of the galaxy.

mosaic image of M33
Hubble’s mosaic image of M33 is overlaid upon a ground-based image of the galaxy. The full Hubble mosaic is represented by the irregularly shaped region, while the cropped version (shown above) is outlined by the rectangle.
NASA, ESA, Digitized Sky Survey 2; acknowledgment: Davide De Martin
star-forming nebula NGC 604
NGC 604 is an enormous star-forming nebula in one of M33’s spiral arms. Spanning almost 1,500 light-years, NGC 604 is nearly 100 times larger than the Orion Nebula in our own galaxy and contains more than 200 hot, massive, young stars (versus the Orion Nebula’s four). This image of NGC 604 is constructed from observations taken with Hubble’s Wide Field and Planetary Camera 2 in 1994, 1995 and 2001.
NASA and the Hubble Heritage Team (AURA/STScI); acknowledgment: D. Garnett (U. Arizona), J. Hester (ASU) and J. Westphal (Caltech)

Blue-colored regions scattered throughout the image reveal numerous sites of rapid star birth in M33. In fact, Hubble’s observations reveal that the Triangulum galaxy’s star formation rate is ten times higher than the average found in the Andromeda galaxy, previously surveyed by Hubble in similar detail. A bright blue patch in the lower left of this image, called NGC 604, is the largest star-forming region in M33 and one of the largest stellar nurseries in the entire Local Group.

The Triangulum galaxy’s orderly spiral structure displays few signs of interactions with nearby galaxies. However, that could change in the future. Only slightly farther away from us than the Andromeda galaxy, about 3 million light-years from Earth, M33 is a suspected gravitational companion to Andromeda, and both galaxies are moving toward our own. M33 could become a third party involved in the impending collision between the Andromeda and Milky Way galaxies more than 4 billion years from now.

M33 has a relatively bright apparent magnitude of 5.7, making it one of the most distant objects that keen-eyed observers can view with the unaided eye (under exceptionally clear and dark skies). Although a telescope will start to reveal some of M33’s spiral features, the diffuse galaxy is actually easiest to examine with low magnification and a wide field of view, such as through binoculars. It is best observed in October.

Although others may have viewed the galaxy earlier, Charles Messier was the first to catalog M33 after observing it in August 1764. In the 1920s, astronomer Edwin Hubble studied dozens of variable stars (those that periodically change brightness) in M33, which helped him to estimate the object’s distance and prove that M33 is not a nebula within our own galaxy, as previously suspected, but actually a separate galaxy outside our own.

For more information about Hubble’s observations of M33, see:

M33 locator chart
This star chart for M33 represents the view from mid-northern latitudes for the given month and time.
Image courtesy of Stellarium

Explore Hubble's Messier Catalog

The following pages contain some of Hubble’s best images of Messier objects.

Messier 1 (The Crab Nebula)

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Messier 2

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Messier 3

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