Distance: 3,000,000 light-years (1 Mpc) Image Size = 42 x 42 arcmin Visual Magnitude = 5.7

X-Ray: ROSAT Ultraviolet: ASTRO-1 Visible: DSS Visible:IAC/RGO/Malin
Near-Infrared: 2MASS Mid-Infrared: IRAS Far-Infrared: IRAS Radio: NRAO

Messier 33 is a small spiral galaxy within the Local Group of Galaxies, which also includes our own Milky Way Galaxy and its sister galaxy the Andromeda Galaxy (Messier 31). The galaxy is located in the constellation of Triangulum. While M33 is located outside of the Milky Way, it is still one of the nearest galaxies to our own. Hence, it is relatively large, extending over a region that is roughly equivalent to the size of the full moon.

Visible: DSS IAC/RGO/Malin

The black-and-white visible-light image (left) and the corresponding color image (right) closely resemble each other. Close inspection of the color image reveals that the stars in M33 are bluish, an indicator that the galaxy is populated primarily by young stars. [Older stars would appear redder.] By clicking on the top-center image in the mosaic at the top of this page, you can also see dust lanes winding through the galaxy. Spiral galaxies typically contain large amounts of dust and gas (in atomic and molecular form), the essential ingredients for future star formation. The DSS image is over-exposed in the central region, but is better for observing the fine detail in the outer regions of the galaxy.

Visible: DSS (left), IAC/RGO/Malin (center), Near-Infrared: 2MASS (right)

If you look closely at the optical images (left and center) and at the near-infrared image (right), you should be able to trace out the relatively faint spiral arm pattern of M33. One spiral arm appears to start just to the west-northwest (2 o'clock position) of the central region, and then arcs up to the north and east (top and left). Another spiral arm starts east (left) of the central region, and traces a graceful loop towards the south and west (bottom right of the photos).

Mid-Infrared: IRAS (left) and Far-Infrared: IRAS (right)

Let us next turn our attention to the longer-wavelength infrared images taken with IRAS (above). In these false color images, red denotes the brightest emission, while blue corresponds to areas of weaker emission. The far-infrared photo (right) clearly reveals a central core of bright emission, corresponding to the center of Messier 33. In addition, other bright IR emission is seen along both of the spiral arms identified above. Interestingly, the most intense emission appears as a circular blob located in the northeast quadrant (upper left), about 12 arcminutes from the image center. What is this object?

Before revealing the answer, we ask you to focus your attention on the mid-infrared picture (left). Broadly speaking, similar features are seen in the mid-IR picture as in the far-IR (to the immediate right). Note, however, that the bright features are smaller in angular size, and are stretched along a line running approximately from upper left to lower right. The smaller size of the features is an indicator that the IRAS 12-micron (mid-IR) detectors had better spatial resolution than the 60-micron (far-IR detectors). This effect is dramatically illustrated in the southern spiral arm tracing an arc from the galaxy center to the 4 o'clock position. The continuous and fuzzy trail of far-IR light breaks up into discrete sources of mid-IR light when seen with better spatial resolution. Emission at both IR wavelengths is primarily due to new stars illuminating dust grains within the spiral arm. The stretching of the emission is due to the fact that the mid-IR detectors were rectangular in shape, rather than the square pixels that are routine in present-day infrared astronomy.

Apart from the galaxy center, the brightest knot of mid-IR emission is once again due to a region located to the northeast (upper left) of center. The bright off-center emission source is located in the same position in both of the IRAS photos. This IR source turns out to be a supergiant HII region where massive young stars are being born. {The term H II refers to the fact that the hydrogen atoms are ionized; that is, they have lost an electron.} These interstellar clouds contain large amounts of molecular gas and dust, the raw materials from which stars are formed. These regions often appear very bright in the Infrared because the newborn stars emit substantial amounts of ultraviolet and visible light, which is immediately absorbed by the surrounding dust and re-radiated in the infrared.

Now that the secret has been revealed, let us re-examine the deep-exposure, visible-light images shown above.

Visible: DSS (left), IAC/RGO/Malin (right)

Remember that the images in this gallery are oriented the same, and have identical sizes. The supergiant HII region appears as a bright and fuzzy white patch of light at about the same (10 o'clock) position in the black-and-white DSS image, about halfway between the galaxy center and the upper left corner of the image. It appears as pinkish nebula towards the upper left edge of the IAC/RGO/Malin color photo.

Radio: NRAO

The Radio image shown above maps the cool hydrogen gas within the galaxy. This gas cannot be seen with an optical telescope.

Ultraviolet: ASTRO-1 UIT

Star formation is concentrated in the arms of spiral galaxies, and since young stars are significant emitters of UV light, we can easily trace the spiral arms of M33 in UV light (above).


The X-ray image (above) shows a central point of emission at the galaxy center and a variety of other point sources of x-ray radiation throughout the field of view. In total, some 50 separate x-ray sources have been identified within Messier 33. Most of these sources are probably associated with Population I (one) stars. Roughly ten of the sources, however, are supernova remnants within M33.

NED technical data for M33
Multiwavelength Astronomy | Multiwavelength Gallery | Solar System | Open Star Clusters| Globular Clusters| Emission Nebulae| Planetary Nebulae| Supernova Remnants| Sa Spiral Galaxies| Sb Spiral Galaxies| Sc Spiral Galaxies| Elliptical Galaxies| Irregular Galaxies| Active Galaxies|