Messier 81 is a magnificent spiral galaxy located in the northern constellation of Ursa Major (which also includes The Big Dipper), and is easily visible through binoculars or a small telescope. It is classified as a type Sb spiral galaxy, intermediate between Sa (e.g., M65) and Sc (e.g., M33). This galaxy is the second nearest spiral galaxy to our Milky Way, beyond only the Andromeda Galaxy (M31). These image sizes are large; the full moon would easily fit within the field of view.
Visible: DSS Visible Color: Robert Gendler
The visible-light images clearly reveal a spiral galaxy. Why is the overall shape of the galaxy not approximately circular (like Messier 51)? Answer
Both visible-light images show the spiral arms emanating from the inner disk of the galaxy, and twisting outwards. If you look closely, you will also see dust lanes running primarily along the arms (particularly in the color photograph). The spiral structure is less obvious than in Messier 33. Note the clumpy knots within the spiral arms, where massive stars are being born, in giant HII regions
Near-Infrared: Spitzer, Mid-Infrared: Spitzer, Far-Infrared: Spitzer
The near-infrared image closely resembles the color visible-light photo. Since near-infrared light is similar to the red portion of the visible-light spectrum, we should not be surprised. Near-infrared radiation is a good tracer for older and redder star. The longer wavelength infrared images from Spitzer are better for tracing the dust distribution within Messier 81. Dust in the galaxy is bathed by ultraviolet and visible light from the surrounding stars. Upon absorbing a UV or optical photon, the dust grain heats up and re-emits the energy at longer infrared wavelengths. The far-infrared image is based on data gathered at a wavelength of 24 microns.
Radio: NVSS (left) and Radio: HI Map (right)
Now turn your attention to the radio data (above, left). The left image represents a radio continuum map made with the VLA telescope in New Mexico. In this configuration, the telescope collects radio waves over a broad bandpass, in a manner similar to the other broadband images in this gallery. The nucleus of the galaxy is seen as a bright central emission peak. If you strain your imagination a bit, you can vaguely trace the spiral structure as a large blue ring, with a diameter approaching the vertical size of the image. An intriguing feature is the bright source of radio emission to the south of the galaxy center. It appears to be almost as bright in radio waves as the nucleus of M81. What is it?
Before answering this question, let us briefly look at a bonus radio image (above, right). This picture looks considerably different than the other radio image, even though the images correspond to the same wavelength! How can this be? Well, the radio emission in the right-side image has passed through a very narrow filter that collects only the light emitted by neutral hydrogen atoms (denoted as HI, in contrast to ionized hydrogen, or HII). Note the central hole in the distribution of neutral hydrogen in M81. Gaseous neutral hydrogen permeates most spiral galaxies, and is an excellent tracer of spiral arms. Moreover, the thin HI gas in a spiral galaxy often extends well beyond the visible-light edge of the disk and arms.
Let us now return to the mysterious source of radio continuum emission located to the south of the galaxy center (left, below).
Radio: NVSS (left) and X-Ray: ROSAT (right)
To help unravel what that source might be, examine the x-ray image (above, right). Once again, we see the galaxy core as the brightest object in the field of view. About 3-4 arcminutes to the south of the galaxy nucleus, you will see a pair of X-ray sources. The brighter of the two, directly south of the galaxy center, corresponds with a cataloged x-ray source that could be a distant quasar. The object we are interested in identifying is the fainter of the pair. It is this third brightest x-ray source in the ROSAT image that is found to be spatially coincident with the secondary peak in the NVSS radio map. In other words, this mysterious object emits radio waves and x-rays.
A search of the NASA Extragalactic Database (NED) reveals that the mystery object is SN1993J, the second brightest supernova visible from the northern hemisphere in 55 years! This supernova reached 10th magnitude in visible light, and was a strong emitter of radio and x-ray emission. Many supernovae produce strong radio and x-ray radiation.
Thinking of scrolling up the page and looking for the supernova in the DSS optical photo? Don't bother! The DSS image was taken in the 1950s, several decades before the supernova exploded!
Visible: TIE (left) and Near-Infrared: 2MASS (right)
However, the supernova can be seen in the more recent TIE and 2MASS images. In the visible-light image (left), can you identify an inverted triangle of stars below the galaxy center, roughly midway to the edge of the image? The supernova is the right-most star in that triangle. And if you look very carefully, you should be able to see it in the very short-exposure near-infrared image (above right). In general, supernovae are best studied at x-ray, UV, visible, and radio wavelengths. They are not typically strong sources of infrared light, especially at the longer IR wavelengths. The supernova 1993J can be seen in the 2MASS picture (barely) because the image is taken at near-infrared wavelengths, and because the host galaxy, Messier 81, is nearby (in an astronomical sense).
Ultraviolet: ASTRO-1 UIT
Finally, take a look at the ultraviolet image (above). It spectacularly traces the spiral structure of M81 and reveals multiple spiral arms. This image was taken in 1990, three years before the supernova cited above.