Messier 101 is a spectacular and nearby spiral galaxy located in Ursa Major, and is also known as the Pinwheel Galaxy. This galaxy is nearly twice the diameter of our own Milky Way Galaxy, and has a less prominent central bulge. The galaxy diameter is about two thirds that of the full moon.
Visible: DSS (left) and Visible: Color Grasslands Observatory(right)
The face-on orientation of Messier 101 allows us to easily identify the primary constituents of a spiral galaxy: a bright nucleus contained within a central bulge, surrounded by a thin disk that contains the spiral arms. Regions of patchy illumination within the arms serve as signposts of recent star formation, and are referred to as giant H II regions. Moreover, dust lanes within the arms are also seen. It is this dust, accompanied by molecular and atomic gas (invisible at optical wavelengths), which provide the raw materials for future star formation. Note how the spiral arms wind all the way in towards the inner disk.
The exposure time for the color photo is of shorter duration and hence the luminosity of the bright nucleus is reduced from that of the DSS image. The hottest stars are blue-white in color and are the youngest and most massive stars. The color image clearly reveals that these are the types of star that are preferentially born within the spiral arms of galaxies.
Visible: DSS (left) and Near-Infrared: 2MASS (right)
The DSS image (above left) is a visible-light photograph. Now, direct your attention to the 2MASS picture (above right), which was obtained at near-infrared wavelengths. In the near-IR, the spiral design of Messier 101 is still obvious. However, the contrast within the spiral arms is considerably reduced because of two reasons. First, near-infrared light is able to pierce through most of the obscuring effects of dark dust within the spiral arms. Second, near-IR wavelengths preferentially detect cooler and redder stars. We learned earlier that the luminosity within spiral arms is primarily due to hotter and bluer stars. In other words, the peak emission in the spiral arms is outside the near-infrared filter and falls within the visible-light region. It is for these reasons that Messier 101 appears more impressive in the visible light photographs than in the near-infrared.
Mid-Infrared: IRAS (left) and Far-Infrared: IRAS (right)
At even longer infrared wavelengths, we begin to directly detect thermal emission (heat) from Messier 101, much of it due to radiation by the dust itself. The mid- and far-IR images above (left and center) were obtained by the IRAS satellite in 1983 and correspond to wavelengths of 12 microns and 60 microns, respectively. Red denotes intense IR emission in both of these photos. The nucleus of Messier 101 is readily apparent in both. Note, however, that the emission peaks tend to be slightly elongated. This is an artifact resulting from the peculiar rectangular shape of the IRAS detectors. The pixelization is a result of the relatively poor spatial resolution in these pictures.
At these wavelengths, the infrared emission is primarily a result of star formation. As the new stars emerge from their cocoons of gas and obscuring dust, their visible and ultraviolet light is absorbed by the dust particles. The dust is heated and then re-radiated at longer wavelengths at the infrared wavelengths depicted above. Hence, the mid- and far-infrared emission corresponds to regions of widespread star formation. These areas include the dense central bulge of the galaxy and patchy regions within the spiral arms. If you examine the IRAS images carefully, you should be able to see an inner spiral arm stretching to the south and east (bottom and left).
In the 60 micron (far-infrared) photo, we find that the brightest source of emission is not the galaxy center, but rather a region located about 4 arcminutes to the southeast (lower left) of the center. Another bright source is seen in both IRAS images and found near the southwest (lower right) of the IRAS images. Within the far-IR photo, both of these regions appear to be contained within the fainter outlines of spiral arms. These bright sources of infrared light are supergiant H II regions, where hot and massive stars are being born. The intense visible and UV light from these stellar nurseries is illuminating the surrounding clouds of dust. Each of these hot spots is clearly seen in the visible-light images examined earlier (scroll up and see for yourself!).
Radio: NVSS (left) and Far-Infrared: IRAS (right)
The images above adopt a similar false color scheme. However, the photograph to the left is a radio image of Messier 101, while the image on the right is the (repeated) far-infrared IRAS picture. Apart from the better spatial resolution in the radio data, the maps display similar features. The center of M101 is revealed as a bright region of both radio and far-IR light. Keeping in mind that the spatial scale and orientation of these pictures is the same, you should be able to identify other luminosity peaks (coded as red) in both images. There are two regions of enhanced emission extending along the spiral arm that extends from the galaxy center towards the east and north (left and top). There are also two other enhancements in the southwest (lower right) quadrant. The IRAS infrared data shows that both are aligned along a spiral arm that extends from the galaxy center to the west and south (right and bottom).
Astronomers have found that radio continuum (broadband) and far-infrared emission is spatially correlated within normal spiral galaxies. This means that peaks in the radio and IR maps tend to lie near each other. Why? The massive stars born in supergiant H II regions burn their fuel (via thermonuclear fusion) very fast, and have short lifetimes (by cosmological standards). Such stars often produce supernova explosions when they die, and these explosive events yield significant amounts of synchrotron radio emission. In the long continuum of astronomical history, the images in this Gallery (all obtained within two decades of each other) essentially represent a near-simultaneous snapshot, revealing both star birth and star death occurring in the same era. Hence, we see both infrared and radio light. The physical emission mechanisms differ, but both arise from the same population of massive stars, albeit at different stages of their life.
Now take one last look at the radio and far-IR images. There is one emission peak in the radio map that does not correspond to any feature in the infrared image. Do you see it? This feature is located in the southeast (lower left) corner of the photograph. What do you think this radio source might be? The fact that it is at the edge of the photo is suggestive that the feature may not be associated whatsoever with Messier 101. It could be a foreground object within our Milky Way Galaxy or a background object in the distant Universe. Some investigative work leads to the answer. A 1990 paper published in THE ASTROPHYSICAL JOURNAL reveals that the mysterious radio source is known as WE1402+54W. There is still some ambiguity on the precise nature of this radio source, but it is likely to be associated with a very distant quasar.
Ultraviolet: GALEX (left) and Visible: DSS (right)
Now let us turn our attention to shorter wavelengths. The ultraviolet image of Messier 101 (above left) traces the spiral structure of the galaxy in much the same manner as the visible-light pictures examined earlier. In particular, note the close correspondence between the patches of light seen in the UV image and those appearing in the visible light seen in the image studied earlier (above right). These supergiant H II regions are areas of intense star formation within molecular clouds located in the inner disk and spiral arms of M101. Massive young stars born in these environments emit significant amounts of ultraviolet light.
X-Ray observations of M101 reveal a possible new class of X-ray sources. These mysterious X-ray sources, marked with green diamonds in the image, have a temperature in the range of one to four million degrees Celsius. The power output of these sources is comparable to or greater than that of neutron stars or stellar-mass black holes fueled by the infall of matter from companion stars. This implies that the region that produces the X-rays in a these sources is dozens of times larger.