The "Whirlpool Galaxy" (Messier 51) is a splendid example of a face-on spiral galaxy gravitationally interacting with a smaller companion galaxy. The spectacular spiral galaxy is also commonly known as NGC 5194, and the smaller companion as NGC 5195, and both are found in the constellation of Canes Venatici.
Visible: Tony and Daphne Hallas (left), Visible: TIE (center) and Near-Infrared: 2MASS (right)
The two visible-light images and the near-infrared image display similar features, with varying sensitivities (due to different photographic exposure times). The long-exposure color image (left) clearly shows the sweeping spiral arms and the fact that the arms include patchy knots of star formation. The companion galaxy, which is classified as an irregular galaxy, lacks the well-defined structure of a spiral galaxy and appears "attached" to the end of a spiral arm in the "Whirlpool Galaxy".
The short-exposure TIE image (center) beautifully reveals the opaque dust lanes within the spiral arms of the spiral galaxy (e.g. at the 1-2 o'clock position in the inner arm). These arms are rich in molecular gas and dust, the raw materials from which stars are born. The blue tint to the arms in this color photograph reflects the fact that spiral arms are the preferential location for the formation of Type O and Type B stars, the hot blue-white stars characterized by large masses and short lifetimes.
The near-infrared image (right) roughly traces the general pattern of spiral arms in NGC 5194. However, near-infrared wavelengths are best for studying older and less massive stars. Hence, the spiral arms, which are dominated by young blue stars, are less prominent in the 2MASS photograph.
Turning your attention to the low-resolution far-infrared image of M51 (above), we see three features: the central core of the spiral galaxy, its two major arms (green) to the upper left and lower right of the center, and the companion galaxy to the north. The spiral arms are barely discerned as emission blobs adjacent to the center of NGC 5194. Far-IR wavelengths are effective for mapping the distribution of dust in galaxies, and as the TIE image showed (above), the "Whirlpool" is rich in dust. [Remember that the IRAS detectors were rectangular, and hence emission features in the far-IR image are stretched along one dimension; in this case, along a line running from the lower left to the upper right.]
Radio: VLA (left) and X-Ray: Chandra (right)
In a similar manner, the radio image (above left) broadly traces the same distribution of light seen in the earlier photos. The spiral arms extending from the galaxy center and the companion galaxy are clearly seen at radio wavelengths. A modest-sized red blob appears to be connected to the end of the southern spiral arm, located about 5 arcminutes to the southeast of the M51 center (8 o'clock position). This is probably a background quasar, and is also seen as a small source in the x-ray image. Many of the distant quasi-stellar sources are strong sources of both radio and x-ray emission. The other radio feature towards the eastern (leftward) edge of the image is not seen at x-ray wavelengths, and its origin is uncertain.
The x-ray image highlights the energetic central regions of the two interacting galaxies. Much of the diffuse glow is from multi-million degree gas. Many of the point-like sources in the x-ray image are due to black holes and neutron stars in binary star systems.
Infrared 15 microns - ISO
Finally, we include a "bonus image" from the European Space Agency's Infrared Space Observatory (ISO). This space-borne observatory operated from 1995-1998, and represented a powerful second-generation capability beyond IRAS. This particular image was taken with a camera operating at a wavelength of 15 microns (15,000 nm), a region referred to as the mid-infrared. This wavelength is intermediate between those of the 2MASS near-infrared and IRAS far-infrared, and has better spatial resolution than IRAS. Mid-IR light is well-suited to studying star formation and tracing dust in spiral galaxies. This image not only shows the galaxy cores and spiral arms, but nicely illustrates the knots of star formation occurring in the arms of M51.
The ultraviolet image from GALEX maps the hotter stars in this galaxy.