Messier 20 is more commonly known as the Trifid Nebula and is a favorite photographic target of amateur astronomers.
We begin our tour by examining the spectacular color image (above), where the Trifid Nebula is revealed by its reddish glow. Messier 20 is an emission nebula and the reddish color is produced by the characteristic light of (fluorescent) hydrogen gas. One of the photographic filters used to obtain this image is tuned to match the wavelength of the hydrogen gas, partly explaining why M20 is reasonably bright in this picture. A bright star cluster lies at the center of the emission nebula, and is partially obscured by lanes of thick dust. These dust threads are dark because they obscure visible light, and are consequently seen in silhouette against the brighter background.
The blue reflection nebula to the north is unconnected with M20 and just happens to appear in the same field of view. The nebula appears blue because starlight is being scattered by interstellar dust particles. This principle also helps to explain why our daytime sky has a blue tint. How?
Visible: DSS (left) Near-Infrared: 2MASS (centerR), and Mid-Infrared: Spitzer
The visible-light DSS image (above left) clearly reveal dust lanes which obscure part of the central regions. The 2MASS image (above center) illustrates the value of studying dusty regions in the near-infrared. These invisible wavelengths are a few times longer than the red light that human eye can see, and can pierce through an obscuring veil of intervening dust. The dust lanes seen in the visible-light images above almost completely vanish. In the mid-infrared image from Spitzer (right) the warm interstellar dust glows. Where dark lanes of dust are visible trisecting the nebula in the visible-light picture, bright regions of star-forming activity are seen in the Spitzer image. Spitzer uncovered 30 massive embryonic stars and 120 smaller newborn stars throughout the Trifid Nebula, in both its dark lanes and luminous clouds. These stars are visible in the Spitzer image, mainly as yellow or red spots. The Spitzer image is a composite of 3.6, 4.5, 5.8, and 8.0 microns.
By clicking on the 2MASS image you can clearly see hundreds of stars that were invisible in the other visible-light images. You should also note the two bright stars in the nearby reflection nebula to the north. One of these bright infrared bright stars also emits sufficient visible light to be seen in the DSS image. The other is barely visible from behind a dust lane. Can you tell which infrared star is most easily seen in the DSS image?
The far-infrared view of Messier 20 (above) was obtained by the IRAS satellite at 100 microns. The detectors used aboard this first space-borne IR satellite had relatively poor spatial resolution (at least by today's standards!). Hence, the nebula appears as a modestly bright blob. At this wavelengths, most of the infrared emission is due primarily to dust within the nebula.
Radio: NVSS (left) and X-Ray: ROSAT
The radio image (above left) of Messier 20 reveals small pockets of emission, with the brightest source (red) to the southwest of the central region of M20. The intensity of the emission features is rather modest. In fact, most of the areas in green-blue are merely mathematical artifacts of data processing. The x-ray photo (above right) is even less impressive. Some weak x-ray emission corresponds to the position of Messier 20, but this nebula clearly lacks widespread violent stellar explosions (supernovae) that would produce substantial x-ray emission.