Distance: 5,000 light-years (1.5 kpc) Image Size = 17 x 15 arcmin Visual Magnitude = 7

Ultraviolet Image
Not Available
X-Ray: ROSAT   Visible: DSS Visible: Color © AAO
Near-Infrared: 2MASS Mid-Infrared: IRAS Far-Infrared: IRAS Radio: VLA

Messier 17 is a diffuse emission nebula in the constellation of Sagittarius, and is known by a variety of other names, including the Omega Nebula. While the light from newborn stars illuminates the gas within the nebula, one can also see dark regions corresponding to the obscuration from interstellar dust.

Visible: DSS and Visible: Color (Malin)

The field of view for each of the visible-light images shown above is about 3/5 the diameter of a full moon. The black-and-white DSS image reveals an irregular shaped cloud of illuminated gas, with darker superimposed regions. The obscured areas are due to dust grains located in the foreground of the nebula. Even the obscured regions are not completely dark, suggesting that the stars are either in the foreground or are in the outer fringes of the dusty regions and are bright enough to be seen through the haze. Because the DSS image is reasonably sensitive, we can also see filaments of bright gas extending over most of the field of view, especially towards the northeast (upper left).

One look at the spectacular color image taken from Australia reveals why Messier 17 is also known as the Lobster Nebula! [Imagine the lobster claws to the northwest, or upper right.] The red color of the nebula tells us that the primary gas being illuminated by the newborn (but hidden) stars is hydrogen. The presence of gas towards the northeast (upper left) is now more easily seen than in the DSS image.

Visible: Color (Malin) and Near-Infrared: 2MASS

Let us now compare the near-infrared view of M17 (upper right) with the color visible-light image (upper left) seen earlier. The first obvious thing to note is the color difference. However, this is a superficial difference due solely to the false-color scheme chosen for the 2MASS near-infrared photo.

The most important difference is dramatic and is seen to the west of center (center-right) of the image. Where is the dark cloud of dust? Rest assured that the dust is still present. The near-infrared light, with wavelengths longer than visible light, is able to pierce through the obscuring cocoon of dust, revealing the previously hidden new stars. Smaller knots of dark dust are so dense, however, that even near-IR light is blocked. Moreover, the 2MASS image shows thick dust along the western (left) and northern (top) edges of the large emission nebula. The color-coding has been chosen to mimic what the human eye sees; namely, hot and massive stars appear bluish-white. What effect does this dust have on these young stars? Close inspection will reveal that the dust reddens the light from these stars, in much the same manner that dust in the Earths atmosphere yields red sunsets.

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

At longer infrared wavelengths (see above), the interstellar dust itself is the source of the emitted light. [At shorter wavelengths of less than about 5 microns, dust is dark and obscures background light.] This thermal emission makes the dust glow in the infrared. The dust particles absorb visible-light and ultraviolet photons from embedded young stars. The dust grains are heated and then re-emit radiation of lower energy (or longer wavelengths); that is, in the infrared. In these false-color photos, the brightest regions are coded as red and the faintest emission is blue. In the mid-infrared, much of the emisison is a direct tracer of star formation. At far-infrared wavelengths, it is a combination of star formation and general heating of dust grains immersed in the nebula.

These images suffer from relatively poor spatial resolution, especially at far-IR wavelengths. Nevertheless, it is possible to see that star formation is concentrated in an asymmetric region in the southwestern portion of Messier 17. In the far-infrared, there is a single emission maximum (red). With moderately better resolution in the mid-IR, the emission peak is clearly separated into two distinct maxima.

Mid-Infrared: IRAS and Radio: VLA

The radio-wavelength image (above right) depicts the distribution of ionized hydrogen; that is, hydrogen atoms with their electrons stripped away (also called hydrogen ions). In the astronomical realm, these regions are referred to as H II regions . The missing electrons are the result of collisions with other atoms and/or radiation from nearby stars. H II regions are the signposts for active star formation, and the peak of the radio emission is clearly in the same relative location as the far-infrared peak.


Finally, we find very little (if any) x-ray emission from M17. The modest peak seen near the image center could be due to weak emission from the gas within the Omega Nebula, or it could originate from an un-cataloged background quasar.

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