Messier 29 (also known as NGC 6913) is an open star cluster in the constellation of Cygnus. While similar to the Pleiades (Messier 45), M29 is ten times farther away and is much less famous than The Seven Sisters.
Visible: DSS, Visible: Color, Visible Color: Colleen Gino (TIE)
The visible-light images above reveal the brightest members of the open cluster. The DSS image is clearly the most sensitive of the three photos shown? How can you tell? The six brightest stars show diffraction spikes, which are artifacts resulting from the internal scattering of light by the telescope optics.
The remaining two visible-light images were taken by amateur astronomers. The middle image is a color composite obtained with a 12-inch telescope in Cambridge, Massachusetts. The exposure times for each filter varied from about 7 to 10 minutes. Note that many of the fainter cluster and background stars are no longer apparent. However, you can readily see a bluish tint to the cluster stars, providing clear evidence that open clusters are comprised primarily of young and hot (bluish-white) stars. The TIE image (above right) was obtained at Mount Wilson, California. This is also a composite of photographs taken through different color filters. In this photo, each exposure was only 20 seconds in length. So why is it just as sensitive as the middle image? Because this image was obtained with a 24-inch diameter telescope, providing four times the collecting area of the other telescope. Once again, the blue-white signature of young stars is evident. However, there is also a scattering of faint red stars throughout the field of view. These cooler stars are almost certainly in the foreground or background of M29.
Visible: DSS and Near-Infrared: 2MASS
Now let us revisit the black-and-white DSS visible-light image (above left) and compare it with the near-infrared photo (above right). The 2MASS image is a composite of three photographs, and has been color-coded so that it resembles the visible-light colors seen in the amateur photographs. The six brightest stars are once again seen to exhibit the colors normally associated with young stars, although the southernmost of the six is obviously less blue (that is, cooler) than the others. By clicking your mouse on the 2MASS image in the matrix at the top of this page, you will see an expanded view. Dont try counting the stars in the image! Near-IR light provides a good view of dust-obscured regions, but since Messier 29 contains very little dust, the overall perception is very similar to the visible-light photos studied earlier.
Far Infrared: IRAS
Do not attempt to adjust your picture. That famous line from the opening of the 1960s classic TV program, The Outer Limits, serves as an appropriate metaphor for the far-infrared image of Messier 29 (above). That eerie blue glow is nothing more than electronic noise akin to radio static, which means only one thing: there is nothing there! Far-infrared light is the result of emission by dust grains, and since star clusters have little (if any) dust content, we should not be surprised to find an empty field.
Now why is this IRAS image blank, while the corresponding photo of the Pleiades still shows some stars and other diffuse light? It all comes back to sensitivity. Both images were obtained in comparable exposure times by the IRAS satellite. However, Messier 29 is roughly ten times farther away. The amount of light received from an object actually varies as the inverse square of the distance. Therefore, in the same exposure time, a given telescope will receive 100 times less light from Messier 29 as from M45. No wonder the field is blank!
Radio: NVSS and X-Ray: ROSAT
The above images were obtained at opposite ends of the electromagnetic spectrum. Believe it or not, the long-wavelength radio image and the short-wavelength x-ray photo reveal the same thing: a blank field. The applied color codes disguises the fact that the intensity variations from pixel to pixel are very small. In a real sense, we are merely seeing random electronic noise throughout the images, none of which is astronomically significant.