Messier 92 is a globular star cluster in the constellation Hercules, located about 12 degrees to the east and north of the more illustrious Hercules Cluster (Messier 13). Several hundred thousand stars are gravitationally bound into a spherical configuration, with a diameter of about 60 light years.
Visible: DSS, Visible: Color, and Visible: TIE
One of the characteristics of globular clusters is the high density of stars in the cluster core, producing a central blur of high luminosity. This is particularly noticeable in sensitive long-exposure photographs like the DSS image (above left). Shorter exposures and/or smaller telescopes will yield images such as those (above center and right) taken by amateur astronomers. While fewer stars are seen, the overall spherical shape of the cluster is still obvious. Note that the three-color composite TIE image is slightly less sensitive than the comparable photo taken at Arizona's Grasslands Observatory , even though both were obtained with 24-inch telescopes. Do you know why?
Visible: TIE and Near-Infrared: 2MASS
The near-infrared image (above right) is a mosaic composed of small individual photos stitched together, each of which is less than 8 seconds of exposure time. Despite being a shorter exposure than the TIE image, the apparent density of stars is similar. Why? Because near-infrared light is particularly useful for identifying older and redder stars, and globular clusters are comprised primarily of old stars.
Mid Infrared: IRAS
The cluster has essentially vanished from the mid-infrared image shown above. At these longer infrared wavelengths, the luminosity is dominated by thermal emission from dust grains. Globular clusters contain very little dust, and hence are nearly invisible at these wavelengths. A weak central emission peak corresponds to the optical position of the cluster core, and is almost certainly the accumulated, but faint, mid-infrared emission from many thousands of stars.
Far-Infrared: IRAS and Radio: NVSS
Now turn your attention to the far-infrared image taken with IRAS (above left). Messier 92 is invisible at these long wavelengths, due to the lack of dust in the cluster. What are the other features scattered throughout the field of view? To help answer that question, it is useful to also consider the radio image (above right). As in the IRAS image, the brightest emission is coded as red. There is a weak central peak in the radio image, corresponding to the cluster position. The strongest radio signature, however, is nearly 6 arcmin to the northwest (upper right). Its position coincides with one of the bright peaks in the far-infrared photograph. Consulting the online NASA/IPAC Extragalactic Database, we discover that this source is a distant quasar, at a redshift of 1.36. Many quasars are strong radio emitters, but not all are bright in the infrared. We conclude that since long-wavelength IR emission is associated with dust, that the identified source is a dust-enshrouded quasar. Many of the other faint blobs in the radio image are essentially are due to random noise, akin to electrical static in a radio.
The rest of the far-IR sources scattered throughout the IRAS image (above left) are not likely to be associated with M92. Instead, the sources of the emission are thin and wispy interstellar clouds of dust within our own Galaxy. This is often referred to as infrared cirrus, since its appearance resembles the cirrus clouds (of ice) that float in Earths upper atmosphere.
Finally, the high-energy (short wavelength) x-ray image of Messier 92 shows weak unresolved emission from the cluster core. The stronger source at the very southwest (lower right) corner of the image has been cataloged as an x-ray source, but has not yet been associated with any visible object.