Distance: 22,800 light-years (7.0 kpc) Image Size = 14 x 14 arcmin Visual Magnitude = 5.8

X-Ray: ROSAT Ultraviolet: FOCA Visible: DSS Visible: © Jason Ware
Near-Infrared: 2MASS Mid-Infrared: IRAS Far-Infrared: IRAS Radio: NVSS

Perhaps the most familiar of globular star clusters, Messier 13 is found in the constellation of Hercules and contains more than 100,000 stars that are bound by gravity into a spherical configuration.

Visible: DSS and Near-Infrared: 2MASS (right)

The density of stars near the core of the cluster is so great that the visible-light and near-infrared images all become a bright central blur. The near-infrared image is a mosaic composed of small individual photos stitched together, each of which is less than 8 seconds of exposure time. We see many stars in the 2MASS image because near-infrared light is particularly useful for identifying old stars, and globular clusters are comprised primarily of old stars.

Mid Infrared: IRAS (left) and Far Infrared: IRAS (right)

Now shift your attention to the longer wavelength IRAS infrared images. In the mid-infrared, the relatively poor spatial resolution of the IRAS detectors results in the smearing of the light from M13 into a central blob. Note that this emission is not spherically symmetric like the underlying star cluster itself. This is an artifact resulting from the rectangular shape of the IRAS detectors. [If these detectors were square instead, the image would be circularly symmetric.]

Now turn your attention to the far-infrared image taken with IRAS. Messier 13 has vanished! Why? Far-infrared light, with wavelengths longer than about 30 microns, is often a product of dust and young stars. The stars emit ultraviolet and visible-light photons that are absorbed by the dust particles. The light is then re-radiated at far-infrared wavelengths, with the energy difference going into heating the dust. Globular clusters have no young stars and very little dust. Therefore, it should not be a surprise to find that there is no measurable far-infrared light from M13.

Radio: NVSS (left) and X-Ray: ROSAT (right)

Now examine the radio image (above left). Once again, the cluster itself is virtually invisible. The blue blobs are primarily random electronic noise, similar to static, introduced into the radio receiver. Radio images often reveal synchrotron radiation, resulting from fast-moving and electrically charged particles (such as electrons) in a spiral motion about magnetic fields. The environment of globular clusters is essentially devoid of strong magnetic fields and fast-traveling electrons, and therefore we should not expect significant synchrotron emission from M13. The only obvious sources of radio emission are individual peaks (red blobs) found in the upper right and lower left corners of the image. To unravel their identity, it is useful to consider the X-ray image first (right).

At these high-energy wavelengths, Messier 13 appears as a weak and concentrated core of emission (yellow/green), coincident with the core of the globular cluster. However, this image alone cannot provide any useful information on the distance to this weak x-ray source, so little can be said about its nature based on this image alone. The emission might be associated with the cluster stars, or it could be associated with nearer (foreground) or more distant (background) objects.

The strongest source of x-ray emission in this image roughly coincides with the brightest source in the upper right (northwest corner) of the radio image. It turns out the distant quasars, or quasi-stellar sources, can be significant emitters of both X-ray and radio radiation. It is likely that the mysterious radio/x-ray source is an un-cataloged quasar in the distant background, located far outside our Milky Way Galaxy. The radio emission emanating from the lower left (southeast corner) of the NVSS image does not have a clear association in the ROSAT x-ray image, and hence its identity remains uncertain.

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