Distance: 41,100 light-years (12.6 kpc) Image Size = 9 x 9 arcmin Visual Magnitude = 7.7

X-Ray: ROSAT Ultraviolet: ASTRO-1 Visible: DSS Visible: Astro 1
Near-Infrared: 2MASS Mid-Infrared: IRAS Far-Infrared: IRAS Radio: NVSS

Messier 79 is a globular star cluster in which hundreds of thousands of star are gravitationally bound into a spherical configuration.


Visible: DSS, Visible: Color - Astro 1 and Near-Infrared: 2MASS

The density of stars near the core of the cluster is so great that the central region in each of the images (above) appears as a bright central blur. Astronomers say that the image is saturated in those regions. Note the relative sensitivity differences among the three images. The black-and-white DSS visible-light photograph is the most sensitive, or deepest. There are two observations that allow us to draw this conclusion. First, the apparent size of the saturated region is largest in the DSS image. Second, the density of outlying stars is greatest in the same image. For a given telescope, deeper exposure times generally reveal more distant (and hence fainter) objects.

The near-infrared photo (above right) has a smaller field of view, and hence has been filled with empty space to maintain the same relative size as the other images in the gallery. Compared to the visible-light images above, the near-IR photo is the least sensitive. This is due in part to the relatively short 7.8 second exposure time. You can peer farther into the cluster core because it is less saturated than in the optical images.


Mid-Infrared: IRAS and Far-Infrared: IRAS

The IRAS mid- and far-infrared images are vastly different than those presented above. The cluster appears to have vanished! Why? It is because thermal infrared emission at these longer wavelengths is normally 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. Hence, there is no source of far-IR light associated with M77.


Ultraviolet: ASTRO-1 UIT and Visible: Color - Astro 1

The ultraviolet image (above left) is from a Shuttle-borne UV telescope. It clearly reveals a central concentration of stars in the cluster core, although it is less impressive than the companion visible-light photograph taken with another telescope on the same Shuttle flight (above right). Globular clusters contain primarily older and redder stars, and hence are most impressive at the visible wavelengths that our eyes can see. The ultraviolet radiation we see originates from white dwarf stars, hot and compact stars nearing the ends of their stellar lives.


Radio: NVSS and X-Ray: ROSAT All-sky Survey

The globular star cluster is virtually invisible at radio wavelengths (above left). The red areas in the image center may be weak radio emission from the cluster core. The green and blue blobs are random electronic noise, similar to static, introduced into the radio receiver. Radio images are often indicative of 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 M79.

The x-ray image (above right) appears to resemble an early video game! In reality, the red pixels are random noise and the cluster is considered to be quiet at these wavelengths.

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