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The Sombrero Galaxy, also known as M 104, is located about 9.2 million parsecs (30 million light years) from the edge of the Milky Way in the direction of the constellation Virgo (Bendo et al. 2006). Its diameter has been estimated as 15.3 kiloparsecs (50,000 light years). This is considerably smaller than the Milky Way. However, the Sombrero is evidently as old as our galaxy and even more massive, with a densely packed population of stars. (See the discussion at the Hubble Heritage site.)

This image of the Sombrero Galaxy was photographed in infrared light by the Spitzer Space Telescope (courtesy NASA/JPL-Caltech/IRAC). It displays key features of the galaxy's structure: the enormous bright central bulge, which is dominated by a super-massive black hole; the dark void surrounding the bulge; and the ring of thick dust that bounds the outer disk. This dust ring, which appears red in the photograph, is the Sombrero Galaxy's major region of active star formation.

The blue-white star Fomalhaut is the brightest luminary of the constellation Piscis Austrinus, located 7.7 parsecs (25 light years) from the Solar System. The star is surrounded by a dense dust ring located at about 150 AU, or 5 times the distance of Neptune from the Sun. The ring's width is estimated at 25 AU. Analogous rings have been observed around other stars, and are normally interpreted as fields of debris similar to the Kuiper Belt in the Solar System. Collisions among the icy or rocky bodies that compose these rings create the dust that renders them visible. The debris itself remains undetectable.

The image of Fomalhaut's dust ring was photographed by the Hubble Space Telescope, using a mask to filter out the glare of the central star (courtesy NASA/ESA/P. Kalas/J. Graham). Fomalhaut itself is therefore unseen. Its position, however, is marked by the dot, which demonstrates that the ring is off-center. The combination of central clearance and non-symmetric orientation have been interpreted for a long time as evidence of at least one massive planet orbiting just inside the ring (Kalas et al. 2005, Quillen 2006). Such a planet has recently been imaged by Hubble (Kalas et al. 2008).

The giant planet Saturn orbits the Sun at an average distance of 9.5 AU (884 million miles). Its spectacular rings consist of ice and dust particles originating from several sources: volcanic eruptions on nearby icy moons warped by Saturn's gravity drag; debris from ancient satellites that have disintegrated through tidal stresses; and the grinding of micrometeorite showers on surviving moons. An overall structure appears in the alternation of brighter and fainter rings divided by dark belts. This concentric pattern is maintained by so-called "shepherd moons" orbiting in the gaps. The rarefied outermost ring, known as the E-ring, has an exterior diameter of about 300,000 miles.

The image above is a composite of photographs taken by the Cassini orbiting spacecraft (courtesy NASA/JPL/Space Science Institute). It represents an eclipse of the Sun by Saturn, as seen from the perpective of Cassini. This unusual angle is responsible for the image's ghostly aspect, which was enhanced by combining ultraviolet and infrared exposures with others in visible light.

See also Barred Spirals and The Galactic Rim.   Click here for References.





All text is copyright Raymond Harris 2006-2008. Image credits appear in the accompanying caption.