The Hubble Space Telescope has produced many pictures of so-called planetary nebula over the past decade, each with its own unique, complex and often remarkably symmetrical structure. The latest example eerily resembles a bit of human anatomy, some giant disembodied eye looking back at us from space.
In fact, the picture is of a dying star, one called the Retina Nebula or, officially, IC 4406.
Planetary nebulae were so named, incorrectly, because they resembled planets in early telescopes. They are vast constructs of gas and dust that have billowed out from dying stars, which still shine hot as death draws near.
In the new picture, the left and right halves seem like mirror images. Researchers aren't sure exactly why this sort of feature is so common in planetary nebulae, but
Most curious about IC 4406 are the dark, web-like strands that make a portion of the nebula look like the retina in a human eye. These are lanes of denser gas and dust 160 times wider than the distance from Earth to the Sun. The lanes are silhouetted against less dense regions where more light from the central star is allowed to penetrate.
The effect is something like a lampshade whose dark patterns block more light than the overall fabric.
The dusty lanes were created by an "instability mechanism" vaguely similar to the process that generates summer clouds when hot air rises, says by Bob O'Dell, a Vanderbilt University researcher who led the effort to produce the photograph.
"The most familiar type of instability is the one that produces those white puffy cumulus clouds or thunderheads on a summer day," O'Dell told SPACE.com.
In the case of the Retina Nebula, however, material is moved laterally across a surface and concentrated into small spots.
"This is much like puddles forming on a prairie after a rain," O'Dell said. "Small local variations in the surface cause the material to concentrate in specific areas. Once concentrated, the neutral material is dense enough to become visible in silhouette."
Overall, the Retina Nebula is shaped something like a donut. Hubble's view of it is from the side -- not from the top looking down into the hole. This means the central star is buried behind the bulk of the gas and dust in the somewhat opaque donut.
The donut of material confines the intense radiation coming from the remnant of the dying star, O'Dell and his colleagues say. Gas on the inside of the donut is ionized (or stripped of electrons) by light from the central star, and so it glows.
Light from oxygen atoms is rendered blue in this image; hydrogen is shown as green, and nitrogen as red.
The nebula was first seen and recorded in the 19th Century. It sits about 1,900 light-years from Earth in the constellation Lupus.
"What the Hubble Space Telescope has allowed us to do is to see the object in much greater detail and to figure out its 3-D structure," O'Dell said.
The image is a composite of data taken by Hubble's Wide Field Planetary Camera 2 in June 2001. It was released today by the Hubble Heritage team, which operates out of the Space Telescope Science Institute on the campus of Johns Hopkins University in Baltimore.
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