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07.02.12

The Boxy Retina Nebula

This boxy, almost rectangular structure, known as the Retina Nebula or IC 4406, shows its infrared glow in this image from NASA's Spitzer Space Telescope. It is found in the constellation Lupis. Estimates of its distance are somewhat uncertain, placing it anywhere from 2,000 to 5,000 light years away from us. 

IC 4406 is a planetary nebula, representing the final stage of a star's life. When stars like our sun reach the end of their nuclear fuel burning lifetimes they make one final grab for glory. The outer layers of these stars, which have swollen to something approaching the size of Earth's orbit, get blown into space forming what has been dubbed a "planetary nebula." The expelled gas from the star glows brightly, illuminated by the ultraviolet light from the surviving stellar core, known as a "white dwarf."

Astronomers think it is common for stars to lose their material in winds that preferentially blow out along their axes of rotation, effectively carving out a cylindrical, sometimes rod-shaped, cavity. When these structures are viewed from the side, as with IC 4406, they take on a rectangular shape, like a pipe would when seen from the side.

The somewhat misleading term "planetary nebula" comes from pioneering astronomer William Herschel.  If the pipe of IC 4406 were viewed end-on, it would take the appearance of a strangely-structured ball.  After observing a number of  such round, fuzzy objects, Herschel thought their appearance was roughly similar to the newly-discovered planet Uranus. Because of that similarity, he applied the description of "planetary" to these nebulae. 

The term has stuck, even though we now know these stellar remnants have little to do with planets. The only connection is that some of the elements recirculated back into interstellar space may one day end up forming new stars and planetary systems.