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Rectangular nebula’s true nature revealed One of the galalxy's oddest nebulas, the Red Rectangle, is actually a double cone of glowing gas from a binary star system, researchers report.
The Red Rectangle shines like a squarish jewel in this Hubble image. The actual structure of the nebula is a double cone. Its rectangular appearance is an optical illusion.
The Red Rectangle shines like a squarish jewel in this Hubble image. The actual structure of the nebula is a double cone. Its rectangular appearance is an optical illusion.Nasa / Esa / Catholic U. of Leuven / UC-Berkeley
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Imagine smoke rings seen edge-on and you may gain some insight to what's going on with the deceptive Red Rectangle, which isn't a rectangle at all.

The optical illusion is the work of a pair of stars that orbit one another and have created a strange series of emissions over thousands of years. The periodic outflows have expanded into space to create circular rungs on two imaginary cones, but we see them from the side rather than as if looking into the mouths of the cones.

The emissions are paired, going in opposite directions, creating a structure something like giant megaphones set back-to-back.

New details of the strange nebula emerge in images from the Hubble Space Telescope, released Tuesday.

Not rectangles
The structure is called a planetary nebula, not because it has anything to do with planets but because in primitive telescopes the class of object looked something like the fuzzy disks of the gas giant planets in our solar system. Cataloged as HD 44179, this nebula is about 2,300 light-years away.

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The Red Rectangle was named by Martin Cohen and Mike Merril in 1973 because of its appearance on old photographic plates. The name stuck, but the two lobes billowing out into space don't really fit the moniker any longer.

"They're not rectangular," Cohen of the University of California at Berkeley said in an e-mail interview over the weekend. "They're 'wineglasses' placed bottom-to-bottom with the star where the bottoms would make contact. Across these outside curves are strands like a ladder of nebulosity crisscrossing the interior of the glasses."

Yes, it's complex. And only now are astronomers starting to figure out what created the bizarre shape. Cohen worked with Hans Van Winckel of the Catholic University of Leuven in Belgium and other researchers to obtain the new Hubble picture. It shows for the first time new structures that may explain how the whole setup came to be.

The secret
"The secret of the Red Rectangle is that it is a double star," said Van Winckel, who led the effort.

At the center of the nebula, two stars orbit each other every 10.5 months. The more massive star is running out of fuel and swelling into what astronomers call a red giant. Our sun will do the same in a few billion years, and the hot sphere will swell unimpeded beyond the orbit of Mars, vaporizing all the inner planets.

Inside the Red Rectangle, the main star ballooned to a point where gravity from the companion star began to siphon material from it, Van Winckel speculates. This effect created a doughnut-shaped disk of dust, called a torus, that orbits both stars. Now the main star is in a new phase, losing more mass. But this mass can't just flow in all directions. It is funneled in two directions, above and below the doughnut but not through it.

"That is why we see the double cone," Van Winckel told "So it is the presence of that companion, with its given orbital characteristics, that made the Red Rectangle the way we see the object now."

The emissions are not continuous, but instead occur in bursts every few hundred years, creating the runglike structure of the nebula.

The red emissions are a result of photoluminescence, in which molecules are excited to glow by light energy (as opposed to just reflecting light). Astronomers don't yet know what gas molecules, or possibly small dust particles, create the color.

Cohen calls the Red Rectangle an "outlier" that by its very strangeness could help astronomers figure out how other nebulas form from dying stars. "The Red Rectangle has become a kind of touchstone in that it embodies several mysteries," he said. "Its stunning geometries are just one facet."

The Hubble observations are expected to help the researchers learn more about how binary star systems develop, especially in the late stages, something that's not well-understood,

"The Red Rectangle shows that we still are far from understanding everything," Van Winckel said.