Press Release

Astronomers find nebula appears larger than it is

By SpaceRef Editor
January 13, 2000
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BLACKSBURG, Va., Jan. 13, 2000ãAstronomers from Virginia Tech announced on Thursday, Jan. 13, at the American Astronomical Society (AAS) meeting in Atlanta that a well-known, luminous cloud of gas in our galaxy called the Rosette Nebula is actually smaller than it appears.

Greg Topasna, a recent Ph.D. graduate of Virginia Tech, and Professors Brian Dennison and John Simonetti used a special camera tuned to detect a particular wavelength of light emitted by hydrogen atoms, much like a radio tuned to receive a particular station.
Astronomers refer to this light from hydrogen as H-alpha. When they pointed their camera at the Rosette Nebula, they discovered that it appears larger than it actually is. The illusion is caused by scattering of H-alpha light from a dust shell surrounding the nebula, the astronomers said. This discovery means that when H-alpha light is used to view nebulae, one must take into consideration that their larger appearance may be an illusion caused by scattered light.

A related process occurs in the Earth’s atmosphere, Topasna said. The sky looks bright during the day, he said, because sunlight is scattered from molecules and particles in the atmosphere. This is analogous to the Rosette Nebula, he said, because some of the light emitted in the hot core of the nebula is scattered by its surrounding shell.

The Rosette Nebula in the constellation Monoceros is about 5000 light years from Earth and can be seen with a modest telescope. It has a symmetrical rose-like appearance, which gives it its name. A cluster of hot stars heats gas within the nebula*s core to nearly 18,000 degrees Fahrenheit. The gas is so hot that it emits light, much like a fluorescent tube. Much of the light emitted in the Rosette nebula is H-alpha light from hydrogen atoms.

The Virginia Tech scientists used a special camera that they developed to detect H-alpha light and light from other atoms in space. The camera is located at a dark mountain site at Virginia Tech’s Horton Research Center in Southwest Virginia. Topasna modified the camera to detect polarization of the light.

“When we found strong polarization in the H-alpha light from what appeared to be the outer parts of the Rosette Nebula, I knew we were onto something,” he said. “Just as scattered sunlight from the daylight sky is polarized, we found that scattering in the dust shell surrounding the Rosette Nebula also causes polarization.” Polarization is a common phenomenon usually present when light is scattered, the astronomers said, and many brands of sunglasses use this property of scattered light to reduce glare. Topasna already has evidence that similar effects are seen around other bright nebulae, such as the North America Nebula, so-called because of its crude resemblance to North America.

The trio is carrying out a highly sensitive survey of hydrogen gas between the stars. Their camera produces digital images that can be manipulated in a computer. Topasna, who implemented, the polarization technique for his Ph.D. thesis, will present his results at the AAS meeting in Atlanta Jan. 13.


Topasna can be reached at 540-231-8757 or via email at [email protected] .

A low-resolution image is available at:

SpaceRef staff editor.