Chandra observation deepens mystery of 'superstar'

Eta Carinae, the most luminous stellar object in our galaxy, was imaged by the Chandra space telescope

October 12, 1999
Web posted at: 3:19 p.m. EDT (1919 GMT)

In this story: Image challenges two-star theory The Great Eruption RELATED STORIES, SITES

By Robin Lloyd
CNN Interactive Senior Writer

(CNN) -- An object that only 150 years ago was the second brightest star in the sky after our sun now burns strangely, emitting more energy from its outer layers than its superhot inner core, a scientist said Tuesday.

The latest space telescope images of Eta Carinae, a massive and now mysterious object, show it is surrounded by a highly energetic outer ring and harbors an inner source that may or may not be a single star but burns at temperatures near 60 million degrees Kelvin.

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Unlike most stars, which emit energy most intensely from their centers, Eta Carinae spews out less X-ray radiation from its center than it does from its outer layers of gas and dust, said Fred Seward of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

"At the center of this, we see what looks like a point source which is in turn surrounded by sort of a fuzzy cloud," Seward said of the images returned by NASA's Chandra X-ray Observatory.

"This is all in the X-ray wavelength, so we think we are seeing very close to the central star," he said. "But it's not clear it's a single star. It could be a double star, for example."

When forced to choose, some astronomers think Eta Carinae is powered by an extremely massive star that may be 100 times as massive as the sun. Such stars produce intense amounts of radiation and are extremely unstable before they explode as supernovae.

From previous telescopes, astronomers learned that lobes of infrared energy surrounded Eta Carinae.

Now, the Chandra images returned in recent weeks show Eta Carinae's shell is composed of layers, with an outer horseshoe ring caused by an outburst that astronomers cannot date at this point.

The ring is about two light years in diameter, with a light year equal to the distance light travels in a year, or 7 trillion miles.

Within that ring is a blue cloud and a white area where the possible star or central source is located.

All three structures may represent shock waves produced by matter rushing away from the object at supersonic speeds.

Image challenges two-star theory

Chandra, launched nearly three months ago by shuttle astronauts, is the first to resolve the inner region of Eta Carinae.

The image it has returned pokes a hole in the currently most favored theory for energy coming from Eta Carinae, said Kris Davidson of the University of Minnesota.

That theory suggests that the energy emitted by the object is due to the collision of shells of material thrown off by two stars circling one another at Eta Carinae's center, he said.

"In such a scenario, you would expect to see a much stronger point source," Davidson said. Instead, Eta Carinae's outer layers appear "brighter" in the Chandra image than its central source.

The Great Eruption

The word "star" is confusing when it comes to Eta Carinae, Seward said.

"There used to be a star there called Eta Carinae," he said. Located in the Carina nebula, the object was bright enough to be seen with the naked eye.

But in the late 1830s, the supposed star got very bright and then faded so it was invisible to the naked eye within the next several years. Astronomers call that event the Great Eruption.

However, Eta Carinae started emitting intense infrared energy in the 1870s, energy that the Hubble Space Telescope recently photographed.

Since 1940, Eta Carinae has begun to brighten again, becoming visible to the naked eye, "only we don't know what's in the center of this," Seward said.

Future Chandra observations of Eta Carinae, planned in the next year and headed up by NASA researcher Mike Corcoran, who had favored the double-star theory, could shed more light on the mystery of the object's center, Seward said.

Whatever is at the center of Eta Carinae, it is a million times as luminous, or energetic, as the sun, indicating that it could be a very massive star, he said.

For now, astronomers say Eta Carinae is the most luminous object known in our galaxy, radiating at the rate of several million times that of the sun.