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Chandra Gratings Peek At Capella

September 2, 1999 ::

"Remember my telling you I had seen a UFO?"
- caller from a small southern California town
"Remember my telling you I had seen a UFO?" The caller was a friend, who lived a few miles away in a small southern California town. (This was several years ago, but yes, there are still small southern California towns.)

"Yes," I said, "you said you had seen it before."

"Right, several times", she said impatiently, "and it's there right now. Low down, in the northwest."

We grabbed our binoculars and rushed outside. Sure enough, there it was. A brilliant light low on the horizon, flashing alternately with red, blue and white colors -- very patriotic. Amazing! We gulped and took a careful look through our binoculars. Although the flashing colors gave the impression that the object was rotating, like a police beacon, the light itself seemed to be point-like. After observing the "UFO" for about fifteen minutes, we noticed that it was slowly setting. It was a star, not the "mother ship" come to rescue us from encroaching development, smog, SUVs, and gophers.

We watched the star's eerie light show for twenty minutes as it set over the Pacific, then we went indoors to search the star maps for its name. We were looking at Capella, forty light years away and the sixth brightest star in the northern sky. Its true color is golden yellow, and it is not one star, but two-- four if you count the dim pair that orbits the central bright pair. The dramatic spectacle we had just observed may have been due to the passage of its light through smog drifting down from Los Angeles.

HETGS Capella first-light observation
HETGS Capella first-light observation
Image: MIT/CXC

Last week, Capella came to our attention again. Scientists from MIT took a peek at Capella while they checked out the performance of the High Energy Transmission Grating Spectrometer which they had built for Chandra.. Capella is interesting to X-ray astronomers because it radiates more energy in X-rays than similar stars, for reasons that are not well understood.

"Within the first hour we had obtained the best X-ray spectrum ever recorded for a celestial source," said MIT Professor Claude Canizares, principal investigator for the instrument and associate director of the Chandra Center. "We can already see unexpected features that will teach us new things about stars and about matter at high temperatures."

High- and Low-Energy Gratings
High- and Low-Energy Gratings being integrated into optical bench
A look at the richly detailed spectrum returned by Chandra's gratings is enough to convince you that astronomers will soon know a lot more about Capella. What had been a jumbled forest of spectral lines, are now a well-resolved stand of "trees" due to magnesium, neon and iron. What does it all mean? The talented team involved in Chandra's Emission Line Project will soon learn even more when they observe Capella later this month with Chandra, the Hubble Space Telescope, the Very Large Array radio telescope, the Extreme Ultraviolet Explorer satellite, and the Beppo-SAX X-ray satellite simultaneously!

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