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Reading Stellar Neon Signs

July 27, 2005 ::
Illustration of a Sun-like Star
Our understanding of the average life of an average star comes primarily from studying one star: our Sun. But, despite being our closest and most studied star, astronomers are still trying to determine some of the Sun's key properties.

Scientists fold the most accurate measurements they have of the Sun's composition, size and brightness into what is called the "solar model". When combined with the best available physics, scientists use the solar model to make precise predictions of the interior structure and the dynamics of the Sun.

Labeled Illustration of Convection in Sun-like Star
Labeled Illustration of Convection in Sun-like Star
Astronomers have known the basic composition of the Sun for many years, but are still working on the fine details of the solar mixture. These details turn out to be important for understanding the Sun. Using paint as an analogy, think of trying to match a new can of paint to a color on a wall. You take a gallon of white paint to the hardware store, show them a sample of the color you want, and ask them to mix it for you. If just the right amount of other colors is added to the white base, you can come up with the perfect match.

Pretend that the Sun is that gallon of paint. Astronomers know that the Sun's primary ingredients are hydrogen and helium, the two lightest elements in nature. In our analogy, those elements are the white base coat already in the paint can.

This X-ray image of the Sun is from the Yohkoh satellite
Chandra X-ray Spectrum of II Pegasi
Just as the person in the hardware store has to add just the right amount of just the right selection of colors, so too must scientists find the precise elements and quantities to fold into the helium and hydrogen base. Scientists, in short, have had some problems getting the right colors and quantities to match the paint on the wall.

Recently, astronomers came up with new and improved methods, using complicated mathematical models in three-dimensions, for measuring abundances (the amount of each paint color in our analogy) on the Sun. They found the Sun had significantly reduced quantities of carbon, nitrogen, oxygen, and neon, which fortunately agreed with the abundances measured in the gas within a few hundred light years of the Sun. Since the Sun is presumed to have been born from just this material, the same amount of carbon and oxygen should be found in both. Thus one long-standing problem was solved.

This X-ray image of the Sun is from the Yohkoh satellite
The Sun is too close and bright for Chandra to observe with its extremely sensitive detectors. This X-ray image of the Sun is from the Yohkoh satellite.
However, a bigger problem remained. The new solar model with smaller amounts of these elements made predictions for well known properties of the Sun that no longer agreed with observations. Astronomers furiously studied the observations and the model to try to reconcile the two. One possible solution lay in the element neon, which plays an important role in regulating the rate of energy flow in the Sun. Theorists found that if, by some chance, the amount of neon in the Sun were to be about 3 times higher than the best measurements to date, the solar model would fall back into agreement with all known data.

Neon cannot be seen in optical light from the Sun, but when it is heated to extremely high temperatures it emits X-rays. A pair of scientists used data from the Chandra X-ray telescope to analyze 21 nearby stars, similar to the Sun, searching for neon. The amount of neon in the nearby stars was found to be 2.7 times higher than previously thought. This result implied that neon is overabundant in the Sun by the same amount, and thanks to Chandra, a problem that could have taken years to untangle may be resolved.

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