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Neon Abundance in Nearby Stars:
Chandra Discovery Solves Solar Paradox
Sometimes you have to leave home to really appreciate it. A pair of
scientists tried a variation on this theme when they used Chandra
observations of stars hundreds of light years from Earth to better
understand the Sun, which is a mere 8 light minutes (93 million miles) away.
The problem was the vexing question as to how much neon the Sun
contains. This seemingly esoteric bit of knowledge turns out to be
important to scientists seeking to understand how the Sun works. And the
Sun, as the nearest star and a fairly average star, is an obvious
starting point toward understanding how most of the other stars in the
Universe work.
Neon, along with atoms of carbon, nitrogen and oxygen, plays an
important role in regulating the rate at which energy flows from nuclear
reactions in the Sun's core to its surface. The character of the energy
flow changes dramatically about 125,000 miles from the surface on the
Sun, where the stately diffusion of heat suddenly converts to a
convective motion much like the unstable air in a thunderstorm (see
illustration).
The location of this turbulent region, called the convection zone, has
been deduced to fairly high precision from the study of oscillations of
the surface of the Sun (a technique called helioseismology in analogy of
the use of oscillations of the Earth to study its interior). The
location of the convection zone can also be deduced to equal precision
from theoretical calculations based on among other things, the abundance
of neon.
This is where astrophysicists get heartburn. The two determinations
disagree. Several scientists have proposed that the paradox could be
resolved if the solar abundance of neon is in fact about three times
larger than the currently accepted value. This value is based on
indirect estimates, since gas at the relatively cool 6,000 degree
Celsius surface temperature of the Sun gives off no characteristic
radiation at optical wavelengths.
However, a gas heated to millions of degrees produces a distinct neon
signal in X-rays. The upper atmospheres, or coronas, of stars like the
Sun have temperatures of millions of degrees, so the solar corona would
seem to be a good place to settle the argument (not with Chandra -- the
bright solar radiation would irreparably damage the telescope).
Unfortunately, the solar X-rays come from numerous localized loops of
hot gas that vary from location to location and time to time,
complicating the interpretation of the data on neon. Jeremy Drake of the
Harvard-Smithsonian Center for Astrophysics in Cambridge, MA and his
colleague Paola Testa of the Massachusetts Institute of Technology in
Cambridge, came up with an ingenious approach to the problem. They used Chandra to measure the neon abundance in 21 Sun-like stars within a distance of 400 light years (see, e.g. the spectrum of II Pegasi in the inset).
The relative amount of neon in these stars was, on average, almost three
times more neon than is measured for the Sun, just the amount needed to
bring the solar oscillation observations and the theoretical model into
agreement. So, for the moment, astrophysicists can feel that their model
of the Sun may be okay after all, and they can continue to boldly
extrapolate this understanding to the rest of the Universe.
| Fast Facts for Neon Abundance in Nearby Stars: |
| Credit |
Spectrum: NASA/CXC/J.Drake & P.Testa; Illustration: NASA/CXC/M.Weiss |
| Category |
Normal Stars & Star Clusters |
| Observation Dates |
The 21 objects in this survey were observed
between September 17, 1999 and August 15, 2002.
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| Observation Time |
Total observation time for the objects in this survey was 485 hours. |
| Obs. IDs |
6-12, 14-17, 601, 604-5, 609, 636, 974, 1252, 1451, 1885, 1887, 1890-2,
1894, 2388, 2527-34, 3403, 3410, 49899
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| Instrument |
ACIS/HETG |
| References | Jeremy Drake and Paola Testa, "The Abundance of Neon in the Local Cosmos and the Sun Inferred from X-ray Observations of Stars,"
Nature, 2005 (in press) |
| Distance Estimate |
Objects in this survey range from 4.2 to 440 light years away. |
| Release Date |
July 27, 2005 |
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