The X-ray Limb
| Nugget | |
|---|---|
| Number: | 135 |
| 1st Author: | Hugh Hudson |
| 2nd Author: | |
| Published: | 2010 August 30 |
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| Previous Nugget: | Fast electrons relaxing |
Introduction
The radius of the Sun, <math>R_\odot</math>, certainly is one of the most important basic pieces of astrophysical information. But how does one know it? Obviously the edge of a solar image (its limb) is pretty well defined, but sound basic physics tells us that it is fuzzy on the scale of about 0.01%. For a mean radius of 969.63<math></math> as obtained from the world-standard "Allen's Astrophysical Quantitites" [1] with a claimed error bar of about 30 ppm (parts per million, thus 0.003%). Do we really know the solar radius that well? How could we, in fact? The answer is that we imagine a fictitious solar atmosphere, one characterized by a stable layered structure comprising features such as the [photosphere], [chromosphere], and [transition region]. This is really not so correct because the solar atmosphere is full of spectacular blemishes, as noted early on by [Galileo].
The significance of the actual number (why do we need to know it so precisely?) lies, for example, in the arcane discipline of [helioseismology]. Here we find exceedingly precise measurements of the fundamental modes of oscillation of the body of the Sun; sound waves essentially can be trapped in the interior and reverberate for long periods of time. The solar tones can be purer than those of a pitch pipe or a tuning fork!