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That is, it's twice the radius where the radius is from the centre of the sun to some edge. But what is that edge?

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4 Answers 4

up vote 6 down vote accepted

Fusion reactions taking place inside the core of the star produce a huge amount of energy, most of which becomes heat. These reactions are not evenly distributed through the star and so there are phenomena such as sun spots and solar flares, however the total amount of energy produced tends to be reasonably constant.

I would say that the edge is defined by the average point where the gravity reaches equilibrium with the pressure of the star's super-heated gases (as a result of internal fusion).

See the picture of the Sun on Wikipedia

That edge/balance will change when the sun begins to run low on hydrogen. At this time, the reactions inside the star will change causing it to become become a giant red star.

I guess you could compare it with the surface of sea water on Earth. It's technically not still and stable, but we can calculate an average value of the sea level. And it is because it's an average value that we can rely on that to determine altitude and earth radius as well.

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Thank you Donald.McLean for completing my answer with more scientific terms. –  Thibault Dec 5 '13 at 15:55
This is a very confusing answer, it defines the edge as the point of hydrostatic equilibrium... but stable stars tend to be at least near hydrostatic equilibrium everywhere, so there is no apparent reason why this definition would unambiguously pick out the "edge". –  Stan Liou Mar 15 at 12:45
The whole of the Sun is in an equilibrium between pressure (gradient) and gravity. –  Rob Jeffries 19 mins ago

Look at the Sun. You shouldn't do this directly with the naked eye, but you can do so through a very dark filter, or project a suitably dark image through a pinhole. You can even find photos of the Sun on the internet.

What you see is a disk, uniformly bright and with a sharp boundary, surrounded by a comparatively much darker sky. The bright region is the part we consider the Sun, and that's how we get the radius.

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A sensibly experimentalist answer. –  Stan Liou Mar 15 at 12:45
Except you would get a wavelength dependent answer. –  Rob Jeffries 18 mins ago

Most literature will define the diameter of the Sun up to the photosphere, the layer of the solar atmosphere you would see if you were to observe the Sun in white light.

The base of the Photosphere is defined as the region where the optical depth is around 2/3, or the region where the plasma becomes transparent to most optical light wavelengths.

Of course the true edge of the solar atmosphere could be considered as the heliopause, where the direct influence of the Sun's magnetic field and solar wind end and interstellar space begins.

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The radius of the Sun depends highly on which wavelength you are looking (well, taking a photo). In each of them, you'll have a well defined sharp boundary as explained by Zsbán Ambrus in his answer, but it is not the same: it varies with the wavelenght.

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