# How much does the stellar radius change if seen through different filters?

I know that the radius of a star should be different depending on the filter being used to do the observation. That is, the bluer the filter, the smaller the measured stellar radius. I know this effect must be pretty small, however, I would like to get more familiar with it. Can anyone point me to some literature or share their own experiences? Would it be a significant difference in Solar radius if we observe the Sun in x-ray vs radio bands?

• Have you tried googling "the sun at different wavelengths?" If so, maybe you can specify if those results were unsatisfactory for you and why. This would help to sharpen your question. – AtmosphericPrisonEscape Nov 8 '19 at 12:28
• photo.stackexchange.com/questions/92387/… may or may not be helpful – user21 Nov 8 '19 at 16:13
• The radius of the Sun is not at all well defined in terms of X-ray and radio emission. – ProfRob Nov 8 '19 at 16:42

The "thickness" of the photosphere - i.e. the depth over which the optical depth changes from being negligible to very large is perhaps a few 100 km at optical wavelengths, compared with $$7\times 10^{5}$$ km for the radius of the Sun. So the exact definition is important at the level of about 1 part in a 1000.