We have alluded so far to stellar sizes, but we have not discussed how they are determined. In this section, we will look at various methods for measuring stellar radii.
The star whose size is easiest to measure is the Sun. This is actually quite useful. We have seen that the Sun is intermediate in its mass and temperature, so its radius is probably a fairly representative stellar radius. The angular radius of the Sun, AO, is 16 arc min. The Sun is at a distance d = 1.50 X 108 km, so its radius, R0, is given by
The Sun is the only star whose disk subtends an angle larger than the seeing limitations of ground-based telescopes. We therefore need other techniques for determining radii. If we know the luminosity (from its absolute magnitude) and the surface temperature (from the spectral type) of a star, we can calculate its radius using equation (2.7). Solving for the radius gives
Example 5.5 Luminosity radius
Estimate the radius of an A0 star. (Use Appendix E
for the stellar properties.)
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