If the radius of a star is \(R\) and it acts as a black body, what would be the temperature of the star at which the rate of energy production is \(Q?\)
\(\left(\sigma~ \text{is Stefan-Boltzmann constant}\right)\)
1. \(\dfrac{Q}{4\pi R^2\sigma}\)

2. \(\left(\dfrac{Q}{4\pi R^2\sigma}\right )^{\dfrac{-1}{2}}\)

3. \(\left(\dfrac{4\pi R^2 Q}{\sigma}\right )^{\dfrac{1}{4}}\)

4. \(\left(\dfrac{Q}{4\pi R^2 \sigma}\right)^{\dfrac{1}{4}}\)

The total radiant energy per unit area, normal to the direction of incidence, received at a distance \(R\) from the centre of a star of radius \(r,\) whose outer surface radiates as a black body at a temperature \(T\) K is given by: (Where \(\sigma\) is Stefan’s constant):
1. \(\dfrac{\sigma r^{2}T^{4}}{R^{2}}\)

2. \(\dfrac{\sigma r^{2}T^{4}}{4 \pi R^{2}}\)

3. \(\dfrac{\sigma r^{2}T^{4}}{R^{4}}\)

4. \(\dfrac{4\pi\sigma r^{2}T^{4}}{R^{2}}\)

A black body at \(227^{\circ}~\mathrm{C}\) radiates heat at the rate of \(7~ \mathrm{cal-cm^{-2}s^{-1}}\).  At a temperature of \(727^{\circ}~\mathrm{C}\), the rate of heat radiated in the same units will be:
1. \(60\)
2. \(50\)
3. \(112\)
4. \(80\)

Assuming the sun to have a spherical outer surface of radius \(r,\) radiating like a black body at temperature \(t^\circ \text{C},\) the power received by a unit surface of the earth (normal to the incident rays) at a distance \(R\) from the centre of the sun will be:
(where \(\sigma\) is Stefan's constant)

A black body is at \(727^\circ\text{C}.\) The rate at which it emits energy is proportional to: