Star $A$ has radius $r$ and surface temperature $T$,while star $B$ has radius $4r$ and surface temperature $T/2$. The ratio of the power radiated by the two stars,$P_A : P_B$,is:

The temperature of a spherical black body is inversely proportional to its radius. If its radius is doubled,then the power radiating from it will be

Two spheres of the same material have radii $1\ m$ and $4\ m$ and temperatures $4000\ K$ and $2000\ K$ respectively. The ratio of the energy radiated per second by the first sphere to that by the second is

In the $MKS$ system,Stefan's constant is denoted by $\sigma$. In the $CGS$ system,the multiplying factor of $\sigma$ will be:

If the radiation emitted by a perfect radiator has maximum intensity at a wavelength of $2900 Å$,the intensity of radiation emitted by it is (Stefan-Boltzmann's constant $= 5.67 \times 10^{-8} W m^{-2} K^{-4}$ and Wien's constant $= 2.9 \times 10^{-3} m K$).

Assume that the solar constant is $1.4 \, kW/m^2$,the radius of the sun is $7 \times 10^5 \, km$,and the distance of the earth from the center of the sun is $1.5 \times 10^8 \, km$. Given Stefan's constant is $\sigma = 5.67 \times 10^{-8} \, W m^{-2} K^{-4}$,find the approximate temperature of the sun in $K$.