As you know, the Sun has a luminosity of 3.9 * 10^26 Watts; its
radius is 7.0 * 10^8 m; and its surface temperature is 5800 K.

(a) When the Sun becomes a red giant five billion years from now, its
radius will be approximately 1.2 * 10^11 m and its surface temperature 
will be about 3000 K.  How many times more or less luminous will the
Sun be then, compared with what it is now?

(b) After that red giant phase, the Sun will become a white dwarf.  Its
radius will be approximately that of Earth's current radius, and its
surface temperature will be about 40,000 K.  How many times more or
less luminous will the Sun be then, compared with what it is now?
Earth's radius is 6.4 * 10^6 m.

(c) Earth is 1.5 * 10^11 m away from the center of the Sun.  How
much flux, in units of Watts per square meter, will Earth receive
from the Sun during the Sun's main sequence, red giant, and white
dwarf stages?