$15.$ Answer the following.

a$)$ What is the surface temperature of Betelgeuse, a red giant star in the constellation of Orion,

which radiates with a peak wavelength of about $970$ nm$?($ Ans: $2990$ K$)$

b$)$ Rigel, a bluish$-$white star in Orion, radiates with a peak wavelength of $145$ nm$.$ Find the

temperature of Rigel's surface. $($Ans: $20000$ K$)$