The `6563 Å H_(2)` line emitted by hydrogen in a star is found to be red shifted by `15 Å`. Estimate the speed with which the star is receding from earth.

Explanation

wavelength of `H_(alpha)` line emitted by hydrogen ,
`lamda =6563 xx 10^(-10)`m
Star's red - shift `(lambda'-lamda, )=15 Å = 15xx10^(-10)` m
Speed of light `C=3xx10^(8) `m/s
Let the velocity of the star receding aways from the earth be v .
the red shift if related with velocity as :
`lamda'lamda=(v)/(c) lamda`
` v=(c)/(c)xx(lmada'-lamda )`
` =(3xx10^(8)xx15xx10^(-10))/(656xx10^(-10) =6.87 xx10^(5) m//s `
there, the speed with the star is receding aways the earth is `6.87 xx10^(5)` m/s.