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

Explanation

`lambda=6563A=6563 xx 10^(-10) m, triangle lambda=15A, c=3 xx 10^(8) m//s`
Frequency shift `trianglev=v.-v=-(Vv)/(c)`
`v=c/lambda, trianglev=-c/(lambda^2) triangle lambda`
`-c/lambda^2 triangle x=-(Vv)/c`
`V=(c triangle lambda)/lambda=(3 xx 10^8 xx 15 xx 10^(-10))/(6563 xx 10^(-10)) =6.86 xx 10^(5)m//s`