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

Given, Red shift , `lambda. - lambda = 15 Å = 15 xx 10^(-10) m`
Wavelength of `H_(alpha)`, line, `lambda = 6563 Å = 6563 xx 10^(-10) m`
Speed of the receding star, `upsilon` = ?
As the star is receding away, `upsilon` will be negative
Since `lambda. - lambda` is positive,
`lambda. - lambda = - (upsilon lambda)/c`
`implies upsilon = -(c(lambda. - lambda))/(lambda)`
or ` upsilon = -(3 xx 10^8 xx 15 xx 10^(-10))/(6563 xx 10^(-10))`
`= - 6.86 xx 10^(5) m s^(-1)`