The $6563 A^{\circ}\; H _{\alpha}$ line emitted by hydrogen in a star is found to be red shifted by $15^{\circ}A$ Estimate the speed with which the star is receding from the earth .

Question

$\begin{array}{1 1} -6.86 \times 10^5\;ms^{-1} \\ 6.86 \times 10^5 \;ms^{-1} \\ -6.86 \times 10^{-5} ms^{-1} \\ 6.86 \times 10^{-5} ms^{-1} \end{array}$