Here, λ = 6563 Å, Δλ = 15 Å, c = 3 × $10^8 \text{ m s}^{-1}$ Since the star is receding away, hence its velocity v is negative (i.e. if Δλ is positive, v is negative) ∴ Δλ = $-\frac{v\lambda}{c}$ or v = $-\frac{c\Delta\lambda}{\lambda}$ = - $\frac{3 \times 10^8 \times 15}{6563}$ = - 6.86 × $10^5 \text{ms}^{-1}$ Here, negative sign shows recession of star.