For stars more distant than 300 light years, astronomers need a different technique. One method involves finding out the direction a Star is moving in, and its actual speed.
To establish the direction, it is much easier to work with the cluster of stars than with a single star. Many stars belong to clusters that consist of hundreds, or even thousands, of stars moving through space. Perspective makes the star in each cluster appear to converge. The angle of their converging lines reveals the direction the cluster is heading in space – towards earth, away from Earth at a 45° angle, and so on.
A Star’s speed can be deduced from its light. The stars motion towards or away from Earth changes the wavelength of the light that it emits – so that it becomes blue if it is coming towards and red if it is moving away (a phenomenon called the Doppler effect).
By combining the rate of change in the star’s spectrum with the direction of movement of the cluster, astronomers can work out its real speed through space, and hence calculate the distance to the cluster.
Picture Credit : Google