Method for the determination of distance of star
This is a spectroscopic method based on inverse square law of intensity. According to this law, the intensity of illumination at a point is inversely proportional to the square of the distance from the source of light. Here we assume that the intrinsic brightness of all the stars is same. We compare the intensity I1 of the faint image of a far away star taken on a photographic plate with the intensity I2 of the bright image of a nearby star. Let r1 and r2 be the respective distances of these two stars.
From inverse square law of intensity,
knowing the distance r2 of the nearby star, the distance r1 of the far away star can be determined.
This method is useful for measuring distances of stars which are more than 100 light years away from the earth.