Question

Discuss the various levels of the cosmic distance ladder and describe how each successive level is “calibrated” using the preceding level. Explain how parallax can be used to determine the distances to the closest stars.

Answer 1

The different levels of cosmic distance ladder are:

(i) Direct Measurement: This is possible only for close stars within a distance of 1000 light years. For measuring and calibrating the distance the method of parallax is used. Here two observations of the star are taken from earth round the year and the resulting shift (parallax) is measured to calculate the distance to the star.

(ii) Standard Candles method: In this method the luminosity of an object is measured with respect to a standard astronomical object whose brightness is known. The luminosities are compared to get details about the distance to the star. This method work for intermediate ranges.

(iii) Redshift : This is based on the Hubble's law of expansion of the universe, which states that the redshift of the light from the stars is directly proportional to how far the star is away from our planet. This method have been used to measure stars upto billion light years away.

Parallax Method

The same star is observed at two different times, it can be over the year too, where once the observation is taken when the earth is in one side of orbit, and another 6 months later when the star is on the another side of the orbit. The shift in star is measured, from which we get the subtended angle, Then the distance to the star is given by the division of distance between the points where we observed the star and the angle subtended.