QUESTION CORNER

Updated - September 23, 2016 04:07 am IST

Published - January 31, 2016 09:00 am IST

OUTER SPACE MEASUREMENT

What technique is used to measure the distance of a celestial object from the Earth and also its distance from other celestial objects?

K.SRINIVASAN, Chennai

Nearby stars are measured with parallax method. The Earth's orbit around the sun has a diameter of about 300 million km.

By looking at a star from two positions of the Earth six months apart, and using trigonometry, distance up to about 400 light years can be measured to reasonable accuracy. There is no direct method currently available to measure the distance to stars farther than 400 light years from Earth. The Cepheid variable technique is one such method which is useful for stars away millions of light-years. Cepheid is a category of stars whose actual brightness is well known. If a Cepheid appears dim its distance thus can be estimated by using inverse square method.

In the case of such distant galaxies, in the order of billions of light years away, the method of Cepheid stars is not useful as it is not possible to identify individual stars. In such cases distance is measured with the help of supernova.

A particular kind of supernovae, Ia Supernovae is used in measuring cosmological distances. This type of supernovae commonly occurs in binary systems. Red shift is another technique used for very far objects. Red shift is the effect where light from an object is “stretched” out so that the wavelength is moved towards the “red” end of the spectrum. Distances are calculated by using Hubble’s Law, more shift indicating the speed with which the object is moving away from us.

S.P.S. JAIN

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