30 March 2012
Last Sunday at ABAA there was an interesting talk and discussion on Stellar Parallax. I had asked Jayanth if he could send a small write up of the talk that he gave on the "Stellar Parallax & Distance measurement to Stars" so that it can be made available to all our blog readers. In his busy schedule Jayanth did manage to find some time to write a summery of the talk on Sunday. Here is the Article and Photos of the session.
What is Parallax?
To visualize this, See any object close to you say a few centimeters away, and look at it alternatively with your left eye closed and then with the right eye. You will notice that the position of the object seems to be different with reference to a far off background objects. You will notice that the object shifts from one position to other alternatively when you see the same once with your left and righ eye. The reason for this is not the object shifting but shifting the place you watch the object from. Your right eye and left eye.
The angles at which your eyes have to look obviously depend on the distance of the object and the distance between your two eyes. nose. The angle gets smaller with larger object to eye distance and smaller eye to eye distance. Therefore the difference in angles can be used to measure the distance to the object if you know the distance between your two eyes!. This difference in angles is called theparallax. The distance between your eyes is called the baseline; the bigger the baseline is, the bigger the parallax is, for the same distance of the observed object to the baseline.
The stars are very far away from us and their parallax is so small that the short baseline from one eye to another is too small to detect or even measure. In order to measure this small parallex of stars, we need a larger base line. A large base line is available which is the diameter of earths orbit = 2 Astronomical Units (AU) = 300 million KM.arth. Simply look where (at which angle) a star is in summer in the sky, and then look again where (at which angle) the same star is in winter in the sky, determine the difference in angles, and you have the parallax for this star. Now you can compute the distance to the star. The following picture should illustrate this:
Here in this calculations Parsec (parallax second) is the distance of a star that would have a parallax of two arc seconds, or, equivalently, the distance from our sun at which the angle between earth and sun (1 AU) would be one arc second. Using this unit t is useful because for large distances involved, distance is inversely proportional to the parallax. Hence a star with a parallax of x arcseconds is 2/x parsec away. This way we can calculate, one parsec is approximately 206,000 AU, or 3.09 × 1013kilometers, or 3.26 light years.
The closest star is 4.2 light years away which means parallax of the closest star will be less than one parsec. In order to measure such small angles, instruments with great precision are needed. Parallax to a star 61 Cygni was measured by astronomer and mathematician Wilhelm Nessel in the year 1838 from Konigsberg observatory and the value was 0.314 arc seconds. The modern value is 0.292 arc Seconds (total angles) which computes the distance to this star close to 10 light years.
Due to the precision of measurements required by this method and the atmospheric distortions limiting the accuracies, 1989 the satellite Hipparcos was launched which has the capability of measuring parallax down to 1 milli arc second. This means Satellite Hipparcos was capable of using Parallax method to estimate distance to near by stars up to 1600 light years. Another satellite named GAIA launched in 2010 is able to measure even more distant stars, up to 32000 light years.
By using radio waves instead of visible light, one can measure even smaller parallaxes, even down on earth, without the need for satellites. Using Very Long Baseline Interferometry (Two radio telescopes with a large distance between them which is usually on different sides of the earth) improves the measurement of angles greatly. The precision is in the order of 100 micro arc second.