Glossary

Procedure:

1. Choose a wall which has some features (posters, windows etc.). Stand at least 10 feet away from the wall, facing towards it.

2. Hold a pen in front of your nose, about 6 inches away from your face. Close one eye and notice a mark (say, mark X) on the wall close to it.

3. Now close the other eye and open the one which was closed. Is the pen seen at the same position? No! It appears to be shifted. This is known as parallax⁴. Notice the mark (say, mark Y) on the wall close to the pen.

4. How far are Marks X and Y from each other?

5. Now repeat steps 2 and 3 while holding the pen in your stretched hand. Did the distance between two marks remain same?

6. You would notice that the distance between marks is more when the pen was closer. Thus parallax is more when the objects are closer.

As you know, the Earth revolves around the Sun. So we get two vantage points from diametrically opposite sides of the Earth’s orbit (positions A and B in Figure 1). When seen from Position A the Star S will appear close to Star Y. After 6 months, when seen from position B, Star S will appear closer to Star X. Very good quality instruments are needed to observe this change in position. The distance of a star from the Sun is calculated using the angle between the the two different lines of sight.

Figure 1: Parallax - difference in the apparent position of an object viewed along two different lines of sight

You might have noticed that stars have subtle colours. Some stars look blueish and some stars look reddish. The colour of a star depends upon the temperature of the star. Temperature of bluish stars such as Vega is higher than the temperature of reddish stars such as Betelgeuse. Now why do stars have different temperatures? The temperature of a star or the amount of energy produced by a star depends upon mass and age of the star.

Watch the following video to learn about how stars are formed, what transformations they undergo, and what happens to them in the end!

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⁴Parallax is a difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle between those two lines.