clix - Unit 3: The Solar System and Beyond

Glossary

Have you watched night sky from a dark place? You must have seen hundreds of stars all over. All these stars are very far away. The stars which appear to be close to each other or sometimes even appear to be forming a certain shape are not at all close. Some stars looks brighter and some look faint. Brightness of a star depends upon how big it actually is and how far it is.

The nearest star, Proxima Centauri, is 4.25 light years¹ from the Sun. This means it takes 4.25 years for light to travel from Proxima Centauri to the Sun (and the Earth). Proxima centauri itself is a small star and hence very faint, not visible to the naked eyes, but it is a part of a system of three stars² called Alpha Centauri, which is 4.37 light years away. The other two stars in Alpha Centauri are approximately the size of the Sun. This star system looks like a single source when seen with the naked eye and happens to be the third brightest star in the night sky. From India, it is seen near Southern horizon during Summer (May to July) and is appropriately called ‘Mitra’ (means friend) in Indian languages. Table 1 gives the masses of the stars as compared to the mass of the Sun (which gives us some idea about how big they are) and distances of some of the closest and brightest stars from the Sun.

Table 1: Distances from the Sun to some of the closest and brightest stars

Name of the star	Mass in multiple of mass of the Sun (M☉)³	Distance in light years	Distance in kilometer if Sun-Earth distance is 1 cm
Alpha Centauri (closest system of three stars, which together make the third brightest object in night sky)	1.1 M☉, 0.9 M☉, 0.1 M☉	4.25	2.7
Barnard's Star (fourth closest star, invisible to the naked eye)	0.1 M☉	6	3.8
Wolf 359 (fifth closest star, invisible to the naked eye)	0.09 M☉	7.7	4.9
Sirius (system of two stars that is the brightest star in the night sky)	2 M☉, 0.98 M☉	8.6	5.4
Canopus (second brightest star in night sky)	8.2 M☉	74	46.8
Rigel (system of three stars that makes the seventh brightest star)	23 M☉, 3.8 M☉, 2.9 M☉	~ 1400	885.4

Notice that some stars such as Rigel are as far as 1400 light years away from our solar system. This means that the light which reaches our eyes when we look at Rigel started 1400 years ago from Rigel. What we see now is how Rigel looked 1400 years ago. Thus stars can show you the past (not the future, as an astrologer might think!).

How do we know these distances? Surely no one can travel so far! The distances to the star are measured using the parallax method. Let us see how it works.

^[1]A light year is a unit of length. A light-year is the distance that light travels in vacuum in one Julian year (365.25 days) which is equal to 9.4605284 × 10¹⁵ meters.
^[2]Just like planets move around a star due to gravitational force, sometimes stars also move around each other due to mutual gravitational force.
^[3]M☉ is the mass of the Sun = 1.99 × 10³⁰ kilograms

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4.1 Stars