Our Universe

Facts about the sun; its structure and composition

When you look into the sky at night, you will find that the sky is sprinkled with countless stars appearing like bright points. Their distance from us is very large which restrict us from knowing much more about them. But still on the basis of observation made through telescopes and other instruments, some of their characteristics are known. These characteristics help us to learn about their size, mass, temperature, composition and evolution.

As the stellar distance is very large so, a unit known as ‘parsec’ is used in addition to the light year unit (ly) to measure the steller distances.

1 parsec (pc) = 3.26 ly = 3 × 10¹³ km

Some of the features of star are discussed as follows:

Brightness and luminosity:

1. If you observe the stars you will find that some stars are brighter while some are faint, some are red while some are blue. They appear to be of different colours due to their temperatures.

2. The stars appear to be of different brightness mainly due to two reasons:

(i) They are at different distances. To understand this, observe the brightness of a candle light and the brightness of a torch light placed at the same distance. The brightness of the torch light will appeared to be more. But when the torch light is observed from sufficiently far distance as compare to the candle light, then its brightness will appear less than the candle light.

(ii) They have difference in their intrinsic brightness.

3. The brightness of the stars is represented by a system known as magnitude scale. It is defined such that the magnitude difference of 5 corresponds exactly to a factor of 100 in brightness. A brightness ratio of 2.5 or corresponds to a magnitude difference of 1.

(i) A star of magnitude 1 is 2.5² = 6.25 times brighter than star of magnitude 3. The star with the lower magnitude is brighter. A star with magnitude -1 is brighter than a star of magnitude 1.

(ii) So in order to compare the brightness of two stars, firstly find the brighter star by comparing their magnitudes. Then 2.5^m-n gives the brightness of the brighter star as compare to the dimmer star. Here m and n are the magnitudes of dimmer star and the brighter star respectively.

(iii) Example:

The magnitudes of sun and moon is –27 and –13 respectively, now compare their brightness.

If you compare the magnitudes of sun and the moon, you will find that the sun has the lower magnitude and thus it is brighter than moon by 2.5^{– 13-(–27)} = 2.5¹⁴ times.

The magnitudes of some of the celestial bodies are as shown below:

4. The luminosity of a star is the total energy emitted by it in one second. The luminosity depends on the temperature and size of the star. Generally the luminosity of the stars is expressed in terms of luminosity of the sun ( ). The luminosity of Sirius is 23.5

Stellar spectra:

• The spectrum of stars is similar as the spectrum of the sun crossed by dark absorption line. The study of these dark lines helps in studying the composition of the stars.

• The intensity distribution in its spectrum decides the apparent colour of the star and thus the temperature of the star. The temperature of stars ranges from about 2000 K for reddish star to about 50000 K for bluish white stars.

Temperature …