## Stars

1) A star is a fiery luminous body which emits light of its own.
2) Stars twinkle at night.
3) Stars are countless in number; about 1022 in a universe.
4) Stars are very big in size but they appear small because they are very far off.
5) The relative positions of stars do not change day by day.
6) Stars appear to be moving from west to east.
7) The temperature of stars is very high.
8) The sun is the nearest star to the earth. Its light reaches the earth in 8.3 minutes.
9) After sun the next nearest star to earth is Alpha centuri. Its distance is 4.3 light years from earth.
10) The temperature of a star is estimated from the colour of its light received on earth. The blue coloured star is at higher temperature than red coloured star.

### Brightness of a star

The brightness of a star is expressed through the system of magnitude. Whereas the observed magnitude of a star is the measure of its brightness when observed from the earth and absolute magnitude of a star is the measure of its brightness at 10 per second distance.
It can be shown that m2-m1 = – 2.5 log(l2/l1).
Where m1 , m2 = magnitudes of two stars of brightness l1 and l2.
If l1=l0, the brightness of a star of zero magnitude i.e. m1 =0, then m2=-2.5log(l2/l0).
For example the star Vega is of zero magnitude and its brightness is l0 =2.52×10-8Wm-2 .
It has been found that the star Sirius has magnitude -1.5 and it is 22 times brighter than our sun.
The stars possessing negative magnitude are brighter than the stars of zero magnitude.

### Stellar spectra

The different stars are of different colours and the spectrum of a star is related to its colour There are seven classes of stellar spectra denoted by letters O, B, A, F, G, K and M. Our sun belongs to G class star.

### Size of a star

The radius of a star can be given by
R = (L/4πδT4)1/2
Where L = luminosity of a star, δ = Stefan’s constant, T = Surface temperature of star.

### Mass of a star

It can be shown that in a binary star system, M1+M2 =4π2a3/T2G
Where, M1, M2 = masses of to star forming a binary system
a = distance between those two stars.
T = period of rotation of each star about the common centre of mass of the binary system.
G = Gravitational constant.
Sirius is a binary star, the masses of the two components of Sirius were found to be 1 and 2.4 times solar masses.

### Density of a star

Density of a star = mass/volume=M/(4/3)πR3. It has been found that the density of stars varies from 5000kg/m3 in case of coolest M stars to about 10 kg/m3 for the hot O stars.

### Birth of a star

When the interstellar dust and gas particles comes closer to form a cloud due to gravitational force of attraction between them, the birth of star beings, if the original cloud has; mass of at least one thousands solar masses. As the contraction of the cloud continues, the pressure and temperature inside the cloud rise.

At a certain stage, the cloud breaks up into large number of fragments of staler size and each fragment continues to shrink and its temperature continues to rise. When the temperature of a fragment reaches millions of degrees, nuclear fusion begins to take place. Due to which the fragment begins to radiate light energy from its surface and it becomes a star.

### Death of a star

In each born star, during the fusion process, the hydrogen is converted into helium. When the contents of hydrogen of a star decrease of a certain proportion, the core begins to contract while the outer regions expend. Due to which, the surface temperature of a star drops and the star becomes giant or supergiant star. At the end, this star produces energy at very high rate to such an extent that it explodes in the form of nova or supernova, throwing out of the major portion of its mass into inters taller space. This is the death of a star. The core left behind may end in three ways.
i) If the original mass of the star is less than two times the mass of the sun, a white dwarf is produced. When it ceases to emit light, it becomes black dwarf.
ii) If the original mass of the star2 to 5 times the solar mass, neutron star is produced.
iii) If the original mass of the star is more than 5 times the solar mass, the black hole star is produced.

### Costellations

1) It is a group of stars.
2) In a constellation, the group of few stars forms a definite shape which resembles the figure of an animal or human being.
3) There are about 88 known constillations.
4) The various constillations are visible during a particular period of the year.

### Supernova Explosion

It occurs very rarely. It helps scientists to study the lifecycle of a star. The last supernova explosion has been observed on February 24, 1987 by astronomer lan Shelfon.