Stars:
Like the Sun, stars are self-luminous bodies. Every star is a huge mass of hot gases hydrogen, helium and dust. And in all stars (including the sun, which is a star) energy is being generated by way of nuclear reactions. What happens is that hydrogen atoms continuously get converted into helium atoms. (It takes 4 hydrogen atoms to make one helium atom). And this con-version results in the release of a large amount of nuclear energy in the form of heat and light. It is this light that makes the star shine. So great and so large are the nuclear reactions that stars are sometimes compared to large hydrogen nuclear energy furnaces.
Pole Star:
Polaris is also known as the Pole Star. It is an important star because the imaginary line passing through the earth's north and south poles, called its axis, points almost exactly in the direction of the Pole star. I therefore, lies immediately above the earth's north pole And as the earth rotates on its own axis from west to east, all stars except the Pole star appear to move in the sky. It therefore appears to be stationary and does not appear to change positions with time. Also known as North star it is 51 light years away.
Viewing the Pole Star:
The Pole Star is always visible in the Northern Hemisphere.
But a person on the equator would not see the pole star at all as it would no longer be above the horizon. The pole star is part of the constellation of stars called the 'Great Bear' or simply the 'Plough'. An imaginary line drawn between the stars at the front of the plough and continued to four times the distance indicates the position of the Pole Star. The Indian name of Pole Star is 'Dhruva Tara',
Will the Pole Star ever change? Yes, Polaris known as the Pole star will lose this epithet. Owing to the precession, the north pole is moving away from the pole star. Precession is the movement in the direction of the poles every 26,000 years. Because the earth is not spherical and it bulges at the equator, the Sun's gravitational pull generates a torque on the earth in an attempt to pull the equatorial bulge into the same plane as the ecliptic. This causes changes in poles every 26,000 years. So a star that is a pole star today will change due to its change of position on precession. Vega, the fourth brightest star (nearly 58 times as bright as the Sun) in the night sky will be the next Pole Star. But that will take another 13,000 years from now!
Is there a 'South Star'? Although there is no South Star but the constellation 'Octans' would be almost directly overhead if viewed from the South Pole.
Stars in Navigation:
The nature of the stars to be fixed with regard to one another and their stationary appearance has been put to use by the navigators. Earliest star charts and catalogues date to 125 BC and are credited to Hipparchus. The stars have been used as navigational tools by the seamen to identify their locations and find their way at the sea. They use the nautical almanacs prepared by mathematicians listing the positions of the stars at various dates and times.
Why do stars shine only at Night? Stars do not disappear during the day. It is only that they are not
viewable from the earth. This is because of the atom-
sphere of the earth that gives the sky its blue colour.
This blue colour dilutes the contrast and the sunlight not
observable during the day.
Stars on the Moon:
If one were to be positioned on the Moon which has no atmosphere the sky looks black. Stars are seen against the black sky even in the day time. Astronauts on the Moon were able to have a look at the stars during the day.
Twinkling of Stars:
The earth atmosphere is composed of many layers and dust particles. A star is virtually a point source. The light from the stars reaches the observer after being deviated from its original path due to the reflection and refraction of the light through various mediums. This generates the phenomenon of twinkling of the stars. Stars do not twinkle in reality. A planet on the other hand shows as a small disk rather than as a point and hence twinkles less than a star.
Stars or Images?
The stars are so distantly placed that the stars that we see at night are the images of light that left these stars thousands of years ago. The light that we see of the Andromeda galaxy, for example, is the lighthat left this galaxy some 2,200,000 light years ago. So a star that is seen today may be the light of the star that has ceased to be one today.Yet, we may continue to seeing these images for years together.
Why the Stars do not collapse?
The Sun (which is a star) is a mass of hot gases nearly 109 times the size of the earth. It is 3,30,000 times as heavy as the earth. It is, therefore, a huge ball of fire. This enormous mass of gaseous matter would be exerting enormous amount of compression pressures on the Sun to make it collapse. Yet it survives. How?
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