Galaxy: Group of billions of stars at a place in the sky is called galaxy. There are about 1012 galaxies in the universe.
Spiral: Milky Way, Andromeda
Elliptical: Fornax, Sculpture
Irregular: NGC 6822, IC 1613
Constellation: Group of few stars having a fixed pattern in the sky is called constellation. There are about 88 galaxies in the universe.
Examples: Ursa major, Ursa minor, Leo, etc.
Meteor: A falling star (actually it is burning rock) and vanishes in the sky due to friction with air is called meteor.
Red giant Star: When maximum amount of hydrogen is converted into helium, outward pressure due to burning of hydrogen will be cease zero. As a result the star contracts continuously and temperature of the core increases, which causes to increase size of the star. Due to the expansion, the temperature of surface starts falling. Finally a star becomes very large in size with red color. This star is called red giant star.
Supernova: At the end of red giant stage a violent explosion occurs with emission of large amount of energy it is called supernova. The star formed by exploding of force of the red giant star is a supernova. When sudden explosion of red giant stars that have mass about three times greater that the sun occurs, supernova is formed.
White dwarf: If the size of star is same or less than our sun, the central core cannot hold the outer shell which escapes out leaving white ball known as white dwarf.
Neutron star: After the explosion of supernova, if the remnant is less than the mass of the sun, it collapse in to itself resulting a rotating ball which is called neutron star.
Black Hole: The remnant of star having high density and gravitational pull through which even light cannot escape is called black hole.
Formation: The red super giant stars contract at the end of their lives and finally convert in to a dense mass. They have such a large magnitude of gravity that even light cannot escape. In this way black hole is formed.
REASONABLE FACTS
1. The earth is suitable for survival of living beings.
Ans- The earth is suitable for survival of living beings because of the following reasons:
– Presence of appropriate temperature
– Presence of excess amount of water.
– Presence of atmosphere containing oxygen.
2. There is vast difference in maximum and minimum temperature in mercury.
Ans- There is vast difference in maximum and minimum temperature (420oC to -180oC) in Mercury due to the absence of atmosphere.
3. Pluto is not considered as a planet of the solar system.
Ans- Pluto is not considered as a planet of the solar system because of the following reasons:
– Orbit of Pluto coincides with the orbit of the Neptune.
– Orbit of Pluto does not lie on the same plane to the plane of orbits of other planets.
– Size of Pluto is also very small as compared to average size of plants.
4. Asteroids are also called minor planets.
Ans- Asteroids are also called minor planets because they revolve around the sun in the same way the planets revolve and they are smaller than major planets.
5. Some stars look much brigher at night in a clear sky.
Ans-Some stars look much brighter at clear night in the sky because they are comparatively near to the earth and are relatively bigger than other stars.
6. A comet is seen only after a long period.
Ans- A comet is seen only after a long periodbecause it takes many years to complete one revolution around the sun.
7. The distance between the sun and the earth is not some in the whole year.
8. The distance between the moon and the earth is not same in the whole year.
Ans: because the moon revolves around the earth in elliptical orbit.
9. The planet Saturn is called as the ringed planet.
Ans- The planet Saturn is called as the ringed planet because the planet Saturn is surrounded by three electrical rings with small divisions having the ice fragments.
10. Though there is H2O, O2 and CO2 in Venus, no life exists on it.
Ans- Though there is H2O,O2 and CO2 in Venus, no life exists on it because of the presence of very high temperature (460oC).
11. Mars is also called as red planet.
Ans-Mars is also called as red planet because of the presence of red colored mineral called limonite (Fe2O3).
12. The earth called inner planet.
Ans- because it lies inner to the belt of asteroids.
13. Meteors are called shooting stars.
Ans- Meteors are called shooting stars because they appear as falling star when falls through the air inside the earth’s atmosphere.
14. Meteors are destroyed before reaching the earth’s surface.
Ans- Meteors are destroyed before reaching the earth’s surface because they are very small and gets burnt due to friction with air when falls in the earth’s atmosphere.
15. Black hole is invisible.
Ans- Black hole is invisible because it is lightless area in space that nothing, not even light, can escape from it due to its extermely large gravitational pull.
16. The moon rise delayed by 50 minutes every day.
Ans- When the earth rotates on its axis in 24 hours, the moon revolves 12o ahead from the original place. To cover this distance, the earth has to rotate 12o more which requires the extra time of about 50 minutes. Therefore, the moon rises delayed by 50 minutes from a particular place every day.
17. Tail is formed when comets come closer to the sun.
Ans- Tail is formed when comets come closer to the sun because of the conversion of superficial lump of ice in to vapour and due to the effect of solar wind.
18. The tail of the comet points opposite to the sun.
Ans- The tail of the comet points opposite to the sun because of the motion of comet and effect of solar wind.
19. A meteor lasts for a very short time.
Ans- A meteor is a small piece of rock or iron or both, which burns and vaporize completely in a few seconds due to the excessive heat produced during atmospheric friction. Therefore, a meteor lasts only for a few seconds.
20. Mercury, Venus and Mars are called terrestrial planets.
Ans- Mercury, Venus and Mars are called terrestrial planets because their structure is similar to that of the earth.
21. Artificial satellites are launched in space.
Ans- Artificial satellites are launched in space for the following reasons:
– To study overall weather patterns of the earth.
– To improve for the quality and speed of mass communication.
– To know the exact position of navigators in the air or in the oceans in any weather.
– To gather all kinds of information about the universe]
– To observe troop movements, spy on military bases, or detect the testing of nuclear weapons in space.
22. Even primitive forms of life cannot exist in Venus.
Ans- Even primitive forms of life cannot exist in Venus because of high temperature of about 464oC.
23. Comet is not actually a star.
Ans- Comet is not actually a star because comet revolves around the sun in an elliptical orbit and is made up of dust particles and cloudy ice.
24. A meteor is finished before reaching the earth’s surface.
Ans- When a meteor comes towards the earth’s surface with a great speed, it burns completely due to the friction between the meteor and atmospheric gases. Therefore, a meteor is finished before reaching the earth’s surface.
25. Our galaxy is called Milky Way or Akash Ganga.
Ans- Our galaxy is called Milky Way or Akash Ganga because a wide band of stars faintly appears as a river of light or milk in the night sky running from the north to the south.
26. Black hole is called black hole.
Ans- Black hole has an extremely large gravitational field. Due to this, nothing even light, can escape from its surface and appears black. Therefore, a black hole is called black hole.
27. Synodic month is longer than sidereal month.
Ans- The time taken by the moon to form next new moon is called synodic month or lunar month and has value of about 29.5 days but the time taken by the moon to complete one revolution around the earth is called sidereal month that has value of about 27.33 days. So, Synodic month is longer than sidereal month.
