Universe

1.            Name the nearest star to the earth?

                Sun is the nearest star to the earth

2.            Light from the sun reaches the earth in 8.3 minutes. Calculate the distance of the sun from

                 the earth?

                (Velocity of light = 3 x 10⁵kms^-1)

                Velocity = distance traveled/time taken or distance traveled = velocity x time taken

                Distance travelled = 3 x 10⁸ x 100 x 8.3 x 60 = 1.5 x 10⁸km

3.            What is solar luminosity? What is its value?

a         )      The amount of solar energy radiated per second in all direction is called solar luminosity.

b        )      Its value is 3.9 x 10²⁶

4.            What is the surface temperature of the sun?

                The surface temperature of the sun is about 6000K

7.            Give the relation between Parsec and light year?

                1 Parsec = 3.26 light year

8.            Name the scale on which the brightness of a body is represented?

                The brightness of a body is represented on magnitude scale.

9.            Two stars have a magnitude difference 6. What is their brightness ratio?

                Their brightness ratio is 1:100

10.          Define luminosity of a star. On what factors does it depend?

a)      The total energy emitted by a star in one second is called its luminosity.

b)      The luminosity of a star depends on the temperature and diameter of the star.

11.          Name the characteristics on which the size of a star is determined?

                The size of a star is determined based on their temperature and luminosity

12.          What is a galaxy?

                A huge group of stars held together by gravitation is called a galaxy

13.          Mention the different types of galaxies?

                Spiral, irregular and elliptical are the three types of galaxies.

14.          Explain the stages of evolution of a sun like star?

                Following are the major stages in the evolution of a star.

                Protostar

The star begins its life with a huge cloud of gaseous hydrogen that contract due to gravity. This results in increased pressure and density and forms a spherical mass at the canter of the cloud and is called the protostar.

Steady state

When the star contracts, it reaches millions of degrees and the fusion of hydrogen nucleus begins. The energy released during the nuclear fusion expands the matter and balances the inward gravitational pull. This stage is called the steady state.

                Red giant

Due to the continuous fusion of hydrogen into helium a core of helium is formed. the envelope expands and cools into a red star and is called the red giant.

                White dwarf

As a star’s envelope expands its core contracts and heats up and reaches a temperature of around 10⁸K. Helium is converted in to carbon. When there is no more fusion, the core cannot contract further and the outer envelope gets detached and thrown into space. The core collapses under the action of gravity.

As the temperature increases, the core develops pressure which prevents further collapse and becomes a white dwarf and shines due to its high temperature.

When it slowly cools, it becomes dimmer and becomes a black dwarf.

15.          Explain briefly the life cycle of a massive star after its red giant stage?

·         After the red giant stage, due to the fusion of helium, a carbon core is produced.

·         This ignites to produce a core of oxygen, magnesium, silicon, iron etc, each inside the layer of another with iron as the inner most core.

·         At this stage, the star explodes and is called the supernova.

·         During supernova, most of the material is thrown out at a great speed of about 10⁴ km/second, leaving behind a sphere of neutrons. This is called the neutron star.

·         Initially it looks like a nebula and gradually mixes with the gaseous clouds to form new stars.

·         The stars thus formed will have elements heavier than hydrogen and helium.

16.          What is the difference between open and globular star clusters?

·         In the open type of star clusters stars appear to be loosely bound and have many blue young stars

·         In globular type, stars appear to be tightly bound and have mostly red colour stars.

17.          What is the evidence for expanding universe?

·         The spectral lines of all galaxies show red shift

·         The spectral lines of a given galaxy show equal red shift.

·         These red shifts show that the galaxies are moving away from us with enormous speeds.

·         This shows that the universe is expanding.

18.          State Hubble’s law. On what basis is it derived?

a)      Hubble’s law states that the velocity of recession of a celestial body is proportional to its distance from us.

b)      It is based on the study of red shift of the various galaxies.

19.          Write a note on Big Bang theory?

·         Big Bang theory refers to the explosion of the primordial fireball with a big bang.

·         This resulted in the expansion of the universe

·         As a result of big bang, matter and radiation cooled, stars and galaxies were formed.

·         Planets came into existence and life came into e1.            Name the theory that can be used to determine the mass of the earth?

                Kepler’s laws of planetary motion can be used to determine the mass of the earth.

2.            Where do you find solar telescopes in India?

                Solar telescopes are found at Kodaikanal and Udaipur.

3.            How do you know the existence of different elements in the sun?

·         Solar spectrum has different Fraunhofer lines as seen from a spectroscope.

·         An analysis of these reveals the existence of different elements in the sun.

4.            What is photosphere?

                The visible disc of the sun is called the photosphere

5.            Describe briefly the structure of corona?

·         The region beyond the chromospheres of the sun is called corona.

·         It extends to millions of km.

·         During a total solar eclipse, it is seen as a white halo around the sun.

·         When it is covered by the photosphere and chromospheres, matter in this region is plasma.

6.            What are granulations and spicules?

                The grain like patterns and thread like structures seen on the photosphere of the sun are called granulations and                 spicules respectively.

7.            What are solar flares?

                Matter steaming out of the sun’s surface like thunderstorms is called solar flares.

8.            What are solar prominences?

                Matter streaming out of the sun’s surface burst like thunderstorm and looping back into the photosphere are                 called solar prominences.

9.            What are aurora?

                The spectacular color display of charged particles ejected at great speeds from the sun when they ionize the air                 molecules are called aurora.

10.          How are we protected from many electromagnetic radiations from the sun?

                The earth’s atmosphere absorbs many of the electromagnetic radiations like ultraviolet rays, X-rays etc. and allows                 only heat, light and radio waves and a small amount of ultraviolet rays. Hence we are protected.

11.          What makes the sun highly energetic?

                The temperature at the core of the sun that is of an order of 20 million Kelvin and the nuclear reactions taking                 place there makes the sun highly energetic.

12.          How does energy from the sun reaches the photosphere?

                Energy from the sun reaches the photosphere through radiation and convection.

13.          Why do stars differ in their color?

                Stars differ in their color due to the difference in their temperature.