1.Name the nearest star to the earth.
2. How far away sun from the earth ?
The distance of the sun from the earth is nearly 1.5 x 108 km
3.What is the mass of sun ?
Mass ofthe sun is 2 x 1030 kg. This is more than 3x105 times that of the earth. Mass of the sun is represented by M .
4.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 105 kms-1)
d = vt
= 3 x 105 X 8.3X 60
distance of the sun from the earth = 1494 X105 kms = 1.5 x 108 km.
5.Define solar luminosity.
The amount of energy radiated per second by the sun in all directions (called solar luminosity & it is equal to 3.9 x 1026 W
6.What is the surface temperature of the sun?
surface temperature of the sun is 5000 - 6000K
7. What is photosphere ?
The visible disc of the sun is called photosphere.
8. What is chromosphere ?
The layer just above the photosphere is called the chromosphere; this extends to about 10,000 km above the photosphere. This region is about 1000 times less dense than the photosphere, but much brighter.
9. What is corona ?
The region beyond chromosphere is called corona and this extends to millions of km. This region is seen as a white halo around the sun during total solar eclipse, when the photosphere and the chromosphere are covered. Matter in this region is in the plasma state and the temperature is of the order of a few million degrees.
10. What is Granulations ?
A careful examination of the photosphere reveals grain like patterns; these are called granulations.
11. What are solar flares?
Occasionally matter streams out of the sun’s surface in bursts like thunder storms; these are called solar flares.
12. What are solar prominences ?
Sometimes there will be huge flares which may loop back into the photosphere; they are called solar prominences. All these phenomena indicate various activities in the inner regions of the sun.
13. Write a note on the structure of the sun with a diagram.
In Figure : 1- Photosphere, 2 – Convective Zone,
3 – Radiation zone 4 – Core,
5 – Solar prominences, 6 – sun spots.
14.What is Aurora ?
The charged particles ejected at great speeds from the sun, ionise the air molecules which results in spectacular colour display. These are seen from polar regions and are called aurora or polar lights.
15.What are sun spots? Explain.
When an image of the sun, obtained by a telescope, is projected on a white screen, dark irregular patches present on the sun can be seen on the screen. They are called sunspots.
16. What is the temperature of sun spots ?
The temperature of sun spots is about 4000K.
17.Give the relation between parsec and light year.
1 parsec (pc) = 3.26ly = 3 x 1013 km
18.Name the scale on which brightness of a celestial body is represented.
The brightness of stars is represented by a system called magnitude scale.
19.Two stars have a magnitude difference of 6. What is the brightness ratio?
Brightness ratio of two stars having magnitude difference of 6 =
2.5X2.5X2.5X2.5X2.5X2.5 = 244.14
20. Define luminosity of a star. Name the factors on which it depends?
The total energy emitted by a star in one second is called its luminosity; the value of luminosity depends on the temperature and diameter of the star.
21. List the temperature , colour of different stars.
2500 - 3500K
3500 - 5000K
5000 - 6000K
6000 - 10,000K
10,000 - 50,000K
22. Name the characteristics on the basis of which, sizes of stars are determined.
sizes of stars are determined from its luminosity and temperature. Majority of stars have radii in the range of a tenth to twenty times solar radius.
23. What is a Binary Star ?
Two stars form a gravitationally bound system, and go round in circles around their common centre of mass is called a binary star. Sirius is a binary star system;
24. What is a galaxy?
A huge group of stars is called a galaxy
25. Name the different types of galaxies.
spiral galaxies , elliptical galaxies & irregular galaxies
26.What is stellar evolution
The process from the birth to the death of a star is called stellar evolution.
27. Explain stellar evolution
Huge gaseous clouds mostly hydrogen exist in space. It is believed that the birth of a star begins when such a gaseous cloud contracts due to gravity. There will be increase in density which leads to increase in pressure. Gradually the cloud settles with a spherical mass at the centre of the cloud. The central portion has a mass of about 99% of the mass of the cloud. The sphere formed at the centre is called a protostar. This represents the cocoon stage of a star. A protostar has a hydrogen core formed at its central portion. With the formation of a protostar contraction and heating up continues. Gradually the temperature increases to millions of degrees. When the temperature is high enough, fusion of hydrogen begins The energy released during nuclear fusion tries to expand the matter. Eventually the outward pressure due to the radiation generated balances the inward gravitational pull. This gives the star a stable state. A protostar thus reaches steady state. The star is now in the youth stage and it is said to be in the main sequence. The youth stage of a star may last for several billions of years depending on its mass. Less massive stars stay longer in the main sequence. The sun has remained in the youth stage for about five billion years. It will continue in the same stage for another five billion years. For massive stars, it may be several million years only.
28. Explain the stages of evolution of a sun like star.
As hydrogen continues to fuse forming helium, a helium core forms at the centre. This is surrounded by hydrogen in the outer shell and hydrogen fuses at a rapid rate. The resulting radiation causes the star’s envelope to expand and cool. The colour of the star changes to red and the star is now called red giant. The sun in its red giant stage, will swallow up mercury and venus and the earth will be burnt in the heat. As a star’s envelope expands, its core contracts and heats up. When the temperature reaches 108 K, helium is converted into carbon in the core. Once the fusion of helium in the core is complete the core can not contract further.Outer envelop of red giant gets detached and thrown into space. It forms a cloud of hydrogen gas called planatary nebula. What happens next depends on the mass of the star. If the star has a mass less than about 1.4 solar masses (known as Chandrasekhar limit), the star collapses under the action of gravity. As the temperature increases, the core develops pressure which prevents further collapse; the star becomes what is called White dwarf. White dwarfs shine due to their high temperature. When further nuclear reactions stop, a white dwarf gradually cools losing its internal energy resulting in decrease of temperature. It becomes dimmer and dimmer until it emits no light. It has then become black dwarf a dark cold chunk of mass. This corresponds to the death of a sun like star.
29. Explain the stages of evolution of a star of mass 10M .
Stars which are 5 times or still more massive than the sun, evolve differently after the red-giant stage have several stages of nuclear reactions ignited one after the other. The carbon nuclei produced during the fusion of helium form a carbon core. This will get ignited to produce a core of oxygen. The chain continues leading to the production of heavier elements. Thus cores of oxygen, magnesium, silicon, etc are formed and the cores lie one inside the other like the layers of an onion. Iron is formed at the inner most core. At this stage, the star explodes and the event is called supernova During supernova, most of the material is thrown out at speeds of about 104 km/s. A sphere of neutrons remains at the centre and this is called neutron star. The material thrown out during a supernova event, will initially look like a nebula and gradually mix with other gaseous clouds, where star formation can take place. The stars thus formed will have elements heavier than hydrogen and helium. In the case of stars with masses about 30 times that of sun, the remnant at the end of supernova, has huge amount of matter compressed into a very small region. This is a region of intense gravitational field and is called a black hole. Any object can become black hole if it can be compressed to a sphere of radius less than a certain minimum. The sun would become a black hole, if compressed to a radius of 3 km. The only recognizable property of a black hole is its mass. Any other property like temperature, pressure or chemical composition cannot be determined. This is because no information in the form of light or radio waves comes out of it. Its presence can be inferred only from the gravitational force it exerts on bodies close to it. Thus there can be three end states in stellar evolution namely - white dwarf, neutron star and black hole. This is decided by the stellar mass. Their presence can be inferred indirectly.
30. What is the difference between open and globular star clusters?
The star clusters are of two types - open and globular clusters based
on their appearance. In open type, stars appear to be loosely bound where as in globular type, stars appear to be tightly bound. The open clusters have many blue stars which are young. The older globular clusters have mostly red stars.
The study of star clusters can verify theories of stellar evolution.
31. What is the evidence for expanding universe?
The red shifts of galaxies show that they are moving away from us with enormous speeds. This demonstrates that the universe is expanding.
32. State Hubble’s law.
The velocity of recession of a celestial body is proportional to its distance from us. This is known as Hubble’s law.
33. Write a note on ‘Big Bang’.
At the beginning of the universe, all matter comprising galaxies and star
and radiation were compressed into a fiery region, with extremely high
temperature and enormous density. This is known as the “primordial fireball”
The fireball exploded with a bang. There was an awesome explosion which
launched the expansion of the universe. This is called the “Big Bang”.
As matter and radiation cooled, stars and galaxies formed. Planets came into existence. Eventually life came into existence on earth. The radiation has gradually cooled. The existence of this radiation was detected in 1965 and this supports the ‘Big Bang’ theory.
34. What is a Neutron star ?
During supernova, most of the material is thrown out at speeds of about 104 km/s. A sphere of neutrons remains at the centre and this is called neutron star.
35. What is a black hole ?
In the case of stars with masses about 30 times that of sun, the remnant at the end of supernova, has huge amount of matter compressed into a very small region. This is a region of intense gravitational field and is called a black hole.
36. What are brown dwarfs ?
If the mass of a contracting gaseous cloud is less than He ignition 0.1 M , it will not become a star. Such objects are called red giant brown dwarfs.
Jupiter may be called a brown dwarf.
37 .What is the age of the Universe ?
The age of the universe is estimated to be 10 to 20 billion years.
38 .What is the age of our sun ?
The age of our sun is estimated to be 4.5 to 5 billion years.
39 .What is the name of our galaxy ? Write its type .
Milky way is the name of our galaxy. Milky way is an example to Spiral galaxy.
40.List the different stages in life cycle of sun like stars & huge stars
Sun like stars
Huge stars( mass more than 5 times that of sun)
Steady state star
Steady state star