No. of Printed Pages : 8
PHE-15
BACHELOR OF SCIENCE (B.Sc.)
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Term-End Examination
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December, 2012 PHYSICS PHE-15 : ASTRONOMY AND ASTROPHYSICS Maximum Marks : 50
Time : 2 hours
Note : Attempt all questions. Marks are shown against each question. You can use log tables or calculator. Symbols have their usual meanings. 1.
2x5=10
Attempt any five parts : (a)
Express the radius of the Earth in unit of
. (Radius of the earth is 6.4 x 106 m). (b)
The apparent magnitudes of two stars are 0.06 and 1.06 respectively. Calculate the ratio of their brightness.
(c)
Compute the magnitude of the faintest object observed with a 1 m optical telescope.
(d)
In the H-R diagram, three stars are found to be having luminosities L1, L2 and L3 such that L1 > L2 > L3. Determine the ordering relation of their radii if the effective temperatures of these stars are the same.
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Explain why sunspots appear dark ? (f) Explain the origin of 21 cm radiation. (g) The estimated life time of the sun on mainsequence is - 101° yrs. Determine the main
(e)
sequence life time of a star of 0.5 Mc., . (h) A galaxy of absolute magnitude M= - 20 is at a distance of 700 kpc. Would it be visible to the unaided eye. 2.
Attempt any two parts : 5x2=10 (a) The apparent magnitude of the sun is - 26.8. Determine its absolute magnitude. It is given that the distance between the sun and the earth is 1.5 x 1011m. (b) Explain the horizon coordinate system used in astronomy. Show the horizon coordinates of a star on a celestial sphere for an observer at latitude 30°N. (c) Estimate the radius of a star in thermal equilibrium of mass 103° kg and average internal temperature 107 K. It is given that kB =1.38 x 10 -23 JK -1, mH - 1.67 x 10 -27 kg and G=6.7 x 10 -11 m3 kg-ls-2.
3.
Attempt any two parts : 5x2=10 (a) Assume that sun radiates like a black body 5 of temperature T. Calculate T using Stefan-Boltzmann law. Take Q=5.67 x 10 -8 Wm -2K -4 and =4 x 1026 W.
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(b)
What is solar wind ? Explain, briefly, the reasons for its occurrence. How are the 1+3+1 Van-Allen radiation belts formed ?
(c)
Draw a labelled H-R diagram. How does it help into estimating the size and mass of a 3+2
star.
4.
5.
Attempt any two parts : (a)
What is interstellar extinction ? How does 2+3 it help in determining the composition of ISM ?
(b)
What is a white dwarf star ? Explain, on 2+3 physical basis the existence of an upper limit on the mass of a white dwarf star. What is this limiting value called ?
(c)
What is a black hole ? Derive an expression 2+3 for Schwarzs child radius of a black hole on the basis of Newtonian mechanics and explain its significance.
Attempt any two parts : (a)
Explain Hubble's scheme of galaxy 4+1 classification. What is the type assigned to the Milky way galaxy ?
(b)
What is a quasar ? Sketch the spectrum of 1+4 a quasar and describe its main characteristics.
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(c) Write down Friedman equation and sketch 4+1 its solutions. Which one of these solutions correspond to the contracting universe ?
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