Bachelor of Science (B.Sc) Term-End Examination December 2014 Physics PDF

01702 No. of Printed Pages : 8 PHE-15 BACHELOR OF SCIENCE (B.Sc.) Term-End Examination December, 2014 PHYSICS PHE-15 : ASTRONOMY AND ASTROPHYSICS Time : 2 hours Maxim...

01702 No. of Printed Pages : 8 PHE-15 BACHELOR OF SCIENCE (B.Sc.) Term-End Examination December, 2014 PHYSICS PHE-15 : ASTRONOMY AND ASTROPHYSICS Time : 2 hours Maximum Marks : 50 Note : Attempt all questions. Values of physical constants are given at the end. Symbols have their usual meanings. You can use non programmable calculators or log tables. 1. Attempt any five parts : 5x3=15 (a) Compare the luminosity of a main sequence star with that of the Sun if the star is 10 times more massive than the Sun. (b) Sketch Hubble's scheme of classification of galaxies. (c) What do you understand by the Bolometric correction ? Why is this correction required ? PHE-15 1 P.T.O.  (d) Determine the magnitude limit in the visible range that can be detected by a 1.04 m telescope. (e) What indicates that the temperature of solar corona is of the order of 106 K ? (f) Explain the importance of Hayashi line in the H-R diagram. (g) Give two arguments in support of the expanding universe as compared to the steady state universe. (h) Write down the complete chain reactions involved in CNO-cycle. 2. Explain Rayleigh's criterion for the limit of resolution of a telescope. What are the factors which determine the resolving power of a telescope ? How does light gathering power of a telescope affect its resolving power ? Obtain the limit of resolution of a 2 m telescope (in arc-sec) for the wavelength of 500 nm. 3+2+1+4=10 OR Explain the universal equatorial system of coordinates with the help of a diagram. Show the equatorial coordinates of the Sun when it is at the summer and winter solstices. 6+4=10 PHE-15 2 3. Starting with the equation of hydrostatic equilibrium and considering the star to be made up of a monoatomic gas, derive the Virial theorem. 10 OR Explain the nebular model for the formation of the solar system. Is the present distribution of angular momentum in the solar system consistent with the nebular model ? Explain. 6+4=10 4. What is meant by a degenerate gas ? How is it different from an ideal gas ? Assuming that the density of a star is uniform and the star is in hydrostatic equilibrium, show that R « (M)-1/3 where M is the mass of the star and R, its radius. What is the significance of this result ? 2+2+4+2=10 OR Derive an expression for the free-fall time. Calculate the free-fall time for a collapsing cloud of neutral hydrogen whose initial number density is 106 m-3. Sketch the rotation curve of the Milky Way Galaxy and discuss its significance. 3+2+2+3=10 PHE-15 3 P.T.O. 5. Distinguish between a Quasar and a Radio galaxy. Assuming H-atoms to be the emitters, calculate the half-width of a spectral line of wavelength 580 nm when the temperature of the gas is 6 x 105 K. 2+3=5 OR Explain the unified model for active galaxies. 5 Physical constants : R® = 7 x 106 m Me = 2 x 1030 kg L® = 4 x 1026 W G = 6.7 x 10-11 m3 kg-1 S-2 kB = 1.38 x 10-23 J K-1 M = 1.67 x 10-27 kg c = 3 x 108 ms-1 1 pc = 3.1 x 1016 m PHE-15 4