BACHELOR OF SCIENCE (B.Sc.) Term-End Examination December, 2009 PHYSICS PHE-15 : ASTRONOMY AND ASTROPHYSICS

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1 BACHELOR OF SCIENCE (B.Sc.) Term-End Examination December, 2009 PHYSICS PHE-15 : ASTRONOMY AND ASTROPHYSICS I PHE-15 Time : 2 hours Maximum Marks : 50 Note : Attempt all questions. The marks for each questions are indicated against it. Symbols have their usual meanings. Use of calculator or log table is allowed. Values of physical constants are provided at the end. 1. Attempt any five parts : 2x5=10 What will the local time in Chennai be when the local time in New Delhi is 12 : 00 noon? The approximate longitudes of Chennai and Delhi are 80 10'E and 77 10'E, respectively. Sketch the coordinates of Pole star in the horizon system. (c) Explain why the temperature of the chromosphere increases with height. PHE-15 1 P.T.O.

2 Draw the main sequence on an H - R diagram. Locate on it, stars of mass 0.5 M and 1.5 M O. What is meant by interstellar extinction? Draw a labelled diagram of a barred spiral galaxy. Write down the nuclear chain reactions involved in CNO cycle. (h) Estimate the age of the universe if Hubble's constant is 70 km s -1 M pc Attempt any two parts : Explain the diffraction limit of resolution of an optical telescope. A 10 m diameter optical telescope operates at 500 nm. Calculate its resolving power. Using Saha's ionisation equation, log (n II pe/n 1) = 2.5 log T - (5040)1' -0.48, R/o. Calculate the fraction of hydrogen atoms ionised at temperature 20,000 K. Take the partial pressure of electrons as 100 dynes cm -2 and the ionisation potential of hydrogen as 13.6 ev. PHE-15 2

3 (c) Distinguish between apparent and absolute 2+3 magnitude of a star. The apparent magnitudes of a - centauri and Betelgeuse are 0.10 and , respectively. Compare the brightness of these stars. 3. Attempt any two parts : Justify the statement with proper 5 derivations that "it is not the pressure but the pressure gradient that supports the star." Describe the stellar classification on the basis 4+1 of stellar spectra. What is the spectral class of a star having prominent hydrogen lines in to spectra? (c) Sketch the binding energy curve of nucleons. In which part of this curve is nuclear fusion a possible source of energy? Write down the reactions of a proton - proton chain Attemp any two parts : 2+3 How do we estimate the life of a star on the main sequence? Estimate this life time of a main sequence star of mass 16 M o if the life time of the sun on the main sequence is 1010 years. Sketch the evolution of a 5M 0 star on the 3+2 H - R diagram and explain the various stages through which it evolves. PHE-15 3 P.T.O.

4 (c) What are the components of interstellar medium? Distinguish between H I and H II regions. Explain how the H I region is deetected in ISM Attemp any two parts : Explain how cepheid variables are used to 5 estimate astronomical distances. Distinguish between a Quesar and a Radio 2+3 galaxy. Assuming H-atom to be the emitter, calculate the half - width of a spectral line of warelength 580nm when the temperature of the gas is 6 x 105 K. (c) Using the expression for total energy per 3+2 unit mass of a particle, derive Friedmanon R 2 K.2 EI 8pG equation in the form : + f R _ R 2 3 Sketch its solution for K>1. Physical constants : kb = 1.38 x J k-1 Mp= 1.6 x kg M = 2 x kg L 0 = 4 x 1026 W Ro = 7x 108 m G= 6.7 x m3 kg-1s-2 /pc = 3.1 x m. - o 0 o - PHE-15 4

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8 5. ch Tt. tl 1-1+IL. -FW "citchlfil Ctli) T Vt TTk 5 (g+ Iirik Ti77: :11-iN r tl vch crcut-r tftz:11 it tell 14. 3T a7 I 2+3 H-TR1:119 3c\4 n3ich F, 580 nm T4-4Ti:ft tg i thcr di x 105 KtI (c) WIT Tit 7WW a)zit otizi ch T 3+2 M g + I i;b1-1:11n (-14-1chui R 2 K.21EI 8pG f +, R 2 11" 3 K>1 77T I Stif6W Pieirtich kb =1.38 x j k-1 Mp= 1.6 x kg M = 2 x 10 3 kg L = 4 x 1026 W RO=7x108 m G= 6.7x m3 kg-is-2 1 pc = 3.1 x 1016 m. - o 0 o - PHE-15 8