BACHELOR OF SCIENCE (B.Sc.)

Transcription

1 No. of Printed Pages : 8 CD (\I BACHELOR OF SCIENCE (B.Sc.) Term-End Examination CD CD December, 2012 PHYSICS PHE-15 : ASTRONOMY AND ASTROPHYSICS PHE-15 Time : 2 hours Maximum Marks : 50 Note : Attempt all questions. Marks are shown against each question. You can use log tables or calculator. Symbols have their usual meanings. 1. Attempt any five parts : 2x5=10 (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. PHE-15 1 P.T.O.

2 (e) Explain why sunspots appear dark? (f) Explain the origin of 21 cm radiation. (g) The estimated life time of the sun on mainsequence is yrs. Determine the main 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 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 JK-1, mh x kg and G=6.7 x m3 kg-ls 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. PHE-15 2

3 (b) What is solar wind? Explain, briefly, the reasons for its occurrence. How are the Van-Allen radiation belts formed? (c) Draw a labelled H-R diagram. How does it help into estimating the size and mass of a star 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. 5. 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. PHE-15 3 P.T.O.

4 (c) Write down Friedman equation and sketch 4+1 its solutions. Which one of these solutions correspond to the contracting universe? PHE-15 4

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6 061RNI a.).? drit 21 cm rcirchtul d 14-1 ti44ir I HIT TO 'TT 31-17f ff a t 10.5M -ar TAT 3T1-04 ITT.q-1-d9mlrl Jft cli.k (h) fa7-4-t wtra.--grrff M= -20 -qt"" ict) 4-1q1avi 700 kpc m1 e&.i 'ER t I ci-eli qt tqf 3ft (rqat f t) f '11? p-r et) : 5x2=10 (a) T T '-if-d-ift9s t I d-ichl fitt wffff-trfmftfc I f ft * 3T.11=4 41 t11.5x1011mti (b) tg +itf041 11sl r fkif-d-7 Myrfw3rurrol- 3-WfT 30 N 'TT ft-4ff.4q-* "T4 4,irtm fctm f f-*7t fkrr-d- (c) Ttzf TITPM-2TT 1030 kg 3 ATM 3-TheTT c K clic4 c11- Trp:Err-qTRIT t arlmff c-mr* 1 rqqr tfw kb =1.38 x 10-23JK-1, mh x kg G=6.7x10-11 m3 kg-1s-2. PHE-15 6

7 3. WTI oh : 5x2=10 (a) 179.-,ki41 c TT c R rcirchrci 5 *01 *I 17T.EFF il1 f9711:11 31-zrfrf Pin C11. I t-hii leictich g=5.67x 10 Wm-2K-4 m 2K 4 at{ Le = 4 x 1026 W I (b) tiyt jai ri *? 1,k4T-T 7,1* miter * N. I ail - (1,-1 rarchor trforf *74 -q-7t f? (c) dikt qc att:fri rc-r alttg f*-14 Omit 4. 'W)1 ITTTT ch : (a) 31:a71-fftc471 rolc1)4-1 c-n c-11 *? 3td1T-d1T Trrizi-rr T 4Nd-I f9.4ift.t.t4 q t74 ctlt? (b) 74 c crib el-q1 )clf *?E 31reiTT tr{ 2+3 fm c414-1-f rnz q ch1 ITT ail t? *Al m g r t? (c) Tun fay ;Eff *? air TR 2+3 ITT cpbul fay c qctigf -c0c,..5 f IT ai\rich mil PHE-15 7 P.T.O.

8 5. ) 34iri 01;k : (a) 4-1.q1 eh r-11). t 011 ikkur ci a-o. i;urr4- k-14-11r. I 4+1 3TWRT,ion i-ictia,41 If-*-T:r coi 4 ktar \TIldlt? (b) cclik-r a-611 dr t? cfallk-1( t Tc).-714.W1 3& q-a-r 3 - ii-k -lici 3-ifirFuuffITT au `1 ch. ] (c) m;ishil -1 4-ila,oifffd 3 T4- (-1 -ci Ad et) TA 4 chl-f TIT (,i fti--t---4 ark c,fais t 44 Id 6ii? PHE-15 8