No. of Printed Pages : 8
PHE-15
BACHELOR OF SCIENCE (B.Sc.) Term-End Examination June, 2012 CO
PHYSICS
CV!
PHE-15 : ASTRONOMY AND ASTROPHYSICS
C:0
Time : 2 hours
Maximum Marks : 50
Note : Attempt all questions. Marks are shown against each question. Symbols have their usual meanings. You may use a calculator or log tables. 1.
Attempt any five parts :
2x5=10
(a)
Express mass of the Earth in unit of M®. Take ME = 6 x 1024kg.
(b)
The apparent magnitude of the sun is — 26.81 and that of another star is —1.81. How much brighter the sun is with respect to that star ?
(c)* What is the diffraction limit of resolution of a 20 cm telescope. For wavelength X = 500 nm ? (d) Two stars of spectral class 0 and A have temperatures 40,000 K and 10,000K, respectively. Draw their black body radiation curves. PHE-15
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(e)
(f) (g)
(h)
2.
The temperature of chromosphere and corona is very high in comparison to that of photosphere. Why then is the photosphere the brightest of the three ? Calculate the energy of electromagnetic radiation having 21 cm wavelength. Let Mi and M2 be the masses of two main sequence stars. If M1 > M2 show that the star of mass M1 will have a shorter life span than the star of mass M2 on the main sequence. Estimate the distance that an emission cloud must have from the center of a Seyfert galaxy (having a central compact object of mass 107 M® ) in order to produce a velocity of 106ms -1.
5x2=10 Attempt any two parts : (a) The distance modulus of a star is -2.5. At what distance (in PC) is it from us ? (b) Draw the celestial sphere showing the ecliptic, vernol equinox and automnal equinox. Determine the deactivation (a) of ecliptic north pole_ (c) Estimate the mass of a star in thermal equilibrium whose average internal temperature is 107 K and radius 7 x108 m. It is given that Boltzman constant, kB =1.38 x 10 - 23Jk -1,mH - 1.67 x 10 -27kg and G= 6.7 x 10-11m3kg —1s — 2.
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3.
5x2=10
Attempt any two parts : (a)
The orbital period of Jupiter's satellite lo is 1.77 days. The semi-major axis of its orbit is 4.22 x 10" cm. Calculate the mass of Jupiter. Assume that the Jupiter is very massive in comparison to To.
(b) Describe briefly the solar corona. Explain 3+2 why its temperature is so high compared with that of the photosphere. (c)
4.
Explain the atomic origin of emission and 3+2 absorption spectra. List and explain two parameters of a star that can be estimated by analyzing these spectra.
Attempt any two parts : (a)
5x2=10
Name the various constituents of Inter stellar 1+4 gas. Describe the detection of graphite and silicate grains in 1SM.
(b) Explain the hydrogen burning process in the 1+4 main sequence stars. Obtain an expression for the main sequence life-time of a star. (c)
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What is a Neutron star ? Explain with the 1+4 help of a diagram, the emission of pulses from a neutron star.
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5.
Attempt any two parts : (a)
5x2=10
State the law which governs the variation 1+4 of the surface brightness in the disc of a spiral galaxy with the distance from the centre of the galaxy. Show that the total light emitted by the disc is 27r red, Id(0), where rd is the scale length of the disc and Id(0) is the surface brightness at r = O.
(b)
Explain the origin of broad-line and narrowline regions in a Seyfert galaxy.
(c)
1+2+2
(i) State Hubble's law. How can this be used to get an estimate of the age of the universe ? (ii) Why is it necessary to propose the existence of dark matter ?
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