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Physics 002 Midterm 2
Time allowed: 70 minutes
23 April, 2002.
Total: 70 marks
A




Table 1: Useful Data

1 AU
1.5 108 km

Suns temperature
5,800 K

Suns absolute magnitude
+4.8

Suns lifetime
10 billion y

H line
656.3 nm




Table 2: Spectral Types and Temperatures of Stars

Spectral type
O5
B0
B5
A0
A5
F0
F5

Temperature (K)
40,000
28,000
15,000
9,900
8,500
7,400
6,600




Spectral type
G0
G5
K0
K5
M0
M5
M8

Temperature (K)
6,000
5,500
4,900
4,100
3,500
2,800
2,400




Part A Short Questions (1 mark each):

Figure 1: HR Diagram with Luminosity Classes

















(1-5) A star of spectral type K0 and luminosity class II has an apparent magnitude of +17. Use Figure 1 to find, to 2 significant figures,
(1)
absolute magnitude of the star



C2.5

(2)
luminosity of the star (in units of solar luminosity)



830

(3)
distance of the star from Earth (in pc)



79,000 pc

(4)
temperature of the star



4,900 K

(5)
radius of the star (in units of solar radius)



40





(b) L/LO = 2.5124.8-(-2.5) = 2.5127.3 = 832 830 ans.

(c) d ans.
(e) Since L T4R2, we have R L1/2T-2. Hence R/RO = 8321/2(4,900/5,800)-2 = 40.4 40 ans.

Remark on the precision of the answers:
Since the data read from Figure 1 may not be precise, the absolute magnitude of the star may range from C2 to C2.8.
Hence the answers to part (a) may range from C2 to C2.8, part (b) from 520 to 1,100, part (c) from 63,000 to 91,000 pc, and part (e) from 32 to 46.

(6-9) Due to the dust in the interstellar medium between a star and Earth, the star will appear to an observer on Earth to be

Yes
No

(6)
pinker than it really is, since hydrogen appears pink.






(7)
bluer than it really is, because of scattering.






(8)
cooler than it really is.






(9)
fainter than it really is.









(6-9) This is called interstellar reddening.

(10-13) Which are true about heat convection in main sequence stars?

Yes
No

(10)
It is not present in upper main sequence stars.






(11)
It prevents the accumulation of helium ash in the core of lower main sequence stars.






(12)
It prevents photons from escaping from the star.






(13)
It is caused by the ionization of hydrogen atoms.









(10) Convection is present in the core of upper main sequence stars because of the large temperature gradient therein.
(11) Convection in lower main sequence stars extends throughout the entire star. Hence helium ash cannot accumulate in the core.
(12) Photons get scattered in the convective zone. When they emerge, the