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Physics 002 Midterm 2 Solution 14 November, 2002.
Time allowed: 70 minutes Total: 70 marks

Name:

Student ID:
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 (16 marks: 3 marks for (1)-(5) and 1 mark for (6))

Use the following chart to answer the questions.

Star
mV
MV
d (pc)
Parallax (arc sec)
Spectral type
Luminosity Class

A
4.2


0.025
A3
IV

B
2.6
-0.3


B2
IV

C

1.8
20

A5
V

D

-6.0

0.005
M2
I

E
-1.5

2.5

A1
V




(1)
How far is Star A from the Earth? Give your answer in pc.



40 pc

(2)
How far is Star B from the Earth? Give your answer in pc.



38 pc

(3)
If MV of the Sun is +4.8, what is the luminosity of Star C? Give your answer in solar luminosity.



15.9 solar luminosity

(4)
Which star has the greatest surface temperature?



B

(5)
Which star has the greatest diameter?



D

(6)
Which star(s) is(are) on the main sequence?



C and E






(1) d = 1/p = 1/0.025 pc = 40 pc Ans.
(2) d = 10 ^ (mv C Mv +5)/5 = 10 ^ (2.6 + 0.3 +5)/5 pc = 38 pc Ans.
(3) L/L0 = 2.512 ^ (4.8-1.8) = 2.512 ^ 3 = 15.85 Ans.
(4) Star B is the hottest because it is a B2 star.
(5) Star D is the largest because it has luminosity class I, ie a supergiant.
(6)
Stars C and E are on the main sequence.




Part B Multiple choice (18 marks)

(1) The nuclear reactions in a stars core remain under control so long as
A.
luminosity depends on mass.


B.
pressure depends on temperature.


C.
density depends on mass.


D.
weight depends on temperature.


E.
temperature depends on mass. Ans.__B__




(2) Interstellar clouds may collapse to form stars if they
A.
have very high temperatures.


B.
encounter a shock wave.


C.
rotate rapidly.


D.
are located near main sequence spectral type K and M stars.


E.
all of the above. Ans.__B__




(3) A star will experience a helium flash if
A.
it has an excessive temperature gradient between its core and envelope.


B.
it ceases to be degenerate when helium ignites.


C.
its core is above the helium ignition temperature.


D.
its degenerate core enables the operation of the pressure-temperature thermostat.


E.
its co