Download Physics 127 Descriptive Astronomy Homework #16

Physics 127 Descriptive Astronomy
Homework #16 Key (Chapter 10)
Winter 2015
10-1. Explain the difference between a star’s apparent brightness and its luminosity.
Apparent brightness refers to how bright a star appears to its observer while luminosity refers to how much energy the
star radiates into space. A star’s apparent brightness depends on its luminosity, but also on its distance from its
observer and the existence of any obscuring medium between the star and its observer.
10-5. The star Zubenelgenubi (from Arabic for “scorpion’s southern claw”) has apparent magnitude
2.75 while the star Sulafat (Arabic for “tortoise”) has apparent magnitude 3.25. Which star
appears brighter? From this information alone, what can you conclude about the luminosities of
these stars? Explain your answer.
Zubenelgenubi is the brighter of the two stars by 0.50 magnitudes (or a factor 1.585). No conclusion is possible about
the relative luminosities of the two stars, since brightness depends upon luminosity, distance (and sometimes the
amount of obscuring material between a star and its observer).
10-7. If a red star and a blue star both have the same radius and both are the same distance from
Earth, which one looks brighter in the night sky? Explain why.?
The blue star, being hotter than the red star, will appear brighter since the two stars are the same size and same distance
from the earth, but the amount of light emitted per unit area from the stars’ surfaces will be greater for the hotter star in
all colors of the spectrum.
10-9. Sketch a Hertzsprung-Russell diagram. Indicate the regions on your diagram occupied by (a)
main-sequence stars, (b) red giants, (c) supergiants, (d) white dwarfs and (e) the Sun.
M
Supergiants
-5
a
g
Red giants
0
n
i
Main sequence
 Sun
5
t
u
10
White dwarfs
d
e
15
O
B
A
F
Spectral Type
G
K
M