Download types of stars, luminosity, and brightness

Online STUDY QUESTIONS #18 — ANSWERS
TYPES OF STARS, LUMINOSITY, AND BRIGHTNESS
1. You do not have enough information to know which star is farther. To know that, you would need to
know the absolute brightness of each star.
1
1
1
the brightness of star A.
= 2=
2
25
D
5
3. Luminosity is an intrinsic property of a star. Luminosity is the energy per second that a star radiates.
2. Star B will be
4. Brightness (energy per sec per square meter) depends on distance whereas luminosity is intrinsic to the
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star.
5. The absolute brightness is the brightness that would be measured at a standard distance of 10 pc.
Apparent brightness is the brightness of a star measured from Earth.
6. Absolute brightness is the luminosity of a star as it would be measured at 10 pc. Luminosity is the
intrinsic energy per sec that a star radiates and does not depend on our distance from the star.
7. Stars are classified by temperature and luminosity.
8. Supergiants are the most luminous and white dwarfs are the least luminous.
9. The main characteristic of main sequence stars is that they have hydrogen fusion in their cores.
10. Stars don't all have the same luminosity for two basic reasons: 1) they do not have the same mass
(the more massive the star, the hotter and the more luminous) and 2) stars go through different stages
of evolution that change the luminosity and size over the time of a star's life.
11. Even if our blue and red stars are radiating the same NUMBER of photons at their peak, the blue star
radiates more blue photons than does the red star, and blue photons have more energy than red photons.
Therefore, the blue star radiates more energy per second, so it is the blue star that is more luminous.
15
T=12,000°
8
Number of photons
Energy per sec per cm 2
10x10
.
6
T=10,000°
4
T=8,000°
12,000 K
.
0
0.0
0.2
0.4
0.6
0.8
10
1.2
2
0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
wavelength in microns
When the blackbody curve is plotted as the number of photons, the curves are all the same in height and
shape (on the right) and differ only in location of the peak in wavelength. When plotted as energy per
second, the 12,000 K star puts out more energy than the 8,000 K star (left). When plotted on the same
diagram, the shapes of the curves are distorted only because of the scale difference. Each blackbody
curve, no matter its temperature, has the same shape.
12. You would expect the same distribution of stars on the HR diagram if Absolute Brightness versus
Temperature were plotted, because absolute brightness is calculated for the same standard distance
from every star, 10 pc.
13. You would NOT expect the same distribution of stars on the HR diagram if Apparent Brightness versus
Temperature were plotted, because apparent brightness depends on the distance to each star, and every
star is located at a different distance from us. So, some stars APPEAR dimmer, even though they are
intrinsically brighter, since they are far away from us, and some stars APPEAR brighter even though
they are intrinsically dimmer because they are close to us. So, you would expect a plot like on the right
below.
Apparent Brightness
bright
dim
O
hot
B
A
F
G
K
M
cool
Temper atur e
14. The groups labeled:
15. An M giant is more luminous than a main sequence A star.
16. The difference is mass. Lower mass stars are cool and not very luminous and spend their main
sequence time on the lower right of the main sequence. High mass stars are very luminous and very
hot and spend their main sequence time on the upper left end of the main sequence distribution.