Download HW7-3

HW7-3: (260) RQ 2, 3, 6; (261) Prob 1;
(261) Learning to Look 1-2; Supp. Q. 4
(260)RQ 2: (a) Why is there a lower end to the main
sequence? (b) Why is there an upper end?
(a) Stars less than 0.08M☉ (8% of M☉) are not hot
enough in their cores to run fusion. These are brown
dwarfs.
(b) Very massive stars have so much fusion that the
outward forces overwhelm gravity and they are
unstable. Stars greater than 100M☉ cannot even form.
Stars more than 25 or 30M☉ engage in mass loss and
lose weight down to 25 or 30M☉.
Sunday, November 25, 12
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HW7-3: (260) RQ 2, 3, 6; (261) Prob 1;
(261) Learning to Look 1-2; Supp. Q. 4
(260) RQ 3: What is a brown dwarf?
A brown dwarf is a “failed star.” They are balls of gas
without fusion. The upper end of brown dwarfs is well
defined: 8% M☉ = 80 Jupiters. There is a not-so-welldefined line between small brown dwarfs and large
planets.
(260) RQ 6: Why do expanding stars become cooler and
more luminous?
As they expand they have more surface area to shine
from, so they are more luminous. The expansion and
extra light output sap energy and lower the surface
temperature.
Sunday, November 25, 12
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HW7-3: (260) RQ 2, 3, 6; (261) Prob 1;
(261) Learning to Look 1-2; Supp. Q. 4
(261) P 1: In the Figure 12-2, how much of the sun’s mass is
hotter than 13,000,000 K?
About 7.3%. Probably a bit more.
Sunday, November 25, 12
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HW7-3: (260) RQ 2, 3, 6; (261) Prob 1;
(261) Learning to Look 1-2; Supp. Q. 4
(261) P 1: In the Figure 12-2, how much of the sun’s mass is
hotter than 13,000,000 K?
About 7.3%. Probably a bit more.
L2L 1: In the photograph of the Pleiades on page 255, there
are no bright red stars. Use the H-R diagram to explain
why the brightest stars are blue. Have there ever been
bright red stars in this cluster?
This is a young cluster. The brightest stars are blue giants:
still on the main sequence. They haven’t expanded yet, so
they’re surfaces haven’t cooled off yet. There have not yet
been any red giants or red supergiants.
Sunday, November 25, 12
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HW7-3: (260) RQ 2, 3, 6; (261) Prob 1;
(261) Learning to Look 1-2; Supp. Q. 4
(261) P 1: In the Figure 12-2, how much of the sun’s mass is
hotter than 13,000,000 K?
About 7.3%. Probably a bit more.
L2L 1: In the photograph of the Pleiades on page 255, there
are no bright red stars. Use the H-R diagram to explain
why the brightest stars are blue. Have there ever been
bright red stars in this cluster?
This is a young cluster. The brightest stars are blue giants:
still on the main sequence. They haven’t expanded yet, so
they’re surfaces haven’t cooled off yet. There have not yet
been any red giants or red supergiants.
Sunday, November 25, 12
5
HW7-3: (260) RQ 2, 3, 6; (261) Prob 1;
(261) Learning to Look 1-2; Supp. Q. 4
(261) L2L 2: Look at the photograph of the star cluster M67
on page 255. Why are there no bright blue stars in this
cluster?
This is a old cluster. The bright
blue giants have already evolved
off the main sequence and have
probably died in a supernova.
Sunday, November 25, 12
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HW7-3: (260) RQ 2, 3, 6; (261) Prob 1;
(261) Learning to Look 1-2; Supp. Q. 4
SQ 4: Look at Table 12-2 on page 245. Larger mass stars
have more fuel to burn. Why do they run out of fuel so
quickly in comparison to low mass stars?
The high mass
stars have
extremely high
luminosities,
so they burn
through their
fuel very
quickly.
Sunday, November 25, 12
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