Giant and supergiant stars are rare (compared to main-sequence stars) because na
ID: 284606 • Letter: G
Question
Giant and supergiant stars are rare (compared to main-sequence stars) because
nature does not make them nearly as often as she makes the main sequence stars
giant and supergiant stars are difficult to see
the giant and supergiant stages are short compared to that of the main sequence stage
helium is very rare
A.nature does not make them nearly as often as she makes the main sequence stars
B.giant and supergiant stars are difficult to see
C.the giant and supergiant stages are short compared to that of the main sequence stage
D.helium is very rare
Explanation / Answer
Answer is option C
When a star exhausts the hydrogen in its core, it becomes a giant or supergiant.
Once a star has used up all the hydrogen in its core, fusion of hydrogen into helium stops. The core starts to contract again (just as it contracted as a protostar, before hydrogen fusion began). As the core contracts, it releases energy. This energy heats up the layer immediately above the contracting helium core.
The layer immediately above the core becomes hot enough to initiate the fusion of hydrogen into helium.
The star now has THREE main layers:
(1) Helium core (inner layer): Releases energy as it shrinks in radius.
(2) Fusion shell: Releases energy as it fuses hydrogen into helium.
(3) Hydrogen envelope (outer layer): Absorbs energy, and swells greatly in size.
These swollen stars, no longer on the main sequence, are now
giants (if M < 8 Msun) or
supergiants (if M > 8 Msun).
A giant's outer hydrogen envelope cools as it expands. A giant becomes very large in radius and very cool -- hence the name RED GIANT, commonly applied to giant stars.
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