Download Physics 127 Descriptive Astronomy Homework #15

Physics 127 Descriptive Astronomy
Homework #15 Key (Chapter 9)
Winter 2015
9-3. Why do thermonuclear reactions occur only in the Sun’s core, not in its outer regions?
Thermonuclear reactions occur only when atomic nuclei are very close to one another, in virtual contact. Because
atomic nuclei are all positively charged they naturally repel one another with an electrical force that grows as the
inverse square of the distance between them if two nuclei approach each other. Two nuclei can get close enough to
each other to undergo a reaction only if they are moving towards each other at a high enough speed (a collision). Such
collisions can occur in the core of a main sequence star since (1) the pressure is high, which crowds the nuclei together,
and (2) the temperature is high which makes the speeds with which nuclei move high thereby making it possible for
two nuclei which, as a matter of chance, are moving directly towards each other, to get close enough to react with each
other before their electrical repulsion for each other brings their mutual motion of approach to a halt. In the outer
regions of the Sun the pressure is not high enough, nor the speeds with which nuclei move, fast enough for such
collisions to ever occur.
9-6. What is a neutrino? Why is it useful to study neutrinos coming from the Sun? What do they tell
us that cannot be learned from other avenues of research?
A neutrino is a tiny, neutral subatomic particle. Their masses are either zero or so close to zero that they are able to
move at speed very close to the speed of light. The interactions of neutrinos with other particles are so weak or nonexistent that neutrinos created in nuclear reactions deep within the core of the sun exit the surface of the sun almost
instantly. Neutrinos are therefore the one probe we have for observing current conditions in the core of the sun.
Neutrinos are created in the thermonuclear fusion reactions which occur in the sun’s core. Our detection of them attests
to the occurrence of these reactions which are otherwise invisible to us.
9-9. Why do astronomers say that the solar cycle is 22 years long, even though the number of
sunspots varies over an 11-year period?
At the end of each 11-year period of variation in the number of sunspots on the sun, the magnetic polarity of the entire
sun, including the sunspots reverses. During one 11-year sub-cycle all of the leading sunspots in the sun’s northern
hemisphere are of the same magnetic polarity with the trailing spots being of the opposite polarity. In the southern
hemisphere the same is true except the polarities of the leading and trailing spots are reversed. Then for the next 11
years all of the polarity conventions of the first sub-cycle are reversed. Hence it takes a full 22 years for both the
relative numbers of sunspots and their magnetic polarity conventions to return to their previous states.
9-11. Why should solar flares and coronal mass ejections be a concern for businesses that use
telecommunication satellites?
Within a few days of their occurrences, such flares and coronal mass ejections sometimes cause the Earth to be
bombarded with solar wind gusts (streams of electrons and positively ionized nuclei) which disrupt the Earth’s
magnetic field and also electrical and telecommunication equipment on the earth’s surface, causing unpredictable and
sometimes damaging electrical current flows in such equipment.