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P446 1-20
1. Name and briefly describe the main regions of the Sun.
The main regions of the Sun are illustrated in Figure 16.2 and summarized in Table
- The “surface”—the part of the Sun that emits the radiation we see—is called
the photosphere. Its radius is about 700,000 km. However, the thickness of
the photosphere is probably no more than 500 km, which is why we perceive
the Sun as having a well-defined, sharp edge (Figure 16.1).
- Just above the photosphere is the Sun’s lower atmosphere, called the
chromosphere, about 1500 km thick.
- From 1500 km to 10,000 km above the top of the photosphere lies a region called
the transition zone, where the temperature rises dramatically.
- Above 10,000 km, and stretching far beyond, is a thin, hot upper atmosphere, the
solar corona.
- At still greater distances, the corona becomes the solar wind, which flows away
from the Sun and moves throughout the entire solar system. (Sec. 6.5)
-Extending down some 200,000 km below the photosphere is the convection zone,
a region where the material of the Sun is in constant convective motion.
- Below the convection zone lies the radiation zone, where solar energy is
transported toward the surface by radiation rather than by convection.
- The central core, roughly 200,000 km in radius, is the site of powerful nuclear
reactions that generate the Sun’s enormous energy output.
2. How massive is the Sun, compared with Earth?
Over 300,000x more massive. (Careful about ever using the word “massive”
when you mean “big”. A “more massive” object technically means “has more
mass”(think of it as weight) than another. A hot air balloon is big, but hardly
3. How hot is the solar surface? The solar core?
5800K; 15 million kelvins
4. What is luminosity, and how is it measured in the case of the Sun?
luminosity = The total rate at which energy leaves the Sun’s surface. It is
measured by multiplying the rate at which solar energy falls on each square
meter of the imaginary sphere that shares earth’s orbital radius
5. What fuels the Sun’s enormous energy output?
Nuclear fusion in the sun’s core.
6. What are the ingredients and the end result of the proton–proton chain in
the Sun?
Four hydrogen nuclei combine to create one helium-4 nucleus (i.e. two
protons, two neutrons), plus some energy (in the form of gamma-ray radiation
and two neutrinos.)
7. Why is energy released in the proton–proton chain?
Because some of the mass in the initial four hydrogen nuclei is converted to
energy in accordance with Einstein’s equation of mass/energy equivalence:
E = mc2
8. How do scientists construct models of the Sun?
By combining every piece of data we do know with our best inferences about
what we don’t. This data includes temperatures, other radiation and how it
changes over time, sunspots and their effects, solar oscillations (waves in the
surface of the sun,) etc.
9. What is helioseismology, and what does it tell us about the Sun?
The study of solar surface waves. These waves can tell us a lot about the
different density layers in the sun…
10. How do observations of the Sun’s surface tell us about conditions in
the solar interior?
…since interior density differences affect how waves move through the sun.
(This is the VERY SAME WAY we can be so confident about the inner
structure of the Earth…for earthquake waves to be generated, then emerge,
from different parts of the earth surface in the unusual patterns that we
observe, the density boundaries must lie at VERY specific depths inside the
11. Describe how energy generated in the solar core eventually reaches
Nuclear fusion generates energy in the sun’s core
- This energy simply radiates (shines) away from the core until it reaches an
opaque layer and heats it.
- This opaque later convects (churns) and moves energy in this way out toward
the photosphere…
- …which then radiates energy out into space.
12. Why does the Sun appear to have a sharp edge?
Because the outer layer of the sun – the photosphere -- goes from opaque to
transparent in a short distance relative to our distance from it. If you were
actually at the “surface” of the Sun, you wouldn’t be able to tell if you were
inside it or not.
13. Give the history of “coronium,” and tell how it increased our
understanding of the Sun.
Scientists used to think the sun’s corona was made of a different element
called coronium because the corona showed completely different spectral
lines than the photosphere. Scientists later learned that the different emission
lines were generated because the gas in the corona is highly ionized (it has lost
many electrons)
14. What is the solar wind?
Electromagnetic radiation and fast-moving particles—mostly protons and
electrons—that constantly escape from the Sun.
15. Why do we say that the solar cycle is 22 years long?
Because the sun’s magnetic field reverses polarity about every 11 years.
Alternating sunspot cycles (that last ≈ 11 years each) have reversed
polarity, though this is not reflected it sunspot shape or location.
-=-=-=-16. What is the cause of sunspots, flares, and prominences?
Magnetic instabilities in the strong fields found in and near sunspot groups
may cause the prominences, although the details are not fully understood,
but appears to be connected with the effect of the sun’s differential rotation
on it’s magnetic field. (As the sun is “wrung out” by its differential
rotation—the rotation varies with depth too!—it’s magnetic field is
twisted and looped. When solar material follows these loops, sunspots and
other solar surface phenomena seem to result.)
17. Describe how coronal mass ejections may influence life on Earth.
The can potentially cause communications and power disruptions on earth.
18. Why are scientists trying so hard to detect solar neutrinos?
Because the gamma-ray energy created in the proton–proton chain is
transformed into visible and infrared radiation by the time it emerges from
the Sun, astronomers have no direct electromagnetic evidence of the
nuclear reactions in the solar core
; Neutrinos created in the proton–proton chain are our best bet for learning
about conditions in the heart of the Sun
19. What is the most likely solution to the solar neutrino problem?
This problem - the predicted number of neutrinos coming from the Sun as a
result of fusion does not match the numbers we actually detect – will
probably only be resolved with a discovery about the properties of the
neutrinos themselves…possibly they oscillate, change into different kinds
of neutrinos on their journey from the sun to the earth.
20. What would we observe on Earth if the Sun’s internal energy source
suddenly shut off? How long do you think it might take—minutes,
days, years, millions of years—for the Sun’s light to begin to fade?
Repeat the question for solar neutrinos.
Depends whom you ask! I’ve heard MANY reputable astronomers say that
a photon generated in the sun’s core during fusion takes millions of years
to reach the surface of the sun, but I could find this tidbit nowhere in the
- NUTRINOS, on the other hand, have 1/100000th the mass of an electron
and are basically too small to interact with any part of any atom’s nucleus,
so it takes ENORMOUS efforts simply to detect one or two. IF we could
reliable detect neutrinos, AND we noticed one day the stream of them
coming from the sun just stopped, that would probably signal the end of
fusion at the solar core, and we’d have maybe a million or so years until
the sun began cooling.