The Sun – Review

1. Name the eight parts of the Sun in order – what unique aspects define each region?

2. The core of the Earth is metallic made of iron and nickel, whereas the mantle contains rocky basalt with elements like silicon and oxygen.  Are the layers of the Sun made of different materials like the layers of the Earth?  If so, what are the different materials – if not, how is it possible to tell the difference? 

3. The density and pressure inside the Sun both increase to great amounts going closer and closer to the core.  In fact the density of the core is about 13 times the density of lead.  How is it possible that this very dense material under great pressure is not a solid or a liquid?

4. What are the most prevalent elements found in the Sun and what is the evidence for the presence of these elements?

5. According to the laws of spectroscopy a sufficiently dense material will emit a continuous spectrum (blackbody radiation).  What part(s) of the Sun are in fact dense enough to do this?

6. According to the laws of spectroscopy a relatively cool and thin gas will absorb particular frequencies of light producing a dark line spectrum.  What part(s) of the Sun are cool enough for this to happen and what are these lines called when referring to the Sun’s spectrum?

7. Convection is the heat transfer process occurring in the convection zone of the Sun.  During this process there is a circulation of material rising and falling that causes heat energy to be transferred from lower layers to higher layers of the Sun.  (a) What causes material to move upward in this process?  (b) What causes material to move downward in this process?  (c) And how is it that energy is transported upward?  (d) In the Sun this process of convection moves energy between what two layers?

8. If you get a Christmas present and don’t know what is inside, you might shake it and listen to what it sounds like to help you determine what it is.  In some ways scientists use a similar idea to determine the properties of the Sun’s interior.  This is called helioseismology.  Explain how it is similar to shaking and listening to a wrapped present.

9. The transition zone marks a rapid increase in temperature going from the chromosphere to the corona.  (a) How was the extremely high temperature of the corona determined?  (b) Even though the temperature is increasing going through the transition zone that is not necessarily equivalent to saying that there is more energy in the corona than the chromosphere – why not?

10. A typical person in the United States might use a total amount of energy – gas, electricity, etc – equal to 70 billion joules per year.  (a) Use the equation E = mc² to determine what amount of mass converted to energy would be enough to power a person for a year.  (b) Given that a typical mosquito has a mass of 5 milligrams – how long could you supply your energy needs if you could convert a mosquito into pure energy?  (c) How does a calculation like this help us understand the energy of the Sun?

11. The solar constant 1380 W/m² means that 1380 watts of light hits a square meter of material held out in sunlight while on Earth.  In science fiction it is thought that in the distant future a society might build a giant “Dyson sphere” to surround a star and capture all of the energy in its light.  Suppose a Dyson sphere is built with radius equal to 1 A.U. (150 million km).  (a) Calculate the area of the inside of the sphere using A = 4πr².  (b) Find the total amount of solar power that would be hitting the inside surface of such a sphere using the solar constant measured on Earth at a distance of 1 A.U. from the Sun.  (c) This power is the total power of the Sun – what is another name for this that astronomers use when describing the Sun and other stars?

12. The Parker Solar Probe, launched in 2018, will explore the Sun by traveling closer to it than any other spacecraft.  Its nearest approach will bring it within 7.0 million km of the Sun.  The space craft will be protected by a heat shield that is a disk of radius 1.2 m made of carbon composite and foam and painted with a pure white coating.  (a) Determine the area of a Dyson sphere with radius 7.0 million km.  (b) Given the power of the Sun 390 yottawatts, determine the number of watts per square meter hitting such a sphere.  This is equivalent to the irradiance of sunlight at that distance.  (c) Calculate the area of the probe’s heat shield using g A = πr² and determine the amount of sunlight power hitting it when the spacecraft reaches that distance from the Sun.  (d) Conceptual question: why is the heat shield painted white?

13. The amount of hydrogen in the Sun is decreasing and at the same time the amount of helium is increasing.  (a) Explain why this is happening.  (b) For every one kilogram of hydrogen that “disappears” from the Sun is there exactly one kilogram of helium that takes its place?  Explain.

14. Part of the proton-proton chain is a reaction where a proton fuses with another proton and produces a deuteron, which consists of a proton bound together with a neutron.  In this process it is as if one of the protons gets “converted” into a neutron.  What happens to the positive charge that was on the proton that “gets neutralized” – does it just disappear, or does it go to some other particle?  Explain.

15. Fusion of protons occurs in the Sun much more often than fusion of helium nuclei.  Why?  Hint: it has to do with charge!

16. Which part of the proton-proton chain actually produces neutrinos?

17. The Sun gives off neutrinos as well as light.  Suppose that you could see neutrinos with your eyes.  It would never get dark – there would be no night and day.  Why would you still see neutrinos after the Sun set?  And what direction would they come from?

18. Given that the Sun produces 1.8 × 10³⁸ neutrinos per second that fly off into space in all directions determine the number of neutrinos that hit one square meter of the Dyson sphere described in question 11. (a).  Given that the human body has an area of about 0.50 m², how many neutrinos hit you when you lie in the Sun at the beach?

19. Is the Sun losing more mass because of nuclear fusion or because of the solar wind?

20. Is the sunspot number that is tracked by scientists equal to the number of sunspots on the Sun?  Why do astronomers keep track of this number and plot its variation with respect to time.

21. As the Sun is nearing a minimum at the end of a sunspot cycle, how can scientists determine that the next cycle has begun?  When the Sun is nearing a maximum during the middle of a sunspot cycle, how can scientists determine that the maximum has been reached?

22. How is the sunspot number for a particular year determined?

23. How is the Sun different from one maximum to the next?  How is it similar at one maximum compared to the next?

24. During what part of the sunspot cycle would sunspots be most likely to appear near the equator of the Sun?

25. Sunspots usually occur in pairs.  Explain why this is so based on the theory that an arc of the Sun’s magnetic field breaks through the surface when sunspots occur.

26. If you could stand on the photosphere of the Sun in between two sunspots holding a compass, which spot would it point to – a “North” spot or a “South” spot.

27. Based on historical records of sunspots it appears that the Sun stayed at a minimum for about 50 years straight during the Maunder minimum.  What does that imply about the Sun’s magnetic field during that time period, given out understanding of the connection between sunspots and magnetism?

28. Why does differential rotation and convection help explain the production of the Sun’s magnetic field?  What is this theory called by scientists?

Answers:

1. Core (fusion occurs only here)
Radiation Zone (energy in form of EMR)
Convection Zone (convection occurs)
Photosphere (visible light emitted into space)
Chromosphere (coolest part, reddish color)
Transition zone (transition of temperature upward going out)
Corona (pale high temperature “crown” of Sun’s atmosphere)
Solar Wind (escaping charged particles “blowing out into space”)

2.  Layers are not different materials – only difference is type of activity and phenomena.

3.  Extreme high temperature prevents materials from being solid or gas.

4.  Hydrogen and helium are by far the most common elements, evidenced by spectral lines.

5.  The body of the Sun (core through photosphere) is dense enough to emit a continuous spectrum.

6. The chromosphere is the coolest part and is a site of photon absorption causing dark “Fraunhofer” lines.

7. a. Hot material has less density and “floats upward”.
b. Cool material has greater density and “sinks”.
c. Cool material gets heated by lower layers, rises, and then cools as energy is given to higher layers.
d. Convection moves energy from the Radiation Zone to the Photosphere.

8. Helioseismology is study of sound-like waves traveling back and forth through the Sun.  The material of the Sun is the medium of these waves and affects the qualities of the sound.

9. a. The corona temperature is determined by spectroscopy analysis of lines and type of radiation.
b. Even though the corona is extremely high temperature it has a very low density, which means there is a relatively small amount of material at that high temperature and accordingly not necessarily a gigantic amount of energy that the high temperature might suggest.

10. a. 7.8 × 10^–7 kg
b. 6.4 years!

11. a. 2.83 × 10²³ m²
b. 3.90 × 10²⁶ W
c. This wattage or power is called luminosity.

12. a. 6.16 × 10²⁰ m²
b. 633000 W/m²
c. 2.86 MW (2860 kW) (like 2800 hair dryers!)
    you could get a sunburn in a matter of seconds
d.  The heat shield is white so that it reflects as much light as possible.  If it were dark it would get hot by absorbing sunlight energy.

13. a. The amount of hydrogen in the Sun is decreasing because it is essentially converted into helium by means of fusion reactions.  This also explains why the amount of helium is increasing.
b. One kilogram of hydrogen converts into about 0.993 kg of helium.  The “missing 0.007 kg, or 7 grams is transformed into energy of amount given by E = mc².

14.  The reaction ¹H + ¹H → ²H + e⁺ + ν produces a positron that has the same charge as the “missing” proton.

15.  Two helium nuclei repel one another 4 times as much as two hydrogen nuclei because the charge is twice as great and each repels the other (2 x 2 = 4!).  A higher temperature would make this more likely because the particles would be moving faster and more likely to fuse.

16.  The reaction ¹H + ¹H → ²H + e⁺ + ν produces one neutrino.

17. After the Sun sets, the neutrinos it emits travel straight through the Earth and would be rising upward out of the ground at midnight!

18. 6.4 × 10¹⁴, about 318 trillion – ouch!

19.  Fusion reactions cause a greater reduction in mass than the loss associated with the solar wind.

20.  The sunspot number is directly related to the number of sunspots but not equal to the number of sunspots.  It is a standardized measure of the prevalence of sunspots and tracking and plotting this value is a convenient way to monitor the solar cycle and the amount of solar activity.

21.  The beginning of a new cycle can be determined by a reversal in the magnetic polarity of sunspots.  The maximum of a cycle cannot be determined until a sufficient amount of time has passed and the plotting of the sunspot numbers reveals a clear downward trend.

22.  Sunspot number for a particular year is the average of all the daily recorded sunspot numbers.

23.  From one maximum to the other the entire magnetic field of the Sun is reversed and opposite in polarity (magnetic North and South are swapped for the Sun’s geographic poles and sunspot pairs).  Each maximum is similar in that there are many sunspots occurring and much activity such as Coronal Mass Ejections, solar flares, prominences, increased irradiance, etc.

24.  Sunspots at the beginning of a cycle are mainly found north and south of the equator and “migrate” toward the equator near the end of a cycle approaching minimum.  The most likely time for sunspots near the equator is just before a solar minimum.

25.  A magnetic field always point from a magnetic north pole to a magnetic south pole.  So, when a loop of the Sun’s magnetic field breaks out and arcs over the surface a “north” sunspot will form at one end and a “south” sunspot will form at the other end.

26.  A compass always points in the direction of the field and so it would point away from a north sunspot and toward a south sunspot.

27.  If there was an extended minimum it implies that the Sun’s magnetic field stopped changing for some reason or that it did not get contorted as it does currently during solar maxima.

28.  The Dynamo Theory posits that charged particles moving in circles or loops inside the Sun are responsible for the production of its magnetic field.  The differential rotation of the various parts – core vs. poles vs. equator – and the convection cells can all transport charged particles that are found in the plasma of the Sun and cause a loop of current analogous to that found in an electromagnet.