ConceptCheck 9-1: The Sun emits most of its energy in the form of visible light.

ConceptCheck 9-2: At the extremely high temperatures and pressures existing in the Sun’s core, hydrogen nuclei can move fast enough to overcome the electrical charge repulsion and fuse together into helium nuclei.

ConceptCheck 9-3: When 1 kg of hydrogen combines to form helium, the vast majority of the mass is used as the substance of helium atoms, with only 0.7% of the original mass left over to be converted into energy.

ConceptCheck 9-4: Astronomers use the current energy output of the Sun to estimate how fast the Sun is consuming its usable fuel and how much fuel it has available to continue at its present consumption rate.

ConceptCheck 9-5: Because pressure in the Sun’s core is due to the downward pushing weight of the overlying mass of material, having less mass pressing down would result in a lower pressure at the core.

ConceptCheck 9-6: When too little energy flows to the surface, the Sun’s core temperature would increase dramatically.

ConceptCheck 9-7: The energy transport process of conduction occurs when energy moves through a relatively dense material by hot material transferring its kinetic energy to nearby cooler material that is in direct contact. The Sun’s density is simply too low for conduction to be an important process by which energy moves from one part of the Sun to another part.

ConceptCheck 9-8: Of these three, only the temperature decreases with increasing distance from the Sun’s central core.

ConceptCheck 9-9: By carefully monitoring how vibrations within the Sun move through the Sun, astronomers are able to deduce the thickness of various zones and the Sun’s internal density at various depths.

ConceptCheck 9-10: The energy transport process of convection causes warmer material to rise to the surface and cooler material to sink back into the photosphere, giving the photosphere a granulelike appearance.

ConceptCheck 9-11: Spicules can have lengths of several thousand kilometers, nearly the distance across the entire United States.

ConceptCheck 9-12: The corona has an extremely low density and can only be observed when the more dominant photosphere is blocked, such as during a solar eclipse.

ConceptCheck 9-13: The solar wind seems to come from places where the corona is most thin, locations called coronal holes.

ConceptCheck 9-14: The photosphere surrounding a sunspot has a temperature of about 5800 K, which far outshines the relatively cooler sunspot region, resulting in the sunspots appearing to be quite dark in comparison.

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ConceptCheck 9-15: The length of time between large numbers of sunspots to few numbers of sunspots and back to large numbers of sunspots averages about 11 years. However, the magnetic character of sunspots flips every 11-year cycle, suggesting an overarching 22-year sunspot cycle.

ConceptCheck 9-16: According to the magnetic-dynamo model, the sunspot cycle is a result of twisting magnetic fields. In the event the Sun was turning faster, the twisting would occur more quickly and the length of the sunspot cycle would decrease.

ConceptCheck 9-17: The glowing plasma tends to follow the pathways created by the Sun’s invisible magnetic field lines, which form curved arches above the Sun’s surface.

ConceptCheck 9-18: Coronal mass ejections are many times more energetic than any other event on the Sun and, when directed at Earth, can cause severe problems with telecommunications, electrical power distribution, and radiation health hazards for astronauts working in space.

CalculationCheck 9-1: According to Einstein’s equation that E = mc², a mass of 5 kg is equivalent to 5 kg × (3 × 10⁸ m/s)² = 45 × 10¹⁶ J, which is equivalent to burning about 100,000 metric tons of coal!

CalculationCheck 9-2: According to the graphs, the Sun’s core temperature is about 30 million °F or 16 million K. At a distance of 50% of the Sun’s radius, the Sun’s temperature has dropped to 8 million °F or 4 million K, which is a drop of about 75%.