Astronomy - Chapter 16 Review Questions

How do we know the age of the Sun? - ANSWERS- Radioactive dating of rocks determines the age of Earth, the Moon, and meteorites to be about 4.5 billion years - Models of the formation of the solar system and observations of the formation of other stars with planets show that the Sun formed at the same time as the other members of our solar system. How we know that the Sun's energy is not supplied either by chemical burning, as in fires here on Earth, or by gravitational contraction (shrinking)? - ANSWERS- The vast amount of energy produced by the Sun over the past 4.5 billion years exceeds the amount that could be supplied by burning or shrinking of the Sun by a significant factor. - Chemical burning would only last a few thousand years, whereas gravitational contraction would provide energy for only about a hundred million years. What is the ultimate source of energy that makes the Sun shine? - ANSWERSThe matter is converted into energy through the fusion of hydrogen into helium. The formulas for the three steps in the proton-proton chain: - ANSWERS Differences between neutrino and neutron - ANSWERS- neutrino's mass is much smaller - neutrino hardly interacts with matter at all, where as a neutron interacts with other particles - neutrino isn't one of the particles to make up an atom - neutrino can "oscillate" (change from one type of neutrino to another between the Sun's core and Earth) What is meant by the statement that the Sun is in hydrostatic equilibrium? - ANSWERSThe pressure and gravity are in balance throughout the Sun, from the very center to the surface. This means the gas pressure at any depth within the Sun can support the weight of all of the gas pressing down upon it, due to gravity. So the Sun neither expands nor contracts but remains as it is; this is true at every point within the Sun as well as for the Sun overall. What do measurements of the number of neutrinos emitted by the Sun tell us about conditions deep in the solar interior? - ANSWERSThe neutrinos are being produced in the solar core by fusion reactions, and measuring their number gives us a sensitive probe into what is happening in the Sun's core. It helps confirm that there are enough proton-proton chain reactions (each of which produces a neutrino) going on in the Sun's core to explain the energy output of the Sun. Do neutrinos have mass? Describe how the answer to this question has changed over time and why. - ANSWERSYes, they do have mass, and they have always had mass. Human just didn't know that at first. When neutrinos were first proposed by Pauli, physicists thought they were massless particles (all energy). Neutrinos produced in the core of the Sun carry energy to its exterior. What is the mechanism for this energy transport called? - ANSWERSSince the majority of neutrinos produced inside the Sun do not interact with other particles as they leave the Sun, the energy they carry is transported as radiation. What conditions are required before proton-proton chain fusion can start in the Sun? - ANSWERSThe Sun must be dense and hot enough in the center for the motion of the protons to overcome their mutual repulsion, with a temperature of at least 12 million K. 2 (main) ways that energy travels through the sun - ANSWERS- Energy is created through fusion of hydrogen into helium at the center of the Sun. This energy first enters the radiative region of the Sun, where photons are absorbed and reemitted in random directions until they make it about 2/3 of the way to the surface. - Once the energy has traveled 2/3 of the way to the surface, it reaches the convection zone where the hot gas flows up and down (like water boiling in a pot) carrying the energy to the solar surface. Why would it be a waste of time to build a special gamma-ray detector to detect gamma rays produced during the proton-proton chain in the core of the Sun? - ANSWERSThe gamma rays produced in the proton-proton chain are quickly absorbed by surrounding atoms in the Sun and reemitted with slightly less energy. This process occurs so many times that by the time the photons are released from the Sun, their energy has dropped from gamma-ray level to visible-light or UV-photon level. Thus, it is extremely unlikely that any gamma rays from the core of the Sun would be detected. Earth contains radioactive elements whose decay produces neutrinos. How might we use neutrinos to determine how these elements are distributed in Earth's interior? - ANSWERSBy detecting the direction from which the neutrinos arrive at detectors in different locations, we can triangulate the location of potential sources within Earth. Note to instructors: Such detection of anti-neutrinos from decay processes within Earth is actually being done by the Kamioka Liquid Scintillator Antineutrino Detector. What are density and mass? - ANSWERSMass is the total amount of material in a body, whereas density is the mass per unit volume. How do we know the Sun is not a burning piece of coal? - ANSWERS- The Sun is too hot to contain solid coal. - Its density is lower than the density of coal. - Even if the Sun were made of coal, burning coal produces energy so inefficiently that the Sun could keep shining at its present rate for only a few thousand years, yet we have geological and fossil evidence that Earth has been warm (and the Sun must therefore have been producing energy) for billions of years. Which of the following transformations is (are) fusion and which is (are) fission: helium to carbon, carbon to iron, uranium to lead, boron to carbon, oxygen to neon? - ANSWERSFusion: helium to carbon; carbon to iron is fusion; boron to carbon; oxygen to neon Fission: uranium to lead How do mathematical computer models allow us to understand what is going on inside of the Sun? - ANSWERSAstronomers know how gravity and pressure work; they understand how pressure, temperature, and density are related for gases; they understand the processing of nuclei in nuclear fusion; and they also know the various mechanisms of heat transfer (convection, conduction, and radiation). This knowledge comes from Earth-bound experiments. Astronomers also observe the Sun to measure how much energy it produces, whether or not it is growing/shrinking, and if it exhibits any other significant large-scale behavior. They then put all of their equations and measurements into a computer model and have it calculate the conditions at each point inside a star like the Sun. If the model star that the computer produces matches all of the observations of our own Sun, then the astronomers are likely describing the physics inside of the Sun properly. If the computer model produces a star that doesn't look like our Sun, then they must check their equations and their initial conditions, and then try again. Why is fission not an important energy source to the sun? - ANSWERSWe know from our study of the Sun's spectrum that there is so little uranium and plutonium (or other heavy fissionable materials) in the Sun that it would not be possible to generate a significant amount of energy by fission of these rare elements. Suppose the proton-proton cycle in the Sun were to slow down suddenly and generate energy at only 95% of its current rate. Would an observer on Earth see an immediate decrease in the Sun's brightness? Would she immediately see a decrease in the number of neutrinos emitted by the Sun? - ANSWERSThe observer would see a decrease in the number of neutrinos almost immediately since neutrinos take only about 2 seconds to travel from the center of the Sun to its surface and another 8 minutes to reach Earth. Light, on the other hand, takes about 105 to 106 years to traverse the distance from the center of the Sun to its surface, so 105 to 106 years would elapse before an observer on Earth saw a decrease in the brightness of the Sun. Everyday examples of radiation and convection - ANSWERS- A floor heater spreads heat throughout a room by convection. Ironically, the style of heater called a radiator doesn't really radiate much energy. - Most of its heat is transported by convection of the air which it heats (you can test this easily by comparing how well it warms your hand if you place it 12 inches off to the side to how well it warms your hand if it is 12 inches above it). - Absorption of sunlight and subsequent radiation of infrared energy by Earth in its daily cycle is an example of radiation. (Heat from a light bulb or from a fire also are examples of radiation.) What mechanism transfers heat away from the surface of the Moon? If the Moon is losing energy in this way, why does it not simply become colder and colder? - ANSWERSRadiation is the mechanism that transports heat away from the surface of the Moon. Since space is nearly a vacuum, the alternative mechanisms of conduction and convection cannot work. The Moon does become colder at night. It also becomes hotter and hotter during the day. Any location on the surface of the Moon is heated by the radiation from the Sun, which raises its temperature. When that location faces away from the Sun, it radiates heat away from the Moon's surface. Overall, the total amount of heat received from the Sun during the day equals the total amount of heat radiated away into space. Earth's atmosphere is in hydrostatic equilibrium. What this means is that the pressure at any point in the atmosphere must be high enough to support the weight of air above it. How would you expect the pressure on Mt. Everest to differ from the pressure in your classroom? Explain why. - ANSWERSThere is less air between the top of Mt. Everest and the outer edge of Earth's atmosphere than there is, say, between a location at sea level and the outer edge of the atmosphere. It takes less pressure to hold up the smaller mass above Mt. Everest. Since the altitude of your classroom is lower than that of Mt. Everest, the pressure in your classroom is higher. The summit of Mt. Everest is above about 70% of the molecules in Earth's atmosphere, and the pressure at the summit is only about 30% the pressure at sea level.