Ch.10 POS

The Sun's interior is extremely dense, so as soon as a photon is produced inside the Sun, it is almost instantly absorbed by matter. It is then emitted in a random direction and possibly at a different wavelength, only to be absorbed again and reemitted. This results in a random walk of the photons as they move toward the surface, where they can escape into space. This situation may sound familiar: it is the effect of a blackbody, which is similar to the simplified view of light bouncing around in a box until it is able to randomly escape from a tiny hole, as shown in the figure. Say you are trying to study the nuclear reactions in the Sun's core by observing photons coming off its surface. How will this random walk affect your results? Choose one or more:

A. Each photon may be a different color than it was when it was created in the nuclear reactions. B. Each photon may be a different energy than it was when it was created in the nuclear reactions. D. Each photon would have been created a long time ago: it would be impossible to use them to study nuclear reactions as they are happening right now in the Sun's interior.

In the Sun, four hydrogen nuclei do not fuse directly into a helium nucleus. The overall reaction involves several steps, and other particles are also produced in the process. Study this animation of the reactions that occur in the Sun, and then select all the particles/elements that are created by the reaction.

A. positron B. neutrino E. gamma ray F. helium

Why don't nuclear fusion reactions, which combine smaller nuclei into a larger nucleus, happen all around us every time atoms come in contact with one another? Gravity attracts particles with mass to one another, but it is extremely weak compared to the electromagnetic force. The strength and direction of the electromagnetic force depends on the charge of the particle. Particles with the same charge repel each other, and particles with opposite charges attract each other. Sort each of the following particles into the appropriate bin according to the electromagnetic force it would feel in the presence of a hydrogen nucleus. The relative charge of each particle is displayed on it. Items (3 images) (Drag and drop into the appropriate area below)

Attracted towards Electron -1 repelled away Proton +1 no force felt Neutron 0

Neutrinos pass straight through solid matter, making them difficult to detect. Luckily, they do react with one type of force—the nuclear weak force. For instance, on extremely rare occasions, a neutrino may interact via the nuclear weak force with a chlorine atom, turning it into a radioactive argon isotope. However, this is rare enough that we would almost never witness this type of reaction in the natural world. Which of these actions would improve our chances of seeing this reaction and thus detecting the presence of a solar neutrino?

Build a detector made of a large amount of chlorine. Go to a place where there are a lot of solar neutrinos.

ExplanationSee Hint The first neutrino detector (Homestake, shown in this image) consisted of a 100,000 gallon tank of a chlorine-containing liquid, built 1,500 meters underground to block out particles other than neutrinos that might affect the results. Calculations from the model of the nuclear reactions expected to occur in the Sun predicted that it would detect about 1 neutrino every day as it turned a chlorine atom into argon. In actuality, only 1 neutrino was detected about every 3 days. This was referred to as the solar neutrino problem. What might this problem imply? Choose one or more:

C. There were fewer neutrinos detected than expected, so something might be wrong with our models. D. The detector may have been missing some neutrinos for some reason.

Energy stored in matter itself—mass energy—can be very powerful. Per Einstein's famous equation E = mc2, energy is equivalent to mass times a constant (the speed of light squared). The speed of light is very large, so just a small amount of mass can result in a very large amount of energy. How can mass turn into energy? Consider the two most common elements in the universe: hydrogen and helium. As shown in this figure, a hydrogen nucleus (where most of its mass is contained) is made of one proton (p). A helium nucleus is made of two protons and two neutrons (n). Nuclear reactions can change what a nucleus is made of. The mass of hydrogen is 1.6726 x 10-27 kg, and the mass of helium is 6.6465 x 10-27 kg. Given this, which of the following nuclear reactions would result in a decrease of total mass, and thus a release of energy, while keeping the same number of particles involved?

Four hydrogen nuclei combine into one helium nucleus.

How do protons ever fuse together in the presence of the electromagnetic force? There is another force involved here called the nuclear strong force. It is the strongest of all the forces, but it only acts over extremely small distances before it becomes too weak to matter. If protons and neutrons are able to get close enough to one another, the strong force provides a powerful attractive force that can bind them together in a nucleus, despite the electromagnetic force. Watch this animation, and then choose the condition that would make it more likely for two nuclei to fuse together.

The nuclei are moving fast with respect to one another.

There is a particle produced in the Sun's nuclear reactions that we can use to directly study what is happening in the interior. Neutrinos are weakly interacting particles, and they have almost no mass and no charge. They are not affected at all by the nuclear strong force. As neutrinos travel through the Sun's dense interior, how will their path change due to the presence of the gravitational, electromagnetic, and nuclear strong forces around them?

They will not change at all.

It was discovered that there are three different types of neutrinos, called flavors, and that neutrinos can spontaneously change from one type to another. Electron neutrinos are produced in the Sun's core, but they can change into a muon or tau neutrino during their trip to Earth. The first detectors were built only to detect electron neutrinos. Can this new information solve the neutrino problem and confirm that our models of nuclear reactions in the Sun are correct? Choose one:

Yes. The existence of three different types of neutrinos would account for the missing neutrinos observed over the number that were predicted.

At the densities of the Sun, hydrogen nuclear fusion can occur at temperatures greater than about 10 million degrees Kelvin (K). Based on these graphs of the Sun's pressure, density, and temperature in its interior, where are the nuclear reactions occurring?

in a central region 20% the size of the entire Sun