Astronomy (short answer questions)
Describe two general ways we learn about the Sun's interior
Astronomers create mathematical models that use the laws of physics, the Sun's observed composition and mass, and computers to predict internal conditions. Therefore, we believe that our models are accurate if they can reproduce the characteristics of the Sun that we can observe. By measuring Doppler shifts of material on the Sun's surface, we observe vibrations of the surface that are created deep within the Sun. We can learn about the densities and other characteristics of the various layers within the Sun by studying how the waves propagate throughout the Sun. Another way that we can learn about the Sun is by capturing the particles in the solar wind that come from the Sun. For example, by detecting solar neutrinos we can learn more about the fusion that is going on within the Sun's core
Might changes in the Sun affect weather or climate on Earth?
During the last Maunder minimum period, the Earth experienced the Little Ice Age. However, it is unknown whether the low solar activity caused this or it was just a coincidence. Also it may be possible that drought cycles or storms are correlated with solar activity. Current climate models however indicate that the magnitude of observed warming can only be explained by including anthropogenic factors.
Imagine you are plunging into the Sun, starting from Earth. Briefly describe what you will see as you descend.
First you will feel the light pressure of particles from the solar wind. As you approach the Sun, you will enter the corona, an extremely hot layer of gas, but so low in density that you won't really feel how hot it is. The next layer you encounter will be the chromosphere, a very hot layer of gas just above the visible surface of the Sun. As you plunge through the "surface" of the Sun, the photosphere, the temperature will be a slightly cooler 5,800 K, compared to the outer layers, and you will see the slightly cooler regions of sunspots and the granulation on the surface caused by the convection underneath. You will then enter this convective layer, feeling regions of hot plasma rising upward to meet you and seeing cooler gas descending from the surface. After passing through this layer, you will reach the radiation zone, where photons are engaged in a random dance as they are continuously absorbed and re-emitted by the hot gas there. You will then reach the source of these photons, the core of the Sun, which is actively involved in nuclear fusion, converting hydrogen into helium and releasing multitudes of photons and neutrinos
Briefly describe why the fact that we detect neutrinos coming from the Sun supports the idea that the Sun generates energy by nuclear fusion
Laboratory experiments and theory show that fusion produces neutrinos. Therefore, scientists predict that neutrinos should come from the Sun if fusion is occurring in its core. Theories predict how many and what type of neutrinos should be observed. Thus, the observations that confirm this prediction support the theory
Process of Science: Why is it important to understand the Sun in order to understand the Earth's radiation belts and space weather?
Solar activity causes the responses in the near-Earth space environment that produce changes in the radiation belts, so an understanding of how the Sun changes is directly relevant to space weather
What is the solar neutrino problem?
Solar neutrinos coming from the Sun have been detected, but early detectors found them in fewer numbers than predicted by theoretical models. This means either that our models of the Sun were not completely correct or that we didn't understand neutrinos as well as we thought we did. We can measure the luminosity that the Sun is producing and therefore determine how much fusion must be going on in its core. The rate of fusion then determines how many neutrinos should be produced by the Sun, and theories estimate how many of these should be detected here on Earth. It turns out that neutrinos can change type when they interact with matter. Newer detectors can find all three types of neutrinos, and now the total number of neutrinos detected matches predictions.
Briefly explain how the Sun became hot enough for nuclear fusion
The Sun formed from a cloud of gas. As it contracted, its gravitational potential energy was converted to thermal energy. The Sun continued to contract until the core became hot enough to sustain nuclear fusion
Describe some of the early theories for why the Sun shines and why they are no longer accepted as viable
The Sun was once postulated to be a cooling ember, but that would have meant the Sun would have been much hotter in its immediate past (just a few hundred years ago) and people could not have lived in such a hot environment. Another idea was that the Sun shone through chemical burning (like a conventional fire on Earth), but this was dismissed because it could not generate and sustain sufficient brightness. A more modern hypothesis was that the Sun shone through the emission of thermal energy resulting from gravitational contraction, but this could only last for about 25 million years, far less than the age of Earth, before the Sun would have contracted to a point.
Process of Science: Explain the reasoning that led to our understanding of nuclear energy being the source of the Sun's light.
The first step was measuring the distance to the Sun which then allowed us to calculate how luminous it is and therefore how much energy is needed to power it. The energy requirements are much larger than chemical reactions (i.e., fire) so this was then ruled out. A longer lived source that could match the energy requirements is gravitational contraction. However, as geologists and paleontologists found evidence for an ancient Earth, astronomers realized that gravitational collapse could not be the dominant energy source of our Sun today. All known energy sources were eliminated and only after the recognition that mass can be converted directly into energy, was the solution of the Sun's light as nuclear energy understood
What is the solar thermostat?
The solar thermostat is analogous to the thermostat at home: it works to maintain a constant temperature. If the solar core were to increase in temperature, the nuclear fusion rate would soar, generating excess energy that increases the pressure and pushes the core outwards. This expansion cools the core back to its normal operating temperature. Similarly, if the solar core were to decrease in temperature, the nuclear fusion rate would plummet and gravity would overcome thermal pressure and contract the core. As the core contracts, it heats up and the core returns to its normal operating temperature.
Briefly explain why sunspots are cooler than surrounding regions of the Sun and why they look dark in photos
They are cooler because their strong magnetic fields suppress convection and prevent hotter material from flowing into them. Because they are cooler, they emit less thermal radiation per unit area and therefore look dark in contrast to brighter surrounding regions.
Process of Science: How do we know what is going on in the center of the Sun so well if we cannot see it or send spacecraft to it?
We can apply our knowledge of how gases behave at different temperatures and densities, which is testable in laboratory environments, to make a mathematical model of the Sun. These models make predictions about how bright and how big the Sun is, which we can then compare with observations. We also use observations of vibrations on the Sun to learn about its interior structure in much the same way we use seismic testing on Earth. Finally, we can test our knowledge of nuclear physics and the fusion process in the core using observations of solar neutrinos.
Briefly describe the phenomena of the sunspot cycle.
We observe the Sun to exhibit a sunspot cycle over a period of 22 years, tied directly to its magnetic activity. At the beginning of the period, sunspots form at higher latitudes. As magnetic activity increases, the sunspots form lower down and in greater numbers. At solar maximum, the height of magnetic activity, we observe many sunspots and solar flares. The corona is even shaped differently, streaming more from the sides of the Sun instead of forming a more spherical shape around the Sun, as when the magnetic field is weaker. As the formation of sunspots approaches the Sun's equator, the polarity of the Sun flips. Thus, the north magnetic pole becomes the south magnetic pole and vice versa. The polarity of the sunspots also changes at this time. The cycle then repeats for another 11 years with the magnetic poles of the Sun flipped. After 11 years the Sun's magnetic polarity flips again, completing the 22-year period.
List at least two ways the sunspot cycle affects us on Earth.
When the Sun is near solar maximum, it undergoes a much higher rate of violent activity in the form of solar flares. These flares are outbursts of charged particles that can affect radio communications on Earth. They also can create more auroras and can be a danger to satellites.
