Astronomy 201 Review 2 Answers What is hydrostatic equilibrium? How does hydrostatic equilibrium maintain the Sun©s stable size? Hydrostatic equilibrium, also known as gravitational equilibrium, describes a balance between gravity and pressure. Gravity works to contract while pressure works to expand. Hydrostatic equilibrium is the state where the force of gravity pulling inward is balanced by pressure pushing outward. In the core of the Sun, hydrogen is being fused into helium via nuclear fusion. This creates a large amount of energy flowing from the core which effectively creates an outward-pushing pressure. Gravity, on the other hand, is working to contract the Sun towards its center. The outward pressure of hot gas is balanced by the inward force of gravity, and not just in the core, but at every point within the Sun. What is the Sun composed of? Explain how the Sun formed from a cloud of gas. Why wasn©t the contracting cloud of gas in hydrostatic equilibrium until fusion began? The Sun is primarily composed of hydrogen (70%) and helium (28%) with the remaining mass in the form of heavier elements (2%). The Sun was formed from a collapsing cloud of interstellar gas. Gravity contracted the cloud of gas and in doing so the interior temperature of the cloud increased because the contraction converted gravitational potential energy into thermal energy (contraction leads to heating). The cloud of gas was not in hydrostatic equilibrium because although the contraction produced heat, it did not produce enough heat (pressure) to counter the gravitational collapse and the cloud continued to collapse. Much of the thermal energy generated from contraction was radiated away at the cloud©s surface despite the increase in interior temperature. As the cloud continued to collapse, the temperature and density at the center finally increased to the point where nuclear fusion was sparked. At this point, the energy being radiated away at the surface matched the energy generated at the core. Once fusion began, the Sun entered a state of hydrostatic equilibrium. Describe the different layers of the Sun. What is interesting about the temperatures of the chromosphere and corona relative to the other layers of the Sun? If we cannot directly see the Sun©s interior, how do we know about the Sun©s internal structure? From the center and proceeding outward, the layers of the Sun are the core, radiation zone, convection zone, photosphere, chromosphere, and the corona. The core is the hottest (about 15 million K) and most dense region of the Sun where nuclear fusion occurs. Energy created in the core will take on the order of a million years to finally reach the surface. This slow migration of photons from the core to the surface is known as radiative diffusion. The radiation zone (about 10 million K) is the region of the Sun where the primary means of energy transport is radiative diffusion. The convection zone describes the region where energy is transported by the rising of hot gas and the falling of cool gas (convection). The photosphere is above the convection zone. The photosphere is the visible portion of the Sun. The average temperature of the photosphere is about 5800 K which corresponds to a peak photon color of about green/yellow. One of the characteristic features of the photosphere is the granulation pattern due to the convection occurring below. The chromosphere basically corresponds to the lowest region of the Sun©s atmosphere. The temperature of the chromosphere is about 10,000 K, which corresponds to ultraviolet radiation. Above the chromosphere is the corona, the uppermost region of the Sun©s atmosphere. The temperature of the corona is about 1 million K and most of the X-rays emitted by the Sun are emitted from the corona. Beginning with the core and extending out to the photosphere, the temperature of each successive region decreases. Interestingly, the temperature then rises in the chromosphere and rises again by a factor of ten in the corona. It is thought that waves carry energy along magnetic field lines to heat the chromosphere and the corona and these waves are generated by turbulence in the convection zone. Models, Sun quakes, and solar neutrino observations allow us to study the Sun©s interior. We can create models according to the laws of physics and check them against observations. Sun quakes describe pressure waves generated in the Sun that create vibrations of the solar surface which can be detected by Doppler shifts. Neutrinos are a byproduct of the fusion reactions occurring within the Sun. Models suggest we should detect a certain rate of neutrinos, which if detected would support the models and increase our confidence in our understanding of the nuclear physics going on in the interior of the Sun. While neutrinos can provide a probe of the interior, there is still much mystery surrounding them and they are very difficult to detect. How does nuclear fusion work? More specifically, what is basically included in hydrogen burning? Nuclear fusion describes the process where nuclei combine to form a nucleus with a greater number of protons and neutrons. Very high temperatures and densities are required for nuclear fusion. Nuclear fusion in the core of the Sun is fusing hydrogen into helium by way of the proton-proton chain. Basically, two hydrogen nuclei fuse into deuterium; deuterium fuses with a neutron to produce helium- 3; two helium-3 nuclei fuse to form helium-4. A lot of energy is produced along this chain of reactions. How is energy transported in the radiation and convection zones? The primary method of energy transport in the radiation zone is radiative diffusion: the slow migration of photons from the core to the surface. Energy is transported in the convection zone by convection: the rising of hot gas and the falling of cool gas. What are Sunspots and what causes them? Describe the Sunspot cycle. Sunspots are dark spots on the surface of the Sun. The plasma within Sunspots is about 4000 K, and therefore they appear dark in contrast to the even brighter surrounding material (5800 K). Sunspots occur in regions with strong magnetic fields and tend to occur in pairs connected by looped magnetic field lines. Since the Sun rotates differentially (rotates more quickly at the equator), the magnetic field lines are dragged into twisted contortions. Trapped gas within a strong magnetic field contortion cools because it is isolated from surrounding material via the magnetic field lines. The Sunspot cycle is the approximately eleven year period over which the variation in the number of Sunspots occurs. Sunspots first appear at mid-latitudes at solar minimum and then become more common near the equator at the next solar minimum. What is solar wind? What is it made of? How does it effect us? Charged particles streaming out of the corona constitute the solar wind. The solar wind consists of roughly equal numbers of protons and electrons, in addition to a small fraction of heavier nuclei. The solar wind escapes primarily through coronal holes, which are regions of the corona where the magnetic field is ªbroken,º unlike the regions around Sunspots. The solar wind has a significant effect on the Earth©s ionosphere and magnetic field, as well as the auroras and telecommunications systems. What causes the granulation features we observe on the Sun? What is limb darkening? What are solar prominences? What are coronal mass ejections? What are spicules? Up close, the photosphere resembles boiling water with a bubbly, almost cellular appearance. This granulation feature is due to the convection occurring below the photosphere in the convection zone. Limb darkening is the darker appearance near the edge of the Sun. It occurs because the observer©s line of sight passes through shallower, cooler, and more absorbing layers of photosphere as it moves away from the center of the Sun©s disk. Solar prominences are loops of hot gas that extend very high above the photosphere. The hot gas of solar prominences follows magnetic field lines. Coronal mass ejections are huge bubbles of gas threaded with magnetic field lines that are ejected from the Sun over the course of several hours. Coronal mass ejections are often associated with eruptive solar prominences in the chromosphere and occur most frequently at solar maximum. Spicules are narrow, usually vertical, short-lived jets of solar material that extend from the chromosphere into the corona. They appear spiky and have relatively short lifetimes of about five to fifteen minutes. Describe the formation theories of our Moon. How is it thought that the moons around Mars came about? Prior to the Apollo missions, the three original theories governing the Moon©s origin were the capture hypothesis, the fission hypothesis, and the binary accretion hypothesis. The capture hypothesis states that the moon was formed elsewhere in the solar system and then captured by the Earth. It is very improbable because conditions would have to be extremely precise and just right requiring that a very unlikely set of orbital coincidences be satisfied. The fission hypothesis states that the moon was ªspun offº of the Earth at a time when it was rapidly rotating and when it was still molten. The binary accretion hypothesis states that the Earth and Moon formed as a ªdouble planet,º and remained rotating about one another. The binary accretion model suggests that the Earth and Moon should have the same composition, which is not the case. After the Apollo missions returned samples of the lunar surface, the above theories were discounted and in the 1970©s the giant impactor theory was proposed. In this scenario, a Mars-sized planetesimal struck the side of the Earth and this glancing blow shot large amounts of Earth©s mantle and large amounts of the impactor into orbit.