Scholars Online Astronomy - Chapter 16: The Sun
Reading: Astronomy, Chapter 16: Our Star, The Sun
- 16.1 The sun is our model for all stars. It is a massive ball of hydrogen and helium gas, and at its core, where the temperature reaches 25,000,000K, thermonuclear reactions fuse hydrogen atoms into helium atoms, releasing incredible amounts of energy that reach Earth as both heat and light. Most of this energy comes from a mass loss that occurs when protons and neutrons created new nucleus. The energy released is given by Einstein's mass energy equivalence: E = mc2. note: While astronomers often call this "hydrogen burning", do not confuse it with combustion, which involves combination of oxygen with other elements. Combustion is a chemical reaction In which atoms are recombined, but no atom changes internally. Hydrogen fusion is a thermonuclear reaction that changes the composition of the atomic nucleus, and therefore changes the element of the atom involved.
- 16.2 All stars therefore have two primary sets of forces acting: thermonuclear energy from the interior causes convection and expansion of gases, creating outward pressure. Gravity acts in the opposite direction, causing a massive gas to collapse. If the star becomes hot enough, it can blow up; it becomes cold enough, it will collapse. Since gas as cool as they expand there is a constant oscillation between expansion and contraction. As long as the star stays within certain boundaries, it remains within hydrostatic equilibrium, and stable.
- 16.3 By using hydrotatic equilibrium oscillations, we can determine the sun's interior structure, similar to the way we use earthquake waves to determine the mantle and core structure of the earth. This information lets us determine the size of the sons helium core.
- 16.4 Another source of information about the Suns core are the solar neutrinos produced when protons morph into neutrons during the fusion reaction. Neutrinos are quantum mechanical particles that interact very weakly with matter, as though they had no mass at all. Because they don't interact with matter, are very difficult to detect. Early attempts found only about one third of the neutrino flux predicted, based on the level of fusion reactions that must occur in the core of the sun in order to produce the heat and energy we observe. In the last decade, astronomers have been able to show that neutrinos can change form. New detectors capable of sensing all three types of neutrinos can now detect solar neutrinos at the predicted.
The solar atmosphere is divided into three layers. The photosphere or lowest layer, is heated from below by convection of the layers of gas between the core and surface, and radiation escaping the core. When we look at the sun, we look straight through the thickness of the layer near the center of the sun, but when we look at the limbs of the sun, we look through more of the photosphere layer. Because this layer is cooler than the underlying interior layers, the limbs of the sun appeared a somewhat darker.
- 16.6 Convection occurs through all layers of the solar interior, bringing hotter materials from the center to the outer layers. This creates a kind of granular convection circulation, resulting in spikes where rising gases leap into areas of lower pressure, expand, cool, and tnen contract and fall back down. Because the cooler gases are denser, the falling areas are darker. The gases of the photosphere spike into the Suns chromosphere, a much less dense layer that acts almost as an atmosphere does around the terrestrial planet's surface.
Above the chromosphere, the solar corona is the outermost region of the Sun's atmosphere. It can only be observed in detail during a solar eclipse, when a brighter disc of the Sun is blocked out by the moon. Because the gases escaping from the sun are expanding rapidly, the corona is much less dense than even the chromosphere. These gases become the solar wind, which blow out word through the solar system into space. Most of these gas particles are high temperatures, so much so that the electrons have been completely stripped off of the atoms leaving the positively charged nuclei in the negative plea charged electrons separated. The flow of these ionized particles maps to the solar magnetosphere. The spectrum of these gases tells us what elements are present in the solar photosophere, chromosphere, and corona.
- 16.8 The fluid nature of the sun and its composition including many ionized particles creates currents of the solar interior and on the solar surface, which of course give rise to magnetic fields, which in turn interact with the ionized particles. Over time, the rotation of the sun creates a complex magnetic pattern that response not only to the rotation, but also to the gases rising from below. Magnetic tubes form through which ionized gases can flow, and when these links are pushed upward by the rising gases, an interesting phenomenon occurs: sunspots. As a gas flows upward it expands, cools, becomes dark, and falls back down to the surface through the tube, creating loops of flowing gas above the solar surface.
- 16.9 Because the magnetic "tubes" form over time and then interact with each other, the sun goes through periods of increasing magnetic chaos. When the magnetic fields become too complex, a kind of shakeout occurs, setting everything back to neutral. During periods of high magnetic field complexity, the sun will have many loops and therefore many sunspots — a period of high solar activity which reaches a maximum, and then begins to slow down. At the point where the solar magnetic field becomes neutralized, there's very little solar sunspot activity, and we see the sun is at minimum. By mapping the number of sunspots, we can determine when solar max and solar min occur, and for the last four centuries of our observations, the sun is kept to an 11 year cycle.
Sunspots are mapped according to their altitude or distance from the solar equator and their distance from the Suns axis of rotation.
By plotting the number of sunspots that occur each year, we can determine the periodicity: about 11 years between two solar Max.
When we plot the altitude of a sunspot north or south of the equator as a function of time, we also see a periodic trend: the earliest sunspots in a cycle begin at high latitudes.
Using this information, we produce a model of the solar magnetic field over time. The differential rotation of the solar gases means that the material at the polls rotates more slowly than the material at the equator. The ion carrying — and therefore magnetic field carrying — particles of solar gas wrap the magnetic field around the sun. This increases the complexity of the solar magnetic field and gives rise to "tubes" or magnetic funnels that carry the ionized gases up into the chromosphere, producing sunspots.
- 16.10 In addition to the sunspot loops, localized high activity areas will produce solar flares that extend into the chromosphere and corona. When these loops or flares occur along the limb and are seen in silhouette, they're called prominences. If the burst of gas escapes the sun altogether heading towards Earth, the ionized particles will be funneled by Earth's magnetic field towards the poles, producing the "northern lights", or auroras. Because of their electromagnetic nature, these flares pose a hazard to planetary communications and to humans who are working in space.
Key Formulae to Know
- Einstein's mass-energy equation:
Read the following weblecture before chat: The Sun
Planetarium Program: Use your planetarium program to explore the surface of the sun. In Starry Night, you should be able to use Favorites → Explorations → Sun; you may also be able to change your Viewing Location to the surface of the Sun. Use the LiveSky options to view the sun "now".
If your program allows, explore solar rotations and run time forward so that the Sun makes an entire rotation. How long does this take?
Website of the Week: Check out The Solar and Heliospheric Observatory site. Look at the pictures of the sun "now". Do you observe any activity? Sketch the position of any sunspots you observe, then return to the site every two days for ten days (total five visits). What happens to the sunspots?
Chat Preparation Activities
- Essay question: The Moodle forum for the session will assign a specific study question for you to prepare for chat. You need to read this question and post your answer before chat starts for this session.
- Go over the list of Key Words and Key Ideas at the end of the chapter. If you don't remember the definition of the key word, review its use (the page number on which it is explained is given).
- Read through the Review Questions and be prepared to discuss them in class. If any of them confuses you, ask about it!
- Mastery Exercise: The Moodle Mastery exercise for the chapter will contain sections related to our chat topic. Try to complete these before the chat starts, so that you can ask questions.
- Required: Complete the Mastery exercise with a passing score of 85% or better.
- Go to the Moodle and take the quiz for this chat session to see how much you already know about astronomy!
Read through the lab for this week; bring questions to chat on any aspect of the lab, whether you intend not perform it or not. If you decide to perform the lab, be sure to submit your report by the posted due date.
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