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Transcript
The Interior of The Sun Jonathan Correa Magdalena Contents: General Features of the Sun Layers: o o o o o o The The The The The The Core Radiative Zone Convection Zone Photosphere Chromosphere Corona Spacecrafts References General Features of the Sun A star is a huge mass of hot, glowing gas. The strong gravitational pull of the Sun holds Earth and the others planets in the solar system in orbit. The Sun’s light and heat influence all of the objects in the solar system and allow life to exist on Earth. The Sun’s radius is 695,508 km, 109 times as large as Earth’s radius. The Sun has a lower density (about 1,409 g/cm3). Which is a quarter of the average density of Earth. It has about 333,000 times the Earth’s mass. It generates 3,83x1026 joules of energy every second. All of this energy is generated in the Core by Nuclear Fusion. The Sun is a second-generation star, meaning that some of its material came from former stars. The first stars were composed only of the hydrogen and helium produced in the early universe. Although hydrogen (94%) and helium (6%) also are the main ingredients of the Sun, it contains heavier elements (0.13%), such as carbon, nitrogen and oxygen as well. Spectroscopy shows us all these values. The Sun is about 4,6 billion years old and we can calculate that it will keep shining for about another 7 billion years. The average distance from Earth is 150 million km. Layers The solar interior is separated into four regions by the different processes that occur there: The Core The core extends from the center to about one quarter of the radius of the Sun. Is in this central region where nuclear reactions consume hydrogen to form helium. These reactions need special conditions of density (about 150 g/cm3, which is more than 13 times the of lead) and temperature (about 15x106 C). The hydrogen nuclei must collide with enough energy to give a reasonable probability of overcoming the repulsive electrical force between thee two positively charged particles. Every second, fusion reactions convert about 700 million tons of hydrogen into helium. Every fusion reaction also produces a neutral particles called “neutrinos”. The Sun creates a huge amount of these particles. In one second 70 billion of neutrinos pass through every square centimeter of Earth that is facing the Sun. The fusion take place through three different steps. Firstly two protons collide to produce deuterium, a positron and a neutrino. In the second step one proton collides with the deuterium to produce a helium-3 nucleus and a gamma ray. And finally, in the third step, two helium-3 collide to produce helium-4 nucleus and two protons. The Radiative Zone This layer extends outward from the edge of the Core to the Convection Zone (between 25-70 percent of the Solar radius). Particles in the radiative zone repeatedly absorb, radiate, and deflect photons. The matter in this zone stays in the same place while the energy moves through it. Because of this, a photon requires about 170,000 years to go from the Sun’s Core to the bottom of the Convective Zone. The density drops from 20 g/cm3 down to 0,2 g/cm3 from the bottom to the top fo the radiative zone. The temperature falls from 7 to about 2 million C over the same distance. The Convection Zone This zone extends from of about 200,000 km to the visible surface. At the base, the temperature is 2 million C which is enough for the heavier ions (such as carbon, nitrogen, etc) to hold onto some of their electrons. These materials at the bottom heat up with blocked radiation and become less dense than surrounding material. Then moves up through the convection zone carrying energy toward the atmosphere of the Sun. When reaches this point (where the density is much less than at the bottom) the energy can radiate into space. The material at the top of the convection zone becomes then colder and denser so falls down to the bottom to pick up more energy. This process takes about ten days. At the surface of the Sun the temperature is about 5,700 K and the density is only 2x10-7 g/cm3. The Photosphere This layer is the visible surface of the Sun. This is not a solid layer but has about 100 km thick (about 0,05% of the radius). The Photosphere is opaque because it contains hydrogen ions which block, absorb, and emit light. The photosphere has different features as well: o Granulation and Supergranulation Under close inspection with a telescope, the photosphere breaks into a million tiny bright points, called granules, with a strongly textured and varying pattern. The granules are small (about 1,000 km across). They are the top of the convection cells where hot fluids rise up. They last only for 20 minutes. The flow within the granules can reach 7 km/s Supergranules are large versions of granules (about 35,000 km across). These features also covert the entire Sun. An individual supergranule last for a day or two. Here, the flow speed is about 0,5 km/s. o Sunspots Large dark spots are often visible in the photosphere. The biggest sunspots exceed Earth in size. The position and the number of this spots changes in a 11-year cycle. This spots are places in the Sun’s surface where the magnetic field is so strong. Appear dark because they are much cooler than their surroundings. The concentrated magnetism in sunspots keeps them cold. They are ten times brighter than the full Moon for example. This spots act as a valve choking off the heat, light and energy flowing outward from the interior. The temperature of is about 3230 C (approximate the half of the surroundings). The Chromosphere The Chromosphere is a thin layer (about 2,0003,000 km) thick, just above the Photosphere. Here the temperature rises from 5510 C to 9700 C near the Corona. The hydrogen emits a deep red color. Calcium ions also produce distinctive radiation here (UV). It is an irregular layer. Here we can find other interesting feature: o Prominences Prominences are dense clouds of material suspended above the surface of the Sun by loops of magnetic field. They can remain in a quiet or quiescent state for days or a week, depending of how quick changes the magnetic field. The Corona The Corona is a very hot layer of the solar atmosphere. At 3,000 km from the solar surface the temperature rises to about 106 K. This was a problem for astrophysics in the past. The Chromosphere and Photosphere are closer to the Sun Core but the Corona is several hundred times hotter than these layers. Now we know that the heat at the Corona is an effect of the solar magnetic field and not of the radiation from the Core. In fact, low frequency magnetic waves dissipate their energy in this layer when they are damped. At these high temperatures both hydrogen and helium are completely stripped of their electrons. Carbon, nitrogen and oxygen are stripped as well. This produced a really strange emission spectrum. This layer emits most of its radiation at very short UV an X-ray wavelengths. Much of this radiation hits Earth’s atmosphere and is absorbed by atoms and molecules. So, scientists use instruments in space to study the Corona. The Corona is visible during total eclipses of the Sun as a pearly white crown surrounding our star. The most important features of the Corona are: o Coronal Loops These features are found around sunspots and in active regions. Usually they are associated with the closed magnetic field lines. Some of them last days or weeks but most change quickly. o Coronal Holes Coronal holes are regions where the corona is dark. Usually associated with "open" magnetic field lines poles. o and are often found at the Sun's Polar Plumes Polar plumes are long thin streamers that project outward from the poles. Like the feature before, are associated with the "open" magnetic field lines at the Sun's poles. Spacecrafts Right now there are a couple of missions studying the Sun. Here we can find some of them: o SOHO SOHO was designed to study the internal structure of the Sun, its extensive outer atmosphere and the origin of the solar wind, the stream of highly ionized gas that blows continuously outward through the Solar System. o Ulysses This mission was designed to sample the solar wind and the heliosphere at latitudes unexplored by any other spacecraft. The Ulysses spacecraft got the Sun's south pole in the fall of 1994 and its north pole in the summer of 1995. Instruments onboard Ulysses measured particles, magnetic fields, and electro-magnetic radiation from radio wavelengths to gamma-rays. o Trace TRACE explores the magnetic field in the solar atmosphere by studying: the 3-dimensional field structure, its temporal evolution in response to photospheric flows, the time-dependent coronal fine structure, the coronal and transition region thermal topology, etc. References “Lecture Notes on Space Physics” Kjell Rönnmark “Solar Physics – NASA” http://science.msfc.nasa.gov/ssl/pad/solar/interior.htm “Instituto de Astrofísica de Canarias” http://www.iac.es/ “Living Reviews” http://solarphysics.livingreviews.org/ “Institute for Solar Physics” http://www.solarphysics.kva.se/ “Sun Science” http://cse.ssl.berkeley.edu/hessi_epo/html/sunmain.htm