ASTR 1205 Unit 7: Our Sun PDF

ASTR 1205 Unit 7 Dr. Bryan Rowsell Unit 7: Our Sun [NASA/Joel Kowsky] 11.1 A Closer Look at the Sun Why does the sun shine? amount of fuel = Sun′ s Lifetime rate of fuel consumption If the Sun...

ASTR 1205 Unit 7 Dr. Bryan Rowsell Unit 7: Our Sun [NASA/Joel Kowsky] 11.1 A Closer Look at the Sun Why does the sun shine? amount of fuel = Sun′ s Lifetime rate of fuel consumption If the Sun was simply on “fire”, the fuel would last about 104 years. This isn’t possible…why not? − No oxygen in space, no fire − the solar system is way older than 10,000 years What if the sun was heated due to gravitational contraction? Fuel would last about 2.5×107 years Again, not likely….why not? − again, the Earth is older than 25 million years Our Star ·7−1· 11.1 A Closer Look at the Sun ASTR 1205 Unit 7 Dr. Bryan Rowsell What about nuclear fusion? The fuel would last over 10 billion years. [NASA/ESA] The Sun in a gravitational equilibrium (or hydrostatic equilibrium) where the outward pressure of the gas is precisely balanced by the inward pull of the Sun’s massive gravity. The Sun still releases a massive amount of energy. energy measured in power is measured in A star’s total power output is called luminosity. This is different than brightness, which is a function of luminosity and distance to the observer, i.e. brightness changes, but luminosity does not. The structure of the Sun can be broken down into the atmosphere and the interior, much like the Earth. Our Star ·7−2· 11.1 A Closer Look at the Sun ASTR 1205 Unit 7 Dr. Bryan Rowsell Atmosphere of the Sun: Coronosphere Outermost layer of very very hot ionized gas (called plasma) but it is not very dense thus not as much thermal energy as one would expect. This is where most of the Sun’s x−rays come from (recall Wein’s Law…λmax for 2×106 K is about 1.5 nm) Chromosphere Middle layer of the atmosphere, about 10,000 K (λmax for 1×105 K is about 290 nm). Photosphere Visible “edge” of the Sun, about 6000 K. Sunspots are found here, large fluxes in magnetic fields. Interior of the Sun: Convection Zone energy generated by the solar core radiates outward, transported by the expanding hot gas and contracting cooled gas, i.e. convection. Radiation Zone energy moves outward via high−energy photons generated in the… Solar Core this where the “magic” happens, nuclear fusion at the highly−compressed, very hot core with compressed plasma. We’ll be looking at some of these aspects of the Sun’s anatomy in the future, starting with… Our Star ·7−3· 11.1 A Closer Look at the Sun ASTR 1205 Unit 7 Dr. Bryan Rowsell 11.2 Nuclear Fusion in the Sun There are two types of nuclear reactions: Large nucleus, in this case 235U, Smaller nuclei come together to spontaneously splits into form larger nuclei, again often daughter particles and other other particles result (to particles such as neutrinos, conserve energy and electrons, positrons, etc. momentum) A hydrogen atom is one proton and one electron. An ion is an atom that has lost or gained electrons. A hydrogen ion is the same thing as a/an: (a) electron (b) neutron (c) proton Our Star ·7−4· 11.2 Nuclear Fusion in the Sun ASTR 1205 Unit 7 Dr. Bryan Rowsell Four Fundamental Forces in Nature 1. Gravity Force: attraction between two objects with mass 2. Electromagnetic Force: opposite charges attract, like charges repel 3. Strong Nuclear Force: binds protons and neutrons together in nuclei but only operates when the particles are very close together 4. Weak Nuclear Force: responsible for subatomic particles changing into other subatomic particles. [XKCD] The core of the Sun is a stew of nuclei and free electrons (i.e. plasma) moving at very high speeds. When two hydrogen nuclei, aka protons, get close enough for the strong force to overcome their electromagnetic repulsion they can fuse together. [SMBC Webcomic] Our Star ·7−5· 11.2 Nuclear Fusion in the Sun ASTR 1205 Unit 7 Dr. Bryan Rowsell These collisions happen again and again, and the daughter nuclei can collide with other protons or other nuclei forming a complex mixture of products and, important to us, energy, i.e. a chain reaction. a near massless, neutral subatomic particle neutrino (ν) moving at near light−speed, barely interacts with matter positron same properties as an electron, but a positive (e+ or β+) electrostatic charge What is the result of this chain, i.e. what goes in what goes out? Thus, the net reaction is: 4 H + → He2+ + 2 𝑒 + + 2 𝜈 + 𝑒𝑛𝑒𝑟𝑔𝑦 Our Star ·7−6· 11.2 Nuclear Fusion in the Sun ASTR 1205 Unit 7 Dr. Bryan Rowsell Let’s do a quick, back of the envelope calculation of the masses here: 𝑚𝑎𝑠𝑠 4 p = 4 × (1.67 × 10−27 𝑘𝑔) = 6.68 × 10−27 𝑘𝑔 𝑚𝑎𝑠𝑠 He2+ = 6.64 × 10−27 𝑘𝑔 𝑚𝑎𝑠𝑠 2 𝑒 + = 2 × (9.11 × 10−31 𝑘𝑔) = 1.82 × 10−30 𝑘𝑔 𝑚𝑎𝑠𝑠 2 ν = 2 × (2.11 × 10−37 𝑘𝑔) = 4.22 × 10−37 𝑘𝑔 You can see the mass of the positron and neutrino are thousands of times less than the helium nucleus, so we can ignore them for now. 𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑚𝑎𝑠𝑠 = 6.68 × 10−27 𝑘𝑔 − 6.64 × 10−27 𝑘𝑔 = 0.04 × 10−27 𝑘𝑔 = 4.0 × 10−29 𝑘𝑔 Law of conservation of mass says we have to start with the same mass we finish with…so what happened to the “missing” mass? energy! 𝐸 = 𝑚𝑐 2 𝐸 = (4 × 10−29 𝑘𝑔)(3.0 × 108 𝑚 ⋅ 𝑠 −2 ) = 4.2 × 10−12 𝐽 This seems small, 0.0000000000042 J, but this is for only a single proton fusion reaction. The Sun fuses about 5.4×1011 kg of hydrogen every second. The hydrogen fusion reaction is sometimes depicted in the overall reaction as shown on 7−6 (upper right). What is misleading about this representation? (a) It makes it seem like 4 hydrogen nuclei fuse together in a single 4-way collision to make 1 helium nucleus. (b) It makes it seem like electrons don’t play an important role (c) It makes it seem like the only new element produced is helium, but other elements are also produced. (d) It makes it seem like only four hydrogen nuclei are needed, but other subatomic particles are also needed. Our Star ·7−7· 11.2 Nuclear Fusion in the Sun ASTR 1205 Unit 7 Dr. Bryan Rowsell Gravitational Equilibrium The Sun shines over its 10 billion year lifetime thanks to the internal balance (i.e. equilibrium) between the outward pressure generated by the hot core (from fusion) and the inward pressure generated by the mass (i.e gravity). The Sun is self−regulating, i.e. it has a built−in thermostat! What would happen inside the Sun if a slight rise in core temperature led to a rapid rise in fusion energy? (a) The core would expand and heat up further. (b) The core would expand and cool. (c) The core would contract as it heats up. (d) The Sun would blow up like a hydrogen bomb. (e) nothing would happen. This is called a negative feedback loop, as the result of the applied stress returns the situation back to values before the stress. Our Star ·7−8· 11.2 Nuclear Fusion in the Sun ASTR 1205 Unit 7 Dr. Bryan Rowsell Radiative Zone (or How does the Energy from Fusion get out of the Sun?) Most of the energy released by fusion in the core, starts its journey out of the core in the form of photons. These photons travel through the next layer of the Sun by randomly “bouncing” around. This layer is called the radiative zone. Photons travel at the speed of light, but the journey through the radiative zone takes well over 100,000 years because the solar plasma is so dense that the photons travel less than a mm between “bounces.” The radiative zone ends when the temperature of the solar plasma drops to about 2,000,000 K, and becomes more likely to absorb photons rather than “bounce” them. This convective zone (see 7−2, 3) are where the plasma at the bottom of the zone absorbs the photons that eventually leak out from the radiative zone resulting in a layer of convection. Our Star ·7−9· 11.2 Nuclear Fusion in the Sun ASTR 1205 Unit 7 Dr. Bryan Rowsell How Do We Know What is Going On in the Sun? 1. Helioseismology Pictured on the right is a Dopplergram of the Sun. Dark = solar plasma moving towards us Light = solar plasma moving away from us Large-scale change in shading from left to right is due to the Sun’s rotation. [NASA/SDO/GSFC] Small-scale changes (mottled appearance) are the Sun’s surface vibrations. Analyzing the vibrations on the Sun’s surface tells us about the interior of the Sun, just like how earthquakes tell us about the interior of the Earth. 2. Making Mathematical Models Data from solar vibrations observations agree with mathematical models of the solar interior. We model the Sun’s composition, temperature, etc. and see how well that model matches experimental observations. 3. Observing Solar Neutrinos Recall the Hydrogen Fusion chain−reaction on 7−6. You’ll see the net reaction released two neutrinos per cycle. Based on this, we can, in principle, detect the number of neutrinos coming from the Sun as evidence the Hydrogen Fusion chain. Neutrinos come in three types: “electron”, “muon”, and “tau”. The Sun produces only “electron” neutrinos. Our Star · 7 − 10 · 11.2 Nuclear Fusion in the Sun ASTR 1205 Unit 7 Dr. Bryan Rowsell Unfortunately, detecting neutrinos is VERY difficult: A neutrino passing through a slab of lead one light−year thick would have a 50% chance of passing through the other side! in other words, neutrinos barely interact with matter! In the 1960s, a detector was built that can detect electron neutrinos. Only 1/3 of the expected neutrinos were observed. Interesting… − is something wrong with fusion model? − is something wrong with the model of neutrino production? In the 1990s, a detector was built that can observe all types of neutrinos, and the correct number was observed. Implications? − model of the Sun’s fusion is good (or at least isn’t far wrong) − model of neutrinos updated…we now know that neutrinos can change types on their way to Earth How long does it take the neutrinos generated by hydrogen fusion in the Sun’s core to escape the Sun? (a) nearly instantaneous (b) a week (c) 100,000 years (same as photons) 11.3 The Earth−Sun Connection The Sun is not a solid sphere, not only is there temperature gradients between different layers of the Sun, but different points on the Sun rotate at different speeds: Material near the poles moves slower while material near the equator moves faster. Our Star · 7 − 11 · 11.3 The Earth−Sun Connection ASTR 1205 Unit 7 Dr. Bryan Rowsell Nearly all the visible light we receive from the sun is emitted by the photosphere. Another way to look at this is the photosphere is where the Sun starts to appear opaque. This results in the production of surface features such as granules and sunspots. We’ve seen granules before, 7−10, the result of convection in the convection zone make the surface of the Sun appear granular. Sunspots appear as temporary dark spots on the photosphere due to their lower temperatures (around 4000 K rather than 6000 K) compared to the surrounding photosphere. [New Solar Telescope] If the temperature of a sunspot is 4000 K, what colour is it? (a) black (b) purple (c) green (d) yellow−orange (e) red Why are sunpots “cold” relative to the rest of the photosphere? Our Star · 7 − 12 · 11.3 The Earth−Sun Connection ASTR 1205 Unit 7 Dr. Bryan Rowsell Sunspot Polarity Image of photosphere (left) and image from magnetometer (right), taken at the same time. [NASA/SDO/GSFC] In the magnetometer image, white = magnetic field lines coming towards Earth, black = magnetic field lines pointing away from Earth. Sunspots appear in pairs of opposing polarity (black and white). The Sun has a magnetic field, though it’s very different from Earth’s magnetic field. Our Star · 7 − 13 · 11.3 The Earth−Sun Connection ASTR 1205 Unit 7 Dr. Bryan Rowsell [Peter Reid/ University of Edinburgh] This image is a depiction of the Sun’s magnetic field lines over a UV image of the Sun. Magnetically active regions: [NASA/SDO/AIA/LMSAL] − appear bright in this UV image − often correspond to sunspot activity − densest magnetic field lines − linked by magnetic field lines Our Star · 7 − 14 · 11.3 The Earth−Sun Connection ASTR 1205 Unit 7 Dr. Bryan Rowsell Magnetic field lines get pushed around by the plasma of the photosphere resulting in many twists and loops in the field lines due to the differential rotation. Why is the Sun’s magnetic field so convoluted/complex? (a) it’s related to the age of the Sun (b) it’s related to the Sun’s rate of nuclear fusion in the core (c) it’s related to the Sun’s differential rotation (d) it’s related to the zodiac sign the Sun is in right now The twisting of the magnetic field lines changes with time and since the magnetic field lines are related to sunspots, this leads to a sunspot cycle. The number of sunspots rises and falls in (roughly) 11-year cycles. Our Star · 7 − 15 · 11.3 The Earth−Sun Connection ASTR 1205 Unit 7 Dr. Bryan Rowsell The latitude of sunspots also changes over the course of a cycle. Often called a butterfly diagram. The start of the cycle has sunspots appearing at ≈ 30° latitudes followed by new sunspots appearing lower (closer to the equator). The end of the cycle has the last few sunspots appear very near the equator, while spots associated with the start of the next cycle again appear at near 30° latitude. The Sun right now: https://sdo.gsfc.nasa.gov/data/ The chromosphere (transition between the photosphere and corona) can contain large features called prominences (when viewed side−on) or filaments (viewed head−on). Does this shape remind you of anything? [NASA/ESA/SOHO] Solar plasma material flows along looping magnetic field lines, sometimes extending into the corona. The corona (or “crown”) is often visible during solar eclipses (see Unit 2) and can extend thousands of km into space. [NASA/Aubrey Gemignani] Our Star · 7 − 16 · 11.3 The Earth−Sun Connection ASTR 1205 Unit 7 Dr. Bryan Rowsell The temperature of the corona is extremely hot compared to the photosphere or chromosphere (not sure why!). This hot temperature means the gas can escape the Sun’s gravity and cause…. Solar Winds are charged particles (mostly protons and electrons) with extremely high wind speeds escaping the Sun’s gravity outward into all directions. When a lot of particles are ejected all at once, this is called a coronal mass ejection (or CME). [NASA/Goddard/Lisa Poje] All forms of light (UV, visible, radio, etc.) travel at the speed of light toward Earth. The particles of the solar wind travel at speeds around 800 km·s−1 taking several days to reach Earth. When they arrive, they interact with the magnetic field of the Earth, causing… https://www.youtube.com/watch?v=6G1MtO1XLPE Our Star · 7 − 17 · 11.3 The Earth−Sun Connection ASTR 1205 Unit 7 Dr. Bryan Rowsell Chapter 11: The Essential Cosmic Perspective End−of−Chapter Questions: 1−35. Solutions found in Bb. Extra Resources: Watch the Crash Course Astronomy video about The Sun [12 min]: https://www.youtube.com/watch?v=b22HKFMIfWo Read and interact with this website for more info about the layers of the Sun, and try out the "Quiz me about this topic" link at the bottom of the page: https://imagine.gsfc.nasa.gov/science/objects/sun1.html Read this website for a quick summary of the layers of the Sun: https://www.nasa.gov/mission_pages/sunearth/science/solar-anatomy.html Read this NASA website about how and why we study our Sun: https://www.nasa.gov/solarscience Watch this NASA video that nicely combines what we learned in Unit 5 about light with Unit 7 about the Sun [2 min]: https://www.youtube.com/watch?v=Wp-dNoHwFSw Watch this NASA video for some mind-boggling footage of solar activity [4 min]: https://www.youtube.com/watch?v=HFT7ATLQQx8 Watch this NASA video for more info on the sunspot cycle [3 min]: https://www.youtube.com/watch?v=sASbVkK-p0w Watch this kinda funny video about helioseismology and the sunspot cycle [5 min]: https://www.youtube.com/watch?v=WWpFH-N2HYE Watch this NASA sea shanty video about the solar wind [2 min]: https://www.youtube.com/watch?v=LP3qzKGh1AM More about auroras: Watch this really well done video about what causes auroras [5 min]: https://www.youtube.com/watch?v=8S_LPFOa-zs Surf the aurora like Chris Hadfield with this video from the International Space Station [5 min]: https://www.youtube.com/watch?v=fVMgnmi2D1w Participate in citizen science, by reporting whether or not you saw an aurora and take advantage of the aurora probability prediction on this website. Click the “Learn” tab for lots more interesting info on what causes auroras, why auroras are important, and how to photograph them: https://www.aurorasaurus.org/ Our Star · 7 − 18 · Problems and Resources