- Slides: 170
The Sun —Our Star Astronomy: The Solar System and Beyond 5 th edition Michael Seeds
The Sun Chapter 11 —Our Star All cannot live on the piazza, but everyone may enjoy the sun. – Italian proverb
The Sun —Our Star • A wit once remarked that astronomers would know a lot more about the sun if it were farther away. – This comment contains a grain of truth. – Astronomers know plenty about stars in general, and there are plenty of stars out there for them to observe, in order to test and refine their theories. – However, there is only one star nearby—close enough for astronomers to see swirling currents of gas and arching bridges of magnetic force that their present theories seem inadequate to completely describe.
The Sun —Our Star • The more closely the sun is observed, the more complex it seems, and the less astronomers seem to know about it. • It pays to look at this paradox from another direction. – The sun is an average star, and there are billions like it in the sky. – The more astronomers can learn from the convenient example of the sun, the more they will know about other stars, whose distances from Earth render detailed study impossible.
The Sun —Our Star • What do astronomers know about the sun? – It is made up of the gases hydrogen and helium, along with a scattering of heavier elements. – It is 109 times Earth’s diameter and 333, 000 times its mass.
The Sun —Our Star – Nuclear reactions produce a tremendous amount of energy in the interior of the sun. – As this energy escapes to space, it stirs the sun’s layers by generating convection currents. – The sun has a powerful and ever-changing magnetic field that causes dark sunspots and eruptions to appear on its surface.
The Sun The Solar Atmosphere —Our Star • Your grand tour of the sun begins with its atmosphere. • Its atmosphere is made up of three layers. – The photosphere is the thinnest, innermost layer. – The chromosphere lies above the photosphere and is a few times thicker.
The Sun The Solar Atmosphere —Our Star – The corona, the highest layer, is huge and extends very far from the sun.
The Sun The Solar Atmosphere —Our Star • As you study the solar atmosphere, you will find that many phenomena are driven by the outward flow of heat energy from the interior of the sun. – Like a pot of boiling soup on a hot stove, the sun’s surface is in constant activity as heat flows up from below.
The Sun The Photosphere —Our Star • The photosphere is the visible surface of the sun. – It is the source of most of the sun’s light. – However, it is very thin—less than 500 km deep.
The Sun The Photosphere —Our Star • To put its thinness in perspective, imagine a model of the sun the size of a bowling ball. – The photosphere would be no thicker than a piece of tissue paper wrapped around the ball. – Due to the relative thinness of the photosphere, astronomers often refer to it as a surface, or ‘the surface of the sun. ’
The Sun The Photosphere —Our Star • Thin as it is, the photosphere emits a great deal of energy. – The temperature at the sun’s surface is about 5, 800 K. – At that temperature, each square centimeter must radiate more energy than a 6, 000 -watt lightbulb. – With all that energy radiating away into space, the sun’s surface would cool rapidly if energy did not flow up from the interior to keep the surface hot.
The Sun The Photosphere —Our Star • The photosphere is dense enough to emit plenty of light but not so dense that the light can’t escape. – Below the photosphere, the gas is denser and hotter and therefore radiates plenty of light. – However, that light cannot escape from the sun because of the outer layers of gas. – So, you cannot detect light from these deeper layers. – Above the photosphere, the gas is less dense and so is unable to radiate or absorb much light.
The Sun The Photosphere —Our Star • Although the photosphere appears to be substantial, it is really a very low-density gas. – Even in the deepest and densest layers visible, the photosphere is 3, 400 times less dense than the air you breathe. – To find gases as dense as the air you breathe, you would have to descend about 7 x 104 km below the photosphere—about 10 percent of the way to the sun’s center. – With a fantastically efficient insulation system, you could fly a spaceship right through the photosphere.
The Sun The Photosphere —Our Star • The spectrum of the sun is an absorption spectrum. – That can tell you a great deal about the composition of the photosphere.
The Sun The Photosphere —Our Star • You know from Kirchhoff’s third law that an absorption spectrum is produced when a source of a continuous spectrum is viewed through a gas.
The Sun The Photosphere —Our Star • The deeper layers of the photosphere are dense enough to produce a continuous spectrum. • However, atoms in the photosphere absorb photons of specific wavelengths, producing absorption lines of hydrogen, helium, and other heavier elements.
The Sun The Photosphere —Our Star • In good photographs, the photosphere has a mottled appearance because it is made up of dark-edged regions. • The regions are called granules. • The visual pattern they produce is called granulation.
The Sun The Photosphere —Our Star • A typical granule is about the size of Texas, and exists for only 10 to 20 minutes before fading away. – Faded granules are continuously replaced by new granules.
The Sun The Photosphere —Our Star • Spectra of these granules show that the centers are a few hundred degrees hotter than the edges. • Doppler shifts reveal that the centers are rising and the edges are sinking at speeds of about 0. 4 km/s. – From this evidence, astronomers recognize granulation as the upper surface of a convecting layer just below the photosphere.
The Sun The Photosphere —Our Star • Most of the convection occurs below the photosphere, but the tops of the cells extend up into its lower reaches. • Convection occurs when hot fluid rises and cool fluid sinks. – For example, a convection current of hot gas rises above a candle flame.
The Sun The Photosphere —Our Star • You can observe convection in a liquid by adding a bit of cool nondairy creamer to an unstirred cup of hot coffee. – As the coffee in the cup convects, the creamer colors some of the currents, so that you are able to see them. – Convection cells form in hot coffee as the surface of the drink cools: the cooler coffee sinks as warmer coffee rises from the bottom to replace it.
The Sun The Photosphere —Our Star – This process is invisible until you pour in the creamer. – When you do, you may see small regions on the surface of the coffee that mark the tops of convection currents. – Viewed from above, these regions look much like solar granules.
The Sun The Photosphere —Our Star • In the sun, the tops of rising currents of hot gas are brighter than their surroundings. – As the gas cools slightly, it is pushed aside by rising gas from below. – The cooler gas, sinking at the edge of the granule, is slightly dimmer. – Consequently, granules have bright centers and dimmer edges.
The Sun The Photosphere —Our Star • Less obvious structures called supergranules appear to be caused by larger, deeper convection currents in the sun’s layers. – These features can be over twice the size of Earth. • The presence of granulation, and the convection that it implies, is clear evidence that energy is flowing upward through the photosphere.
The Sun The Chromosphere —Our Star • Above the photosphere lies the chromosphere. – Solar astronomers define the lower edge of the chromosphere as lying just above the visible surface of the sun, its upper regions blending gradually with the corona. – You can think of the chromosphere as being an irregular layer with an average depth of less than Earth’s diameter.
The Sun The Chromosphere —Our Star • As the chromosphere is roughly 1, 000 times fainter than the photosphere, you can see it with your unaided eyes only during a total solar eclipse—when the moon covers the brilliant photosphere. – Then, the chromosphere flashes into view as a thin line of pink just above the photosphere.
The Sun The Chromosphere —Our Star • The pink color is produced by the combined light of the red, blue, and violet Balmer emission lines of hydrogen. – The word chromosphere comes from the Greek word chroma, meaning ‘color. ’
The Sun The Chromosphere —Our Star • As the chromosphere is roughly 1, 000 times fainter than the photosphere, you can see it with your unaided eyes only during a total solar eclipse—when the moon covers the brilliant photosphere. – Then, the chromosphere flashes into view as a thin line of pink just above the photosphere.
The Sun The Chromosphere —Our Star • Astronomers know a great deal about the chromosphere from its spectrum. – As the chromosphere produces an emission spectrum, Kirchhoff ’s second law tells you the chromosphere must be an excited, lowdensity gas. – The density is about 108 times less dense than the air you breathe.
The Sun The Chromosphere —Our Star • Spectra reveal that atoms in the lower layers of the chromosphere are ionized and atoms in the higher layers are even more highly ionized. – That is, they have lost more electrons. • From the ionization state of the gas, astronomers can find the temperature in different parts of the chromosphere.
The Sun The Chromosphere —Our Star • Just above the photosphere, the temperature falls to a minimum of about 4, 500 K and then rises rapidly to the extremely high temperatures of the corona.
The Sun The Chromosphere —Our Star • This fact is a little surprising. – You would expect the outer layers of the sun to get progressively cooler. – You will learn later what makes the chromosphere and corona hotter than the photosphere.
The Sun The Chromosphere —Our Star • Solar astronomers can take advantage of some elegant physics to study the chromosphere. – The gases of the chromosphere are transparent to nearly all visible light. – However, atoms in the gas are very good at absorbing photons of specific wavelengths. – This produces certain dark lines in the absorption spectrum of the photosphere.
The Sun The Chromosphere —Our Star • A photon having one of those wavelengths that is emitted in a deeper layer of the sun is very unlikely to escape through the chromosphere without being absorbed. – If a photon of one of these particular wavelengths reaches Earth, you can be sure that it came from higher in the atmosphere.
The Sun The Chromosphere —Our Star • A filtergram is a photograph made using light in one of those dark absorption lines. – Filtergrams reveal detail in the upper layers of the chromosphere. • A filtergram made at the wavelength of the Hα Balmer line is displayed. – It reveals complex structure in the chromosphere.
The Sun The Chromosphere —Our Star • Spicules are flamelike jets of gas rising upward into the corona that last 5 to 15 minutes. – They appear to be cooler gas from the lower chromosphere extending upward into hotter regions.
The Sun The Chromosphere —Our Star • Seen at the limb (edge) of the sun’s disk, spicules blend together to look like flames covering a burning prairie. – Filtergrams of spicules located at the center of the solar disk show that they spring up around the edges of supergranules.
The Sun The Chromosphere —Our Star • Spectroscopic analysis of the chromosphere reveals that it is a lowdensity gas in constant motion, where the temperature increases rapidly with height. – Just above the chromosphere lies even hotter gas.
The Sun The Solar Corona —Our Star • The outermost part of the sun’s atmosphere is called the corona, after the Greek word for ‘crown. ’ • The corona is so dim that it is not visible in the daytime sky because of glare from the brilliant photosphere.
The Sun The Solar Corona —Our Star • During a total solar eclipse, the moon covers the photosphere and the corona shines with a pearly glow not quite as bright as the full moon. – Special telescopes on earth, and in space, can block the light from the photosphere and image the corona out beyond 20 solar radii, almost 10 percent of the way to Earth.
The Sun The Solar Corona —Our Star • The images show streamers in the corona that follow the lines of force of the sun’s magnetic field.
The Sun The Solar Corona —Our Star • The spectrum of the corona can tell you a great deal about the coronal gases and simultaneously illustrate how astronomers can analyze a spectrum. • Some of the light from the outer corona produces a spectrum with absorption lines the same as the photosphere’s spectrum. – This light is just sunlight reflected from dust particles in the corona.
The Sun The Solar Corona —Our Star • In contrast, some of the light from the corona produces a continuous spectrum that lacks absorption lines. – That happens when sunlight from the photosphere is scattered off free electrons in the ionized coronal gas. – As the coronal gas has a temperature over 1 million K, the electrons travel very fast. – The reflected photons suffer large, random Doppler shifts that smear out solar absorption lines to produce a continuous spectrum.
The Sun The Solar Corona —Our Star • Superimposed on the corona’s continuous spectrum are emission lines of highly ionized gases. • In the lower corona, the atoms are not as highly ionized as at higher altitudes. – This tells you that the temperature of the corona rises with altitude.
The Sun The Solar Corona —Our Star – Just above the chromosphere, the temperature is about 500, 000 K. – In the outer corona, it can be as high as 2 million K or more.
The Sun The Solar Corona —Our Star • The corona is very hot, but it is not very bright. – Its density is very low, only 106 atoms/cm 3 in its lower regions. – That is about a trillion times less dense than the air you breathe. – In its outer layers, the corona contains only 1 to 10 atoms/cm 3, less dense than the best vacuum on Earth. – Due to this low density, the hot gas does not emit much radiation.
The Sun The Solar Corona —Our Star • Astronomers have wondered for years how the corona and chromosphere can be so hot. – Heat flows from hot regions to cool regions. – So, how can the heat from the photosphere— with a temperature of only 5, 800 K—flow out into the much hotter chromosphere and corona?
The Sun The Solar Corona —Our Star • Observations made by the Solar and Heliospheric Observatory (SOHO) satellite have mapped a magnetic carpet of looped magnetic fields extending up through the photosphere. – Unseen turbulence below the surface may be whipping these fields about and churning the low density gases of the chromosphere and corona, heating the gas. – In this instance, energy appears to flow outward as the agitation of the magnetic fields.
The Sun The Solar Corona —Our Star • Gas follows the magnetic fields pointing outward and flows away from the sun in a breeze of ionized atoms called the solar wind. – Like an extension of the corona, the low-density gases of the solar wind blow past Earth at 300 to 800 km/s, with gusts as high as 1, 000 km/s. – Earth is bathed in the corona’s hot breath.
The Sun The Solar Corona —Our Star • Due to the solar wind, the sun is slowly losing mass. • However, it is a minor loss, amounting to only about 107 tons per year, only about 10 -14 of a solar mass per year. • Later in life, the sun, like many other stars, will lose mass rapidly.
The Sun The Solar Corona —Our Star • Do other stars have chromospheres, coronae, and stellar winds like the sun? • Ultraviolet and X-ray observations suggest that the answer is yes. – The spectra of many stars contain emission lines in the far ultraviolet that could only have formed in the low-density, high-temperature gases of a chromosphere and corona.
The Sun The Solar Corona —Our Star – Additionally, when gas is as hot as it is in the corona, it emits X rays. – Many stars are sources of X rays, which means that those stars have coronas too. – This observational evidence gives astronomers good reason to believe that the sun, for all its complexity, is a typical star.
The Sun Helioseismology —Our Star • Almost no light emerges from below the photosphere. • Thus, you can’t see details in the solar interior. • However, solar astronomers can use vibrations in the sun to explore its depths in a process called helioseismology.
The Sun Helioseismology —Our Star • Random movements in the sun caused by convection or eruptions constantly produce vibrations. – Some of those vibrations resonate like sound waves in organ pipes. – These vibrations are just sound waves of very long period. – The periods range from about 3 to 20 minutes. – A vibration with a period of 5 minutes is strongest.
The Sun Helioseismology —Our Star • Astronomers can detect these vibrations by observing Doppler shifts in the solar surface. – As a vibrational wave travels down into the sun, the increasing density and temperature curve its path. – This causes the wave to return to the surface, where it makes the photosphere heave up and down.
The Sun Helioseismology —Our Star • By observing these movements, astronomers can determine which vibrations become stronger and which become weaker. – The strength of different wavelengths of vibration is determined by the temperature, pressure, and density of the layers that the vibrations travel through.
The Sun Helioseismology —Our Star • Just as geologists can study Earth’s interior by analyzing vibrations from earthquakes, solar astronomers can use helioseismology to explore the sun’s interior. • To understand the process better, think of a duck pond. – If you stood on the shore and looked down at the water, you would see ripples arriving from all parts of the pond.
The Sun Helioseismology —Our Star – Each duck produces ripples. – Thus, in principle, you could analyze the ripples at your feet and draw a map showing the position and velocity of every duck on the pond. – Of course, it would be difficult to untangle all the ripples. – So, you would need lots of data and a big computer.
The Sun Helioseismology —Our Star • Solar astronomers need large amounts of data for helioseismology. – The Global Oscillation Network Group (GONG) uses a network of telescopes spread around the world to observe the sun continuously as Earth turns. – Spacecraft such as SOHO can also observe the sun uninterrupted by the cycle of day and night.
The Sun Helioseismology —Our Star • Analysis of such data has allowed astronomers to map the temperature, density, and rate of rotation inside the sun. – Helioseismology has been able to detect great currents of gas flowing below the photosphere and to detect the formation of a sunspot before it appeared in the photosphere. – It can even locate sunspots on the far side of the sun, sunspots that are not visible from Earth.
The Sun Building Scientific Arguments —Our Star • How deeply into the sun can you see? – Scientific arguments usually involve observations. – It is always important to know how observations are made. • When you look into the layers of the sun, your sight does not really penetrate into the sun. – Rather, your eyes record photons that have escaped from the sun and traveled outward through the layers of the sun’s atmosphere.
The Sun Building Scientific Arguments —Our Star • If you observe at a wavelength at the center of a dark absorption line, the photosphere and lower chromosphere are opaque. – Photons can’t escape to your eyes. – The only photons you can see come from the upper chromosphere.
The Sun Building Scientific Arguments —Our Star • In contrast, if you observe at a wavelength that is not easily absorbed (a wavelength between spectral lines), the atmosphere is more transparent. – The photons from deep inside the photosphere can escape to your eyes.
The Sun Building Scientific Arguments —Our Star • However, there is a limit. – At a certain depth, the sun’s atmosphere is opaque for almost all wavelengths, few photons can escape, and you can’t see deeper. • By choosing the proper wavelength, solar astronomers can observe to different depths. – However, the corona is so thin and the gas below the photosphere so dense that this doesn’t work in these regions.
The Sun Building Scientific Arguments —Our Star • Now, it is time to build a new argument. – How can you observe the corona and the deeper layers of the sun?
The Sun Nuclear Fusion in the Sun —Our Star • Like soap bubbles, stars are structures balanced between opposing forces that individually would destroy them. • The sun is a ball of hot gas held together by its own gravity. – If it were not for the sun’s gravity, the hot, pressurized gas in the sun’s interior would cause it to explode. – Similarly, if the sun were not so hot, its gravity would compress it into a small, dense body. – In this section, you will explore the way in which the sun generates its heat.
The Sun Nuclear Fusion in the Sun —Our Star • The sun is powered by nuclear reactions that occur near its center. – The energy produced keeps the interior hot and keeps the gas in the interior totally ionized. – That is, the electrons are not attached to atomic nuclei. – Thus, the gas is an atomic soup of rapidly moving particles colliding with each other at high velocities.
The Sun Nuclear Fusion in the Sun —Our Star • Nuclear reactions inside stars depend on atomic nuclei, not whole atoms. – How exactly can the nucleus of an atom yield energy? – The answer lies in the force that holds the nuclei together.
The Sun Nuclear Binding Energy —Our Star • The sun generates its energy by breaking and rearranging the bonds between the particles inside atomic nuclei. – This is quite different from the way you generate energy by burning wood in a fireplace. – The process of burning wood extracts energy by breaking and rearranging chemical bonds between atoms in the wood.
The Sun Nuclear Binding Energy —Our Star • Chemical bonds are formed by the electrons in atoms. • You have learnt that the electrons are bound to the atoms by the electromagnetic force. – Thus, the chemical energy released when bonds are broken originates in the electromagnetic force.
The Sun Nuclear Binding Energy —Our Star • There are only four known forces in nature: the force of gravity, the electromagnetic force, the weak force, and the strong force. – The weak force is involved in the radioactive decay of certain kinds of nuclear particles. – The strong force binds together atomic nuclei. – Thus, nuclear energy comes from the strong force.
The Sun Nuclear Binding Energy —Our Star • There are two ways of generating nuclear energy from atomic nuclei. • Nuclear power plants generate energy through nuclear fission reactions that split uranium nuclei into less massive fragments. – A uranium nucleus contains a total of 235 protons and neutrons, and it can split into a range of fragment sizes containing roughly half as many particles.
The Sun Nuclear Binding Energy —Our Star • The graph shows the binding energy that holds different nuclei together.
The Sun Nuclear Binding Energy —Our Star – When uranium splits, the fragments are lower in the diagram than the original uranium nucleus. – Any nuclear reaction that sinks lower in the diagram produces energy.
The Sun Nuclear Binding Energy —Our Star • Stars make energy in nuclear fusion reactions that combine light nuclei into heavier nuclei. – The most common reaction fuses four hydrogen nuclei (single protons) into a helium nucleus (two protons and two neutrons).
The Sun Nuclear Binding Energy —Our Star – Because a helium nucleus is lower in the diagram than a hydrogen nucleus, energy is released.
The Sun Nuclear Binding Energy —Our Star • Both fusion and fission reactions move downward in the diagram. – Thus, both produce energy by releasing the binding energy of atomic nuclei.
The Sun Hydrogen Fusion —Our Star • The sun fuses four hydrogen nuclei to make one helium nucleus. – As one helium nucleus contains 0. 7 percent less mass than four hydrogen nuclei, it seems that some mass vanishes in the process. – That mass is converted to energy, and you can figure out how much by using Einstein’s famous equation, E=mc 2
The Sun Hydrogen Fusion —Our Star • You can symbolize the hydrogen fusion reaction with a simple equation. 4 1 H → 4 He + energy – 1 H represents a proton, the nucleus of the hydrogen atom. – 4 He represents the nucleus of a helium atom. – The superscripts indicate the approximate weight of the nuclei.
The Sun Hydrogen Fusion —Our Star • The actual steps in the process are more complicated. – Instead of waiting for four hydrogen nuclei to collide simultaneously, a highly unlikely event, the process can proceed step by step in a chain of reactions called the proton–proton chain.
The Sun Hydrogen Fusion —Our Star • The proton-proton chain is a series of three nuclear reactions that builds a helium nucleus by adding protons one at a time.
The Sun Hydrogen Fusion —Our Star • As this process is efficient at temperatures above 10, 000 K, it is the mechanism by which energy is produced in the interior of the sun, where temperatures reach 15, 000 K.
The Sun Hydrogen Fusion —Our Star • The three reactions in the chain are as follows. 1 H + 1 H → 2 H + e + + v 2 H 3 He + 1 H → 3 He + γ + 3 He → 4 He + 1 H
The Sun Hydrogen Fusion —Our Star • In the first reaction, two hydrogen nuclei (two protons) combine to form a heavy hydrogen nucleus called deuterium, emitting a particle called a positron (a positively charged electron, symbolized e+) and another called a neutrino (ν). 1 H + 1 H → 2 H + e + + ν
The Sun Hydrogen Fusion —Our Star • In the second reaction, the heavy hydrogen nucleus absorbs another proton and, with the emission of a gamma ray (γ), becomes a lightweight helium nucleus (3 He). 2 H + 1 H → 3 He + γ
The Sun Hydrogen Fusion —Our Star • Finally, two light helium nuclei combine to form a normal helium nucleus and two hydrogen nuclei. 3 He + 3 He → 4 He + 1 H
The Sun Hydrogen Fusion —Our Star reaction needs two 3 He • As the last nuclei, the first and second reactions must each occur twice for each 4 He produced.
The Sun Hydrogen Fusion —Our Star • The net result of this chain reaction is the transformation of four hydrogen nuclei into one helium nucleus plus energy. • The energy appears in the form of gamma rays, positrons, and neutrinos.
The Sun Hydrogen Fusion —Our Star • The gamma rays are photons that are absorbed by the surrounding gas before they can travel more than a few centimeters. – This heats the gas.
The Sun Hydrogen Fusion —Our Star • The positrons produced in the first reaction combine with free electrons. – Both particles vanish, converting their mass into gamma rays, which are also absorbed and so help keep the center of the star hot.
The Sun Hydrogen Fusion —Our Star • The neutrinos, however, are particles that travel at nearly the speed of light and almost never interact with other particles. – The average neutrino could pass unhindered through a lead wall 1 light-year thick. – Consequently, the neutrinos do not help heat the gas but race out of the star—carrying away roughly 2 percent of the energy produced.
The Sun Hydrogen Fusion —Our Star • Creating one helium nucleus makes only a small amount of energy—hardly enough to raise a housefly 0. 001 inch into the air. • Only by concentrating many reactions in a small area can hydrogen fusion produce significant results. – For instance, a single kilogram (2. 2 lb) of hydrogen, converted entirely to energy, would produce enough power to raise an average-sized mountain 10 km (6 miles) into the air.
The Sun Hydrogen Fusion —Our Star • The sun has a voracious energy appetite and needs to produce 1, 038 reactions per second, transforming 5 million tons of mass into energy every second, just to balance its own gravity. – It might sound as if the sun is losing mass at a furious rate. – However, during its entire 10 -billion-year lifetime, the sun will convert less than 0. 07 percent of its mass into energy.
The Sun Hydrogen Fusion —Our Star • You can see that nuclear fusion is very powerful, especially if you calculate that the fusion of a milligram of hydrogen, roughly the mass of a match head, produces as much energy as burning 30 gallons of gasoline.
The Sun Hydrogen Fusion —Our Star • However, the nuclear reactions in the sun are spread through a large volume in its core. • Any single gram of matter produces little energy. – A person of normal mass who is eating a normal diet produces about 4, 000 times more heat per gram by burning calories than the matter in the core of the sun. – The sun produces a lot of energy because it contains a lot of grams of matter.
The Sun Hydrogen Fusion —Our Star • Fusion reactions can occur only when the nuclei of two atoms get very close to each other. • As atomic nuclei carry positive charges, they repel each other with an electrostatic force called the Coulomb force. – Physicists commonly refer to this repulsion between nuclei as the Coulomb barrier.
The Sun Hydrogen Fusion —Our Star • To overcome the Coulomb barrier, atomic nuclei must collide violently. – Violent collisions are rare unless the gas is very hot. – In that case, the nuclei move at high speeds and collide violently.
The Sun Hydrogen Fusion —Our Star • So, nuclear reactions in the sun take place only near the center—where the gas is hottest and most dense. – A high temperature ensures that collisions between nuclei are violent enough to overcome the Coulomb barrier. – A high density ensures that there are enough collisions, and thus enough reactions, to meet the sun’s energy needs.
The Sun Energy Transport in the Sun —Our Star • Now, you are ready to follow the energy from the core of the sun to the surface. – The surface of the sun is relatively cool—only about 5, 800 K—whereas the center is about 15 million K. – So, energy must flow outward from the core.
The Sun Energy Transport in the Sun —Our Star – As the core is so hot, the gas acts as a black body and emits gamma rays. – Each time a gamma ray encounters an electron, it is deflected or scattered in a random direction. – As the gamma ray bounces around, it slowly drifts outward toward the surface. – This process, repeated innumerable times each second for individual gamma rays, carries energy outward in the form of radiation.
The Sun Energy Transport in the Sun —Our Star • Astronomers refer to the inner parts of the sun as the radiative zone.
The Sun Energy Transport in the Sun —Our Star • Imagine picking a single gamma ray and following it to the surface. – As the ray is scattered over and over again by the hot gas, it drifts outward into cooler layers, where the gas tends to emit photons of longer wavelength. – The ray is eventually absorbed and reemitted as two X rays.
The Sun Energy Transport in the Sun —Our Star • Now, you must follow those two X rays as they bounce around. – As they drift outward into even cooler gas, they in turn become a number of even longer-wavelength photons.
The Sun Energy Transport in the Sun —Our Star • The packet of energy that began as a single gamma ray is broken down into a large number of lower-energy photons. • It eventually emerges from the sun’s surface as about 1, 800 photons of visible light.
The Sun Energy Transport in the Sun —Our Star • However, something else happens along the way. – The packet of energy eventually reaches the outer layers of the sun, where the gas is so cool that it is not very transparent to radiation.
The Sun Energy Transport in the Sun —Our Star • The energy is backed up like water behind a dam, causing the gas to churn in convection. – Hot blobs of gas rise and cool blobs sink. – In this region, known as the convective zone, the energy is carried outward as circulating gas.
The Sun Energy Transport in the Sun —Our Star • The granulation visible on the photosphere is clear evidence that the sun has a convective zone just below the photosphere, carrying energy up to the surface.
The Sun Energy Transport in the Sun —Our Star • Sunlight is nuclear energy produced in the core of the sun. – The energy of a single gamma ray can take a million years to work its way outward—first through radiation and then through convection —on its long journey to the photosphere.
The Sun Energy Transport in the Sun —Our Star • It is time to ask the critical question that lies at the heart of science. – What is the evidence to support this theoretical explanation of how the sun makes its energy? – The search for that evidence will introduce you to one of the great problems of modern astronomy.
The Sun Neutrinos from the Sun’s Core —Our Star • The center of a star seems forever hidden. • The sun, though, is transparent to neutrinos because these subatomic particles almost never interact with normal matter. – Nuclear reactions in the sun’s core produce floods of neutrinos that rush off into space. – If astronomers could detect these neutrinos, they could probe the sun’s interior.
The Sun Neutrinos from the Sun’s Core —Our Star • As neutrinos almost never interact with atoms, you never feel the flood of over 1012 solar neutrinos that flows through your body every second. – Even at night, neutrinos from the sun rush through Earth as if it weren’t there, up through your bed, through you, and onward into space. • You are lucky to be transparent to them. – It also means that they are extremely hard to detect.
The Sun Neutrinos from the Sun’s Core —Our Star • Certain nuclear reactions, however, can be triggered by a neutrino of the right energy. • In the late 1960 s, chemist Raymond Davis, Jr. began using such a reaction to detect solar neutrinos.
The Sun Neutrinos from the Sun’s Core —Our Star • Davis filled a 100, 000 -gallon tank with the cleaning fluid perchloroethylene (C 2 Cl 4). – Theory predicts that, about once a day, a solar neutrino will convert a chlorine atom in the tank into radioactive argon—which can be detected later by its radioactive decay.
The Sun Neutrinos from the Sun’s Core —Our Star • To protect the detector from cosmic rays from space, the tank was buried nearly a mile deep in a South Dakota gold mine. – Of course, the mile of rock overhead has no effect on the neutrinos.
The Sun Neutrinos from the Sun’s Core —Our Star • The result of the Davis experiment startled astronomers. – The cleaning fluid detected too few neutrinos—not one neutrino per day as predicted by models of the sun, but about one every three days.
The Sun Neutrinos from the Sun’s Core —Our Star • The experiment was refined, tested, and calibrated for over three decades. – However, it did not find the missing neutrinos. • Other detectors were built. – They too counted very few neutrinos coming from the sun.
The Sun Neutrinos from the Sun’s Core —Our Star • The missing neutrinos were one of the great mysteries of modern astronomy. – Some scientists argued that astronomers didn’t correctly understand how the sun and stars make their energy. – Other scientists wondered if there was something about neutrinos that could explain the problem.
The Sun Neutrinos from the Sun’s Core —Our Star • Astronomers had great confidence in their theories of the sun’s interior. • Helioseismology confirmed those theories. – So, they did not abandon their theories immediately.
The Sun Neutrinos from the Sun’s Core —Our Star • As the 21 st century began, scientists were able to solve the mystery. – Physicists now know of three kinds of neutrinos, which they call ‘flavors. ’ – The Davis experiment could detect, or ‘taste, ’ only one flavor: electron-neutrinos.
The Sun Neutrinos from the Sun’s Core —Our Star • Theory hinted the electron-neutrinos produced in the core of the sun might oscillate among the three flavors as they rushed out through the sun, and across space, to Earth.
The Sun Neutrinos from the Sun’s Core —Our Star • Observations begun in 2000 confirm theory. – Some of the electron neutrinos produced in the sun transform into tau- and muon-neutrinos, which most detectors cannot count.
The Sun Neutrinos from the Sun’s Core —Our Star • This solution to the solar neutrino problem is exciting because neutrinos can’t oscillate unless they have mass. – Neutrinos were long thought to be massless. – However, if they have even a small mass, they are so common their gravity could affect the evolution of the universe as a whole—an idea you will learn about later.
The Sun Neutrinos from the Sun’s Core —Our Star • The detection of neutrino oscillation also excites astronomers because it confirms theories that describe the interior of the sun and stars.
The Sun Building Scientific Arguments —Our Star • Why does nuclear fusion require that the gas be very hot? – This argument has to include some basic physics of atoms and thermal energy. • Inside a star, the gas is so hot it is ionized —which means the electrons have been stripped off the atoms and the nuclei are bare and have a positive charge.
The Sun Building Scientific Arguments —Our Star • For hydrogen fusion, the nuclei are single protons. – These atomic nuclei repel each other because of their positive charges. – So, they must collide with each other at high velocity to overcome that repulsion and get close enough together to fuse.
The Sun Building Scientific Arguments —Our Star • If the atoms in a gas are moving rapidly, then it must have a high temperature. • So, nuclear fusion requires that the gas have a very high temperature. – If the gas is cooler than about 10 million K, hydrogen can’t fuse because the protons don’t collide violently enough to overcome the repulsion of their positive charges.
The Sun Building Scientific Arguments —Our Star • It is easy to understand why nuclear fusion in the sun requires high temperature. • Now, expand your argument. – Why does it require high density?
The Sun Solar Activity —Our Star • Solar activity refers to short-lived features on the sun that change over minutes, days, or years. – As you use spectroscopic analysis to explore this aspect of solar astronomy, you will discover that solar activity is shaped by magnetic fields and driven by the powerful flow of energy rising from the sun’s interior.
The Sun Observing the Sun —Our Star • Solar activity is often visible with even a small telescope. • However, you should exercise great caution in observing the sun. – Sunlight is very intense and, when it enters your eye, it is absorbed and converted into heat.
The Sun Observing the Sun —Our Star • Equally dangerous is the infrared radiation in sunlight. – Your eyes can’t detect the infrared. – However, it is converted to heat in your eyes and can burn and scar the retina.
The Sun Observing the Sun —Our Star • It is unsafe to look directly at the sun. • It is even more dangerous to look at it through any optical instrument such as a telescope, binoculars, or even the viewfinder of a camera. – The light-gathering power of such an optical system concentrates the sunlight and can cause severe injury.
The Sun Observing the Sun —Our Star • You should never look at the sun with any optical instrument unless you are certain it is safe. – The figure illustrates a safe way to observe the sun with a small telescope.
The Sun Observing the Sun —Our Star • In the early 17 th century, Galileo observed the sun and saw spots on its surface. – Day by day, he saw the spots moving across the sun’s disk. – These are sunspots. – He rightly concluded that the sun was rotating.
The Sunspots —Our Star • The dark spots that you see at visible wavelengths only hint at the complex processes that go on in the sun’s atmosphere. – To explore those processes, you must turn to the analysis of spectra at a wide range of wavelengths.
The Sunspots —Our Star • There are various points to note about sunspots. • They are cool spots on the sun’s surface caused by strong magnetic fields.
The Sunspots —Our Star • The Zeeman effect gives astronomers a way to measure the strength of magnetic fields on the sun.
The Sunspots —Our Star • Also, they follow an 11 -year cycle not only in the number of spots visible but in their location on the sun.
The Sunspots —Our Star • The cycle can vary over centuries and appears to affect Earth’s climate.
The Sunspots —Our Star • Finally, there is clear evidence that they are part of a larger magnetic process that involves all layers of the sun’s atmosphere.
The Sunspots —Our Star • The sunspot groups are merely the visible traces of active regions. – What causes this magnetic activity? – The answer appears to be linked to the waxing and waning of the sun’s magnetic field.
The Sun’s Magnetic Cycle —Our Star • Sunspots are magnetic phenomena. • So, the 11 -year cycle of sunspots must be caused by cyclical changes in the sun’s magnetic field. – To explore this idea, you need to begin with the sun’s rotation.
The Sun’s Magnetic Cycle —Our Star • The sun does not rotate as a rigid body. – It is a gas from its outermost layers down to its center. – Some parts of the sun rotate faster than other parts.
The Sun’s Magnetic Cycle —Our Star – The equatorial region of the photosphere rotates faster than do regions at higher latitudes. – It rotates once every 25 days. – At a latitude of 45°, one rotation takes 27. 8 days.
The Sun’s Magnetic Cycle —Our Star • Helioseismology shows that deeper layers of gas rotate at different speeds. • Different parts of an object rotating at different rates is called differential rotation. – The magnetic cycle in the sun is linked to its differential rotation.
The Sun’s Magnetic Cycle —Our Star • The sun’s magnetic field appears to be powered by the energy flowing outward through the moving currents of gas. – The gas is highly ionized, so it is a very good conductor of electricity. – The dynamo effect occurs when a rapidly rotating conductor is stirred by convection to produce a magnetic field.
The Sun’s Magnetic Cycle —Our Star • This process is believed to produce Earth’s magnetic field. • Helioseismology provides evidence that the process also produces the sun’s magnetic field at the bottom of the convection currents that lie below the sun’s photosphere. – The details of the process are still poorly understood. – However, the sun’s magnetic cycle is clearly related to the creation of its magnetic field.
The Sun’s Magnetic Cycle —Our Star • The magnetic behavior of sunspots provides an insight into how the magnetic cycle works. – Sunspots tend to occur in groups or pairs. – The magnetic field around such a pair resembles that created by a magnet. – That is, one end of the field is magnetic north and one end is magnetic south.
The Sun’s Magnetic Cycle —Our Star – At any one time, sunspot pairs south of the sun’s equator have reversed polarity compared with those north of the sun’s equator. – At the beginning of the next 11 -year sunspot cycle, the new spots appear with reversed magnetic polarity.
The Sun’s Magnetic Cycle —Our Star • This magnetic cycle is not fully understood. • The Babcock model, named for its inventor, explains it as a progressive tangling of the solar magnetic field. – As the electrons in an ionized gas are free to move, the gas is a very good conductor of electricity, and any magnetic field in the gas is ‘frozen’ into the gas. – If the gas moves, the field must move with it.
The Sun’s Magnetic Cycle —Our Star • Thus, the sun’s magnetic field is frozen into its gases.
The Sun’s Magnetic Cycle —Our Star • The differential rotation wraps the field around the sun —like a long string caught on a hubcap. • Rising and sinking gas currents twist the field into ropelike tubes, which tend to rise.
The Sun’s Magnetic Cycle —Our Star • Where these magnetic tubes burst through the sun’s surface, sunspot pairs occur.
The Sun’s Magnetic Cycle —Our Star • The Babcock model explains the reversal of the sun’s magnetic field from cycle to cycle. – As the magnetic field becomes tangled, adjacent regions of the sun’s surface are dominated by magnetic fields that point in different directions.
The Sun’s Magnetic Cycle —Our Star • After about 11 years of tangling, the field eventually becomes so complex that adjacent regions of the solar surface begin changing their magnetic fields to agree with neighboring regions. – Quickly, the entire field rearranges itself into a simpler pattern. – Differential rotation begins winding it up to start a new cycle.
The Sun’s Magnetic Cycle —Our Star • However, the newly organized field is reversed. – The next sunspot cycle begins with magnetic north replaced by magnetic south. • Thus, the complete magnetic cycle is 22 years long, and the sunspot cycle is 11 years long.
The Sun’s Magnetic Cycle —Our Star • The Babcock model may in fact be incorrect in some details, but it gives you a framework on which to organize all the complex solar activity. • This is the power of a scientific model. – Even though the models of the sky and the atom, which you have learned about earlier, were only partially correct, they served as organizing themes to guide your thinking.
The Sun’s Magnetic Cycle —Our Star • Similarly, although the details of the solar magnetic cycle are not yet understood, the Babcock model gives you a general picture of the behavior of the sun’s magnetic field. – Further observations will help astronomers refine the model and make it a better description of what really happens in the sun.
The Sun’s Magnetic Cycle —Our Star • If the sun is truly a representative star, you might expect to find similar magnetic cycles on other stars. – However, they are too distant for spots to be directly visible. • Some stars, though, vary in brightness over a period of days in a way that reveals they are marked with dark spots believed to resemble sunspots.
The Sun’s Magnetic Cycle —Our Star • Other stars have spectral features that vary over periods of years, suggesting that they are subject to magnetic cycles much like the sun’s. • In fact, some stars display sudden flares that resemble solar eruptions. – Once again, the evidence tells you that the sun is a normal star.
The Sun Chromospheric and Coronal Activity —Our Star • The solar magnetic fields extend high into the chromosphere and corona, where they produce beautiful and powerful phenomena.
The Sun Chromospheric and Coronal Activity —Our Star • There are three important points to note about these phenomena. • One, all solar activity is magnetic.
The Sun Chromospheric and Coronal Activity —Our Star • There are no such events on Earth because Earth’s magnetic field is weak. • Also, Earth’s atmosphere is not ionized and is thus free to move independently of the magnetic field. – On the sun, however, the weather is a magnetic phenomenon.
The Sun Chromospheric and Coronal Activity —Our Star • Two, tremendous energy can be stored in arches of magnetic field. – These are visible near the limb of the sun as prominences and, seen from above, as filaments.
The Sun Chromospheric and Coronal Activity —Our Star • When that stored energy is released, it can trigger powerful eruptions. – Although these eruptions occur far from Earth, they can affect us in dramatic ways.
The Sun Chromospheric and Coronal Activity —Our Star • Finally, in some regions of the solar surface, the magnetic field does not loop back. – High-energy gas from these regions flows outward and produces much of the solar wind.
The Sun Building Scientific Arguments —Our Star • What kind of activity would the sun have if it didn’t rotate differentially? – This is a really difficult question because only one star is visible close up. – Nevertheless, you can construct a scientific argument by thinking about the Babcock model.
The Sun Building Scientific Arguments —Our Star • If the sun didn’t rotate differentially, then the magnetic field might not get twisted up, and there might not be a solar cycle. – Twisted tubes of magnetic field might not form and rise through the photosphere to produce prominences and flares, although convection might tangle the magnetic field and produce some activity.
The Sun Building Scientific Arguments —Our Star • Is the magnetic activity that heats the chromosphere and corona driven by differential rotation or by convection? – It is hard to guess but, without differential rotation, the sun might not have a strong magnetic field and high-temperature gas above its photosphere.
The Sun Building Scientific Arguments —Our Star • This is very speculative. • However, sometimes, in the critical analysis of ideas, it helps to imagine a change in a single important factor and try to understand what might happen. – For example, redo the argument. – What do you think the sun would be like if it had no convection inside?