Chapter 11 Our Star The Sun the Sun is a G2 star a yardstick for modern astronomy compared to other stars it s run of the mill but its mass is over 300 000 times that of Earth with a radius over 100 times that of Earth it produces more energy in a second than all of the electric power plants on Earth could generate in a half million years 11 1 The Structure of the Sun Is a Matter of Balance At each point within the Sun the outward push of pressure is balanced by the in ward pull of gravity The energy radiated from the surface of the Sun balances the en ergy produced in its interior If gravity were stronger than pressure the Sun would collapse if pressure were stronger it would blow itself apart the pressure at any point within the Sun s interior must be just enough to hold up the weight of all the layers above that point this bal ance is called hydrostatic equilibrium deeper inside the sun the weight of the material above is greater and pressure increases this also means higher density and temperature The sun is a remarkably stable object The same data can be plotted in two ways using powers of 10 shows the full range of data a linear plot shows how quantities rise sharply at the center of the Sun the luminosity of the Sun seems to be increasing with time very slowly to remain balanced it must produce just enough energy in its interior to replace the energy lost from its surface the sun produces 3 85 x 10 26 watts of energy each second through nuclear re actions at its core found from studies in theoretical and nuclear physicists not tele scopes most hydrogen atoms have one proton while other atoms have a mixture of pro tons and neutrons since like charges repel the closer they are the stronger the force protons in the nucleus are constantly repelling each other with tremendous force but strong nuclear force counteracts this outward pressure holding the nu cleus together very strong but only acts over a very short distance a hundred thou sandth the size of an atom the energy required to tear apart a nucleus is enormous compared to the energy required to free an electron when a nucleus is formed this enormous energy is released nuclear fusion combining two less massive atomic nuclei into a more massive atomic nucleus can only happen if nuclei are close enough for the strong nuclear force to overcome the electric repulsion in the sun the primary process is the fusion of hydrogen into helium a process called hydrogen burning Slow moving protons approach each other but are pushed apart by electric re pulsion but the faster they are going the closer they can get At high enough temper atures protons may move rapidly enough to overcome the electric repulsion and fuse releasing energy and leaving behind a deuterium nucleus a positron and a neutrino mass can be converted to energy and energy can be converted to mass mod eled by E mc 2 E is energy m is mass and c is the speed of light in nuclear reactions subtracting the mass of the outputs from the mass of the in puts gives us the mass that was turned into energy some mass in nuclear fusion is converted and released as energy energy is produced in Sun s core extreme conditions density 150 times that of water temp of about 15 million K so nuclei can overcome repulsion and fuse in hotter denser gases collision happen more often so fusion is sensitive to temp and density half of Sun s energy is produced in the inner 9 of the radius hydrogen is the most abundant element in the universe most abundant source of nuclear fuel most efficient mass to energy method also the easiest type of atom to fuse because the electric repulsion is just a single proton against another repulsion is relatively weak compared to element with larger nucleus so fusion happens at much lower temperature in the cores of low mass stars like the Sun hydrogen burns in a process called the proton proton chain the first step in the proton proton chain two hydrogen nuclei fuse during which one of the protons turns into a neutron to conserve energy and charge a pos tively charged positron and neutral neutrino are emitted the new nucleus has a proton and neutron its still hydrogen because it has only one proton but it has more than the normal number of neutrons so it s an isotope of hydrogen called deuterium a positron is the antiparticle of an electron it has all the same properties as an electron except the charge is opposite when matter electrons and antimatter positrons meet they annihilate each other all their mass is converted to energy in the form of photons light this is the emitted positron in the sun and the photons carry away part of the energy released when the two protons fused heating the sur rounding gas neutrinos have no charge very little mass they interact weakly with ordinary matter so much that they escape from the Sun without any particular interactions the second step in the proton proton chain another proton slams into the deuterium nucleus forming the nucleus of a helium isotope consisting of 2 protons and a neutron energy is released as a gamma ray photon the third step in the proton proton chain two helium isotope nuclei collide and fuse producing an ordinary helium nucleus and ejecting two protons this energy release shows up as kinetic energy of the helium nucleus and ejected protons in the end four hydrogen nuclei have combine to form one helium nucleus some energy released by hydrogen burning in the core of the Sun escapes di rectly into space in the form of neutrinos but most of the energy heats the solar inte rior Energy transport determines the Sun s structure Sun is made of gas so convec tion and radiation are important conduction mainly for solids radiation transfers energy from hotter to cooler regions via photons the energy carriers in the sun hotter regions contain more and livelier photons than cooler ones so they migrate to the emptier cooler regions this is how radiation carries energy out ward from the Sun s core depends on how freely photons can move within a star the degree to which matter impedes the flow of photons is opacity which depends on the density composition and temperature of the material and wavelength of photons radiative transfer is efficient in inner part of sun that has low opacity carries en ergy from core outward radiative zone extends 70 of the way out toward the Sun s surface despite the region s low opacity photons travel short distance before they re absorbed emitted or deflected by matter like beach ball in a