Astronomy 111 – Lecture 18 Our Sun - An Overview

Tuesday, November 30, 2004 Astronomy Lecture 182 Basic Concepts Size and Composition of Sun Age and Energy Problem Models of the Sun The Atmosphere of the Sun The Future of the Sun

Tuesday, November 30, 2004 Astronomy Lecture 183 Basic Sun Data Distance to Earth –Mean Distance : 1 AU = 149,598,000 km (light travel time to Earth = 8.32 min) –Maximum : 152,000,000 km –Minimum : 147,000,000 km Mean angular diameter : 32 arcmin Radius : 696,000 km = 109 Earth radii Mass : x kg = 3.33 x 10 5 Earth Masses

Tuesday, November 30, 2004 Astronomy Lecture 184 Basic Sun Data Composition (by mass) –74% Hydrogen, 25% Helium, 1% other elements Composition (by number of atoms) –92.1% Hydrogen, 7.8% Helium, 0.1% other elements Mean Density : 1.4 g cm -3 Mean Temperatures –Surface 5800 K, Centre 15.5 Million K (!) Total Luminosity : 3.86 x W (1 sec  enough energy for 1 million years for humankind)

Tuesday, November 30, 2004 Astronomy Lecture 185 How old is the Sun ? Really problematic : Sun does not come with a time-stamp, shows no wrinkles etc… Start with Earth & Solar System –Date geological formations, rocks Various geochronometrical methods using radioactive decay properties  >4.2 billion years –Find oldest meteorites  billion years Conclusion : Sun at least that old !

Tuesday, November 30, 2004 Astronomy Lecture 186 What fuel does the Sun use ? A truly gigantic energy crisis emerges: –Sun is billions of years old. –Life on Earth as well.  Sun must shine roughly like today for already a long time. How does it create all the energy ? What keeps the Sun so hot ?

Tuesday, November 30, 2004 Astronomy Lecture 187 The Solar Energy Source First guesses (early 18 th century) : –Chemical Processes (‘burning’ fuel) Maximum burning time : 10,000 years ! –Using gravitational energy (Kelvin-Helmholtz mechanism, mid-1800s) Slow contraction of Sun releases energy, heats gas up -> energy radiated into space Maximum contraction time : 25 million years ! Only important at very early stage (birth of Sun)

Tuesday, November 30, 2004 Astronomy Lecture 188 A new Energy Source Problem : –Need more energy released per atom Solution : –Albert Einstein : E = mc 2 m = mass, c = speed of light, E = energy Mass and energy are equivalent. Why does that help ? –Speed of light is a large number !

Tuesday, November 30, 2004 Astronomy Lecture 189 Thermonuclear Fusion Turning mass into energy : –Transforming Hydrogen into Helium –Mass balance : 4 1 H atoms x kg 1 4 He atom x kg Mass lost : x kg Extremely efficient process : –Fusing 1 kg of Hydrogen releases same energy as burning 20,000 metric tons of coal !

Tuesday, November 30, 2004 Astronomy Lecture 1810 Thermonuclear Fusion How much hydrogen must be converted to give solar luminosity ? –600 million metric tons per second ! –Sounds enormous, but….. –Sun has enough fuel for at least 9 billion years Solution to energy crisis ! Problem : How does fusion work in detail ?

Tuesday, November 30, 2004 Astronomy Lecture 1811 Thermonuclear Fusion ‘nuclear’ – regarding nuclei of atoms ‘fusion’ – putting together (NOT fission) ‘thermo’ – need enormous temperatures as nuclei do not fuse easily due to electric repulsion

Tuesday, November 30, 2004 Astronomy Lecture 1812 Introducing the Proton-Proton Chain Thermonuclear Fusion

positron neutrino 2H2H positron neutrino 2H2H photon 3 He 4 He photon 3 He

Tuesday, November 30, 2004 Astronomy Lecture 1814 Models of the Sun Solar surface temperature ~ 5800 K Temperature required for fusion ~ 10 million K Fusion can only occur at core of the Sun ! Can we understand and describe the conditions inside the Sun ? Use theoretical models ! –Start with well-known principles and laws of physics

Tuesday, November 30, 2004 Astronomy Lecture 1815 Models of the Sun Observational Fact 1 : –The Sun does not change size over long periods of time, keeps its shape quite well. Principle of Hydrostatic Equilibrium –Pressure and Gravity maintain a balance.

Tuesday, November 30, 2004 Astronomy Lecture 1816 Hydrostatic Equilibrium Gas Pressure Gravity

Tuesday, November 30, 2004 Astronomy Lecture 1817 Models of the Sun Conclusions : Pressure increases with depth Temperature increases with depth

Tuesday, November 30, 2004 Astronomy Lecture 1818 Models of the Sun Observational Fact 2 : –The Sun does not heat up or cool down over long periods of time, keeps its temperature quite well. Principle of Thermal Equilibrium –Energy Generation and Energy Transport maintain a balance.

Tuesday, November 30, 2004 Astronomy Lecture 1819 Models of the Sun Modes of Energy Transport : –Heat conduction  very inefficient for gases –Convection : ‘circulation of fluids’, upwelling of hot gases –Radiative diffusion : Photons carrying away energy

Tuesday, November 30, 2004 Astronomy Lecture 1820 Convection cooler water sinks Hot blob rises

Tuesday, November 30, 2004 Astronomy Lecture 1821 Photon Random Walk

Tuesday, November 30, 2004 Astronomy Lecture 1822 Models of the Sun

Tuesday, November 30, 2004 Astronomy Lecture 1823 Models of the Sun Construct computer model to describe state of solar material from core to atmosphere How can we test those models ? Do they describe the interior well ? How can we ‘see’ into the Sun ?

Tuesday, November 30, 2004 Astronomy Lecture 1824 Testing the Models Test 1 : Helioseismology –Sun vibrates (‘rings like a bell’) –Use waves to test interior structure –Same Principle as Geologist/Geophysicists with Earthquakes –Nice analogy : Ripeness of melons

Tuesday, November 30, 2004 Astronomy Lecture 1825 Astronomers probe the solar interior using the Sun’s own vibrations Helioseismology is the study of how the Sun vibrates These vibrations have been used to infer pressures, densities, chemical compositions, and rotation rates within the Sun

Tuesday, November 30, 2004 Astronomy Lecture 1826 Testing the Models Test 2 : Catching solar neutrinos –By-products of fusion –Unlike photons, neutrinos escape solar interior very easily –‘Ghost-like’ particles : very, VERY hard to detect solar neutrinos every second through 1m 2 on Earth

Tuesday, November 30, 2004 Astronomy Lecture 1827 Catching Neutrinos

Tuesday, November 30, 2004 Astronomy Lecture 1828 The Solar Atmosphere Core of Sun hidden because gases become opaque Outermost layers show remarkable structures (‘atmosphere’) Tiny and much less dense than interior Nonetheless most important for life on our planet

Tuesday, November 30, 2004 Astronomy Lecture 1829 The Solar Atmosphere

Tuesday, November 30, 2004 Astronomy Lecture 1830 The Photosphere Almost all of the visible light emanates from that small layer of gas. Temperature decreases upwards in photosphere. Sun darker at the edge ?

Tuesday, November 30, 2004 Astronomy Lecture 1831 The Photosphere

Tuesday, November 30, 2004 Astronomy Lecture 1832 The Photosphere Cooler layers further out  Notice absorption lines in spectrum ! Cool ? –Still about 4400 K at the upper edge of the atmosphere. –Comparison : Siberian winter night to hot tropical day in Hawaii.

Tuesday, November 30, 2004 Astronomy Lecture 1833 The Photosphere Solar Granulation : Convection cells of gas in photosphere 4 million granules cover the solar surface Each granule covers area ~ Texas & Oklahama combined

Tuesday, November 30, 2004 Astronomy Lecture 1834

Tuesday, November 30, 2004 Astronomy Lecture 1835

Tuesday, November 30, 2004 Astronomy Lecture 1836 The Solar Chromosphere Above the photosphere is a layer of less dense but higher temperature gases called the chromosphere Spicules extend upward from the photosphere into the chromosphere along the boundaries of supergranules

Tuesday, November 30, 2004 Astronomy Lecture 1837 The Solar Chromosphere Spectrum : Emission lines !  Temperature must rise again : –Top of chromosphere : 25,000 K Approximately 300,000 spicules exist at one time, each last about 15 minutes Covering ~ 1 % of solar surface Phenomenon related to Sun’s magnetic field

Tuesday, November 30, 2004 Astronomy Lecture 1838 The outermost layer of the solar atmosphere, the corona, is made of very high- temperature gases at extremely low density The solar corona blends into the solar wind at great distances from the Sun The Solar Corona

Tuesday, November 30, 2004 Astronomy Lecture 1839 The corona ejects mass into space to form the solar wind

Tuesday, November 30, 2004 Astronomy Lecture 1840 Activity in the corona includes coronal mass ejections and coronal holes

Tuesday, November 30, 2004 Astronomy Lecture 1841 Sunspots are low-temperature regions in the photosphere

Tuesday, November 30, 2004 Astronomy Lecture 1842

Tuesday, November 30, 2004 Astronomy Lecture 1843

Tuesday, November 30, 2004 Astronomy Lecture 1844 Sunspots are produced by a 22-year cycle in the Sun’s magnetic field

Tuesday, November 30, 2004 Astronomy Lecture 1845

Tuesday, November 30, 2004 Astronomy Lecture 1846 The magnetic-dynamo model suggests that many features of the solar cycle are due to changes in the Sun’s magnetic field

Tuesday, November 30, 2004 Astronomy Lecture 1847

Tuesday, November 30, 2004 Astronomy Lecture 1848 The future of the Sun Equilibrium holds due to energy generation What happens when Sun runs out of fuel ? Drama at the End of the Solar Life A story for another day…. When will that happen ? –~ 4.0 – 5.0 billion years from now –Puuuh, we are safe !

Tuesday, November 30, 2004 Astronomy Lecture 1849