Chapter 8 The Sun Our Star

Transcription

1 Note that the following lectures include animations and PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode (presentation mode). Chapter 8 The Sun Our Star Outline I. The Solar Atmosphere A. Heat Flow in the Sun B. The Photosphere C. The Chromosphere D. The Solar Corona E. Helioseismology II. Solar Activity A. Sunspots and Active Regions B. The Sunspot Cycle C. The Sun's Magnetic Cycle D. Magnetic Cycles on Other Stars E. Chromospheric and Coronal Activity F. The Solar Constant 1

2 Outline (continued) III. Nuclear Fusion in the Sun A. Nuclear Binding Energy B. Hydrogen Fusion C. The Solar Neutrino Problem General Properties Average star Spectral type G2 Only appears so bright because it is so close. Absolute visual magnitude = 4.83 (magnitude if it were at a distance of 32.6 light years) 109 times Earth s diameter 333,000 times Earth s mass Consists entirely of gas (av. density = 1.4 g/cm 3 ) Central temperature = 15 million 0 K Surface temperature = K SOHO Source: SOHO 2

3 Very Important Warning: Never look directly at the sun through a telescope or binoculars!!! This can cause permanent eye damage even blindness. Use a projection technique or a special sun viewing filter. Sun s Energy Energy produced in the Sun 600,000,000 tons of hydrogen every second 100 million nuclear bombs every heartbeat The Solar Atmosphere Only visible during solar eclipses Apparent surface of the sun Heat Flow Solar interior Temp. incr. inward 3

4 The Photosphere Apparent surface layer of the sun Depth 500 km Temperature 5800 o K Highly opaque (H - ions) Absorbs and re-emits radiation produced in the solar interior The solar corona Density and Temperature Energy Transport in the Photosphere Energy generated in the sun s center must be transported outward. In the photosphere, this happens through Cool gas sinking down Convection: Bubbles of hot gas rising up 1000 km Bubbles last for min. 4

5 Granulation is the visible consequence of convection The Chromosphere Region of sun s atmosphere just above the photosphere. Visible, UV, and X-ray lines from highly ionized gases Temperature increases gradually from 4500 o K to 10,000 o K, then jumps to 1 million o K Filaments Transition region Chromospheric structures visible in Hα emission (filtergram) The Chromosphere (2) Spicules: Filaments of cooler gas from the photosphere, rising up into the chromosphere. Visible in Hα emission. Each one lasting about 5 15 min. 5

6 The Layers of the Solar Atmosphere Visible Photosphere Sun Spot Regions Ultraviolet Corona Chromosphere Coronal activity, seen in visible light The Magnetic Carpet of the Corona Corona contains very low-density, very hot (1 million o K) gas Coronal gas is heated through motions of magnetic fields anchored in the photosphere below ( magnetic carpet ) Computer model of the magnetic carpet Solar Magnetic Field Source:SOHO 6

7 The Solar Wind Constant flow of particles from the sun. Velocity km/s Sun is constantly losing mass: 10 7 tons/year ( of its mass per year) Helioseismology The solar interior is opaque (i.e. it absorbs light) out to the photosphere. Only way to investigate solar interior is through Helioseismology = analysis of vibration patterns visible on the solar surface: Approx. 10 million wave patterns! Sun Spots Cooler regions of the photosphere (T 4240 K). Only appear dark against the bright sun. Would still be brighter than the full moon when placed on the night sky! 7

8 Sun Spots (2) Active Regions Visible Ultraviolet Face of the Sun Solar Activity, seen in soft X-rays Magnetic Fields in Sun Spots Magnetic fields on the photosphere can be measured through the Zeeman effect Sun Spots are related to magnetic activity on the photosphere 8

9 Sun Spots (3) Magnetic field in sun spots is about 1000 times stronger than average. Magnetic North Poles Magnetic South Poles In sun spots, magnetic field lines emerge out of the photosphere. Magnetic Field Lines Magnetic North Pole Magnetic South Pole Magnetic Field Lines Star Spots? Image constructed from changing Doppler shift measurements Other stars might also have sun spot activity: 9

10 The Solar Cycle After 11 years, North/South order of leading/trailing sun spots is reversed 11-year cycle Reversal of magnetic polarity => Total solar cycle = 22 years The Solar Cycle (2) Maunder Butterfly Diagram Sun spot cycle starts out with spots at higher latitudes on the sun Evolve to lower latitudes (towards the equator) throughout the cycle. The Sun s Magnetic Dynamo The sun rotates faster at the equator than near the poles. This differential rotation might be responsible for magnetic activity of the sun. 10

11 Magnetic Loops Magnetic field lines The Sun s Magnetic Cycle After 11 years, the magnetic field pattern becomes so complex that the field structure is re-arranged. New magnetic field structure is similar to the original one, but reversed! New 11-year cycle starts with reversed magnetic-field orientation The Maunder Minimum The sun spot number also fluctuates on much longer time scales: Historical data indicate a very quiet phase of the sun, ~ : The Maunder Minimum 11

12 Magnetic Cycles on Other Stars H and K line emission of ionized Calcium indicate magnetic activity also on other stars. Prominences Relatively cool gas (60,000 80,000 o K) May be seen as dark filaments against the bright background of the photosphere Looped Prominences: gas ejected from the sun s photosphere, flowing along magnetic loops Eruptive Prominences (Ultraviolet images) Extreme events (solar flares) can significantly influence Earth s magnetic field structure and cause northern lights (aurora borealis). 12

13 Space Weather ~ 5 minutes Solar Aurora Sound waves produced by a solar flare Coronal mass ejections Coronal Holes X-ray images of the sun reveal coronal holes. These arise at the foot points of open field lines and are the origin of the solar wind. Temperature Scales o F = 9/5( o K) o F = 9/5( o C) o K o C o F o K = o C

14 Energy produced in the Sun 600,000,000 tons of hydrogen every second 100 million nuclear bombs every heartbeat Density and Temperature of the Sun Energy flow in the Sun Radiative zone Convective zone 14

15 Gravity of Sun attracts the matter, What keeps the sun from collapsing onto itself? Gravity and Sun Gas Pressure and Temperature Pressure = density x temperature x constant Hydrostatic Equilibrium The tug War Who is the winner? 15

16 What kind of fuel can give such high temperatures? Nuclear Fusion The Proton- Proton (p-p) Chain E = mc 2 Converting Mass into Energy E = mc 2 4 H atoms = x kg -1 He atom = x kg Mass Lost = x kg 0.7% of mass converted to energy E = mc 2 = (0.048 x 10-27kg ) (3 x 10 8 m/s) 2 = 4.3 x joule Lights up a 10-watt bulb for a one-half of a trillionth of a second 10 7 times larger than burning in a chemical reaction 16

17 Comparing fusion with burning Converting 1 kg of Hydrogen into Helium E = mc 2 = (0.007kg) (3 x 10 8 m/s) 2 = 6.3 x joule 20,000 metric tons of coal (2 x 10 7 kg) is needed to produce this much energy Lifetime of the Sun Luminosity of the Sun 3.9 x joules per second 3.9 x joules per second 6.3 x joules per kilogram = 6 x kilograms per second 600 billion metric tons every second Mass of the Sun = x kg x kg 6 x kg per second = 3.3 x second 3.3 x seconds or 3.15 x 10 7 seconds per year = 1.05 x years So How Long will the Sun Live? T Life = /M sun 2 If M sun = 1, then T Life = years 17

18 Energy Production Energy generation in the sun (and all other stars): Nuclear Fusion = fusing together 2 or more lighter nuclei to produce heavier ones. Nuclear fusion can produce energy up to the production of iron; Binding energy due to strong force = on short range, strongest of the 4 known forces: electromagnetic, weak, strong, gravitational For elements heavier than iron, energy is gained by nuclear fission. Energy Generation in the Sun: The Proton-Proton Chain Basic reaction: 4 1 H 4 He + energy 4 protons have 0.048*10-27 kg (= 0.7 %) more mass than 4 He. Need large proton speed ( high temperature) to overcome Coulomb barrier (electromagnetic repulsion between protons). T K = 10 million 0 K Energy gain = m*c 2 = 0.43*10-11 J per reaction. Sun needs reactions, transforming 5 million tons of mass into energy every second, to resist its own gravity. The Solar Neutrino Problem The solar interior can not be observed directly because it is highly opaque to radiation. But neutrinos can penetrate huge amounts of material without being absorbed. Early solar neutrino experiments detected a much lower flux of neutrinos than expected ( the solar neutrino problem ). Recent results have proven that neutrinos change ( oscillate ) between different types ( flavors ), thus solving the solar neutrino problem. Davis solar neutrino experiment 18

19 New Terms sunspot granulation convection supergranule limb limb darkening transition region filtergram filament spicule coronagraph magnetic carpet solar wind helioseismology active region Zeeman effect Maunder butterfly diagram differential rotation dynamo effect Babcock model prominence flare reconnection aurora coronal hole coronal mass ejection (CME) solar constant Maunder minimum weak force strong force nuclear fission nuclear fusion Coulomb barrier proton proton chain deuterium neutrino 19