What can we learn from light? Hydrogen Lines Temperature Energy Chemical Composition Speed towards or away from us All from the! Lower E, Lower f, λ Visible! Higher E, Higher f, λ Visible Hydrogen Spectrum Lines: Series Spectral Classification Get spectral type from line features, predict Subdivisions within each letter: 0-9 0 is, 9 is Sun is a G2 star (hotter than a G8 star) Long λ = E Short λ = E Spectral Classification O, B, A, F, G, K, M Oh Be A Fine Girl/Guy Kiss Me Only Boring Astronomers Feel Good Knowing Mnemonics Subdivisions 0-9 Sun is a G2 star Predict temperature to 5% Intensity Actual Spectrum from SDSS Balmer Lines Wavelength 1
Chapter 6 Recap Atoms, electron energy levels, absorbing & emitting light Temperature & color Types of spectra: absorption, emission, continuous Spectral Classes: OBAFGKM Doppler shift & speed Overview: The Sun Properties of the Sun Sun s outer layers Photosphere Chromosphere Corona Solar Activity Sunspots & the sunspot cycle Flares, prominences, CMEs, aurora Sun s Interior The Sun as an energy source Fusion in the Sun Nearest Distance ~ _ AU Temperature ~ K From peak intensity Sun s Properties Radius ~ 7 x 10 5 km About x Earth s radius Mass ~ 2 x 10 30 kg About x Earth s mass Sun s Properties Composition of the Sun Sun is gaseous Violent, bubbling up close Too hot to be solid or liquid 73% 25% 2% other stuff (near surface) The outer layers 2
Photosphere: Surface Innermost visible layer, only 400 km thick (0.06% of radius) Temperature: 6,000 K Defines of the Sun lines Photosphere isn t actually smooth! Features called are about 1000 km across Convection causes granules : transfer of energy through currents Granules Up-Close: like boiling water Dark-colored areas => gas Light-colored areas => gas Chromosphere: sphere of Middle layer, temperature 10 4-10 5 K Chromosphere: Hot hydrogen atoms form a red streak when viewed during a solar eclipse 3
Corona or of the Sun Very low density 10 million to 10 billion x less dense than Earth s atmosphere However, corona is extremely hot! About K Known from spectrum Solar activity is like weather Sunspots Solar Flares Solar Prominences Coronal Mass Ejections (CMEs) All are related to magnetic fields, vary on an -year cycle Sunspots Dark spots on the photosphere Sunspot Number Number & locations change daily Can use observations to determine how fast the Sun than other parts of the Sun s surface (4000K vs. 5800K) Solar Solar Sunspots One of Galileo s major observations More than 30 sketches from summer of 1612 Sunspot number about every 11 years: the sunspot cycle Data goes back to the 1600 s 4
Sunspot Cycle Sunspots usually appear within 30º of equator Where they form changes during a cycle Sunspots often come in Butterfly Diagram One is positive, the other negative, like a magnet Connected by loops of bright gas We think most solar activity is related to changing magnetic fields Fig. 7-14a, p. 135 Fig. 7-14b, p. 135 Fig. 7-7c, p. 135 5
Fig. 7-14d, p. 135 Fig. 7-14e, p. 135 Other Solar Activity: Solar Prominences Solar Flares Coronal Mass Ejections All occur in associated with strong fields Solar Flares Bursts of X- rays and charged particles 5-10 minutes Solar Prominences Erupt high above the Sun s surface Can be quite stable (hoursdays) Usually form near! Coronal Mass Ejections Rare bursts, more energetic than flares or prominences Speeds up to 1,000 km/s 50,000 km above Sun s surface 6
Solar Wind Not moving air, like on Earth Stream of from coronal holes 900,000 mph, reaches Earth in about 4 days Solar Wind Charged particles get trapped in Earth s magnetic field and cause auroras The Sun Today www.spaceweather.com Structure of the Sun Corona Core Core: Energy generated by nuclear Radiation Zone Radiation Zone: Energy transported upward by photons Inner 25% K ~ K Very dense! Photons spend a long time here 7
Convection Zone Convection Zone: causes granules Energy transported upward by rising hot gas Outer 30% K Convection ( hot gas) takes energy to surface What is the Sun s structure? From inside out, the layers are: How does the Sun shine? The Sun has its own energy source Main difference between a star and a Not well understood until 1940 s Need to explain & Lifetime of the Sun Need a vast, constant source of energy Sun is at least years old (from fossils) Most ideas could not sustain the energy rate needed 8
Luminosity of the Sun Energy output: 3.9 x 10 24 Joules/sec A 100-Watt light bulb emits 100 Joules per second Possible Solar Heating Mechanisms Gravitational Collapse Gives lifetime of about years Combustion (burning/fire) Gives lifetime of about years Need 4.6 billion years! Nuclear Power on Earth Nuclear Fission: nuclei This is what happens in nuclear power plants. Nuclear Fusion: nuclei This has only been used for weapons. Fission Fusion San Onofre Fission Plant, CA Hydrogen (fusion) bomb, Pacific Ocean, 1962 Big nucleus into smaller pieces (Nuclear power plants) Small nuclei to make a bigger one (Sun, stars) How does nuclear fusion occur in the Sun? Energy from the Sun: The Sun has hydrogen at its core, at ~15 million K Basic Particles: Protons (in nucleus), positive charge Neutrons (in nucleus), no charge Electrons, negative charge Particles in nucleus have binding energy Core is so hot, the hydrogen is in plasma form: the electrons are free 9
High temperature enables nuclear fusion to happen in the core: Overcome repulsion Speed Interaction Speed Interaction Nuclear Fusion 4 hydrogen nuclei ( ) must collide Not very likely Helium nucleus is built up in steps This sequence of steps is called the - Where Stars Get Their Energy End product of fusion process is a nucleus 1 He is massive than 4 H by a factor of 0.007 (0.7%) Where did that mass go? E =mc 2 c is the speed of light c = 3 x 10 8 m/s so c 2 = 9 x 10 16!!!!! A small amount of mass can produce a lot of energy IN 4 protons Proton-proton chain has 3 steps OUT 4 He nucleus 2 gamma rays 2 positrons 2 neutrinos Total mass is 0.7% lower 10
P-P chain 3 steps: Fusion in the Sun How does the energy from fusion get out of the Sun? 2 1 H -> 2 H + e + + ν ( ) 2 H + 1 H -> 3 He + γ ( ) 3 He + 3 He -> 4 He + 2 1 H ( ) Most of the stuff in the Sun has not yet undergone fusion! (and that s a good thing ) Chapter 8: The Family of Stars We already know how to determine a star s surface temperature chemical composition motion Next, we will learn how we can determine its distance luminosity radius mass Measuring the Distance to Stars The best method for measuring distances of nearby stars is called. This involves observing a star from. How do we know how far away the stars are? 11
Parallax on Earth The apparent shift in position of a against more distant background objects, due to the changing of the observer Parallax on Earth Parallax happens because the observer changes her Distance between observations = The Distance to the Stars We observe a star, on opposite sides of Earth s In 6 months Earth has moved Baseline = A closer star will appear to move more with respect to background stars Distance vs. Parallax Parallax Most stars have a very small parallax angle: p p is usually measured in Distances to stars are measured in either: light years, or parsecs. (1 pc = 3.26 LY) Big distance (object is far away) => parallax Small distance (object is close) => parallax parsec = PARallax of one arcsec 12
Parallax and Distance The Parsec Lecture-Tutorial: Pages 35-37 Work with a partner or two Read directions and answer all questions carefully. Take time to understand it now! Come to a consensus answer you all agree on before moving on to the next question. If you get stuck, ask another group for help. If you get really stuck, raise your hand and I will come around. 13