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•Lecture 7 Our Star What is a Star? • Large, glowing ball of gas that generates heat and light through nuclear fusion in its core. • Stars shine by Nuclear Fusion: – For the sun and most of the stars: powered by the reaction: • 4 Hydrogen nuclei (4 protons) 1 Helium nucleus (2 protons and 2 neutrons) + energy • The same process in H bomb • E=mc2 at work: the total mass of helium is only 99.3% of the mass of 4 protons put together; the rest, 0.7% of the rest mass is turned into energy the powers stars Sun: Our Star Distance: 1.49 x 108 km = 1 A.U. Mass: 1.99 x 1030 kg Radius: 6.96 x 105 km Density: 1.41 g/cm3 Luminosity: 3.8 x 1026 watts How to calculate Sun radius : • The angle diameter of Sun = 32’ (arcsec). 8 1.49 x 10 • The Earth-Sun distance = 1 A.U.= km Radius: 6.96 x 105 km How to calculate Sun mass : Fcentripetal M z v 2z M zMs G Fgravitational 2 1AU . . (1AU . .) vz 30km / s 1AU . . 1.49 x108 km G 6.67 x1011 m3kg 1s 2 Mass: 1.99 x 1030 kg AMO: available solar energy for satellite solar cells. AM 1.5: AMO reduced by gases in earth’s atmosphere. 12-23 UV absorbed by O2 & O3 IR absorptions of H2O & CO2 How to calculate Sun luminosity : • Solar constant is the intensity of radiation (the flux of energy riching 1m 2 ) at AMO ( air mass zero) 2 1 S s 1370 Jm s Ls 4 (1AU . .) S s 3.86 x10 W 2 26 How to calculate Sun temperature : • Stefan – Boltzman law: Ls 4 R T 2 s 4 T 5800K Sun density Composition of the Sun Percent by Mass • H 73% • He 25% • O 0.8% • C 0.3% • N 0.1% Hydrostatic equilibrium The Sun’s Internal Structure Layers of the Sun Core of the Sun The core is in the center, where fusion occurs. • • • • Temperature: Pressure: Plasma Nuclear Fusion 14 million K 1 billion atm Transition region – Convection to Radiation – 300 km thick • Convection zone is similar to the Earth’s mantle. • It lies between the photosphere and the radiation zone. How does the Light Comes Out? Photons are created in the nuclear fusion cycle. They collide with other charged particles and change their direction (random walk). They also decrease their energy while walking. It takes ~10 million year to get outside. The random bouncing occurs in the radiation zone (from the core to ~70% of the Sun’s radius). At T<2 million K, the convection zone carries photons further towards the surface. How does the Light Comes Out? Solar Neutrino Neutrino is a subatomic particle. It is a by-product of the solar proton-proton cycle. It barely interacts with anything. Counts of neutrino coming from the Sun are crucial to test our knowledge about solar physics. Neutrino observatories use huge amounts of different substances to detect nuclear reactions with neutrino. So far theory predicts more neutrino than is seen. Photosphere Photosphere is the visible surface of the Sun. It consists of gas and is far less dense than air over the Earth’s oceans. • 6000 K • Sunspots Sunspots • Dark areas on the photosphere • Cooler –appear dark • 11.2 year cycle SOHO NASA ESA – Dynamic Magnetic field SOHO/MDI NASA & ESA Chromosphere Chromosphere is a zone below corona, where the Sun’s UV radiation is produced (T~10,000 K). • • • • Hotter Less dense Flares Prominences SOHO/EIT NASA/ESA Chromosphere SOHO/EIT NASA/ESA Corona • The uppermost layer of the atmosphere • Very hot: • 0.5 - 2 Million K • very low density: • 1-2 million km thick – Visible during eclipse • Coronal mass ejections The solar wind The solar wind is a stream of photons, ions, and subatomic particles outward from the surface. YOHKOH Dynamic Magnetic field • • • • Sunspots Flares Prominences Coronal mass ejections Effects on the Earth • Magnetic storms • Auroras • Climate – 1645-1715 few sun spots – "Little ice age" Basic Properties of a Star • Mass: from 0.1 solar mass to > 100 solar mass • Size: from 0.1 solar radius to > 1000 solar radius • Luminosity: from 0.01 solar luminosity to a million solar luminosity • Life time: from a few million years to longer than tens of billion years – Sun: 10 billion years lifetime (~5 billion years old now) • Surface Temperature: from 3000 degree above absolute zero (3000 K 4900 F) to 30,000 K (54000 F) – Sun: ~6000 K • Color: from violet to red – Astronomers classify stars based on their colors, or spectral types: from hot (violet) to cool (red) are: O, B, A, F, G, K, M Oh Be A Fine (Girl/Gal/Girl) Kiss Me The Hertzsprung-Russell Diagram Luminosity or The energy output Temperature or color Main Sequence Stars • For more than 90% of a typical star’s lifetime, it is on the Main Sequence of H-R Diagram – With stable Hydrogen burning in the core – The luminosity, temperature and mass of main sequence star follow simple relation: • High mass stars: – Hotter, more luminous, bigger, and have shorter lifetime • Low mass stars: – Cooler, less luminous, smaller, and have longer lifetime – For main sequence stars, if we know the color of the star, then we know its MASS, temperature and lifetime as well. Main Sequence Stars The most important property: Mass of a star • To the first order, the mass of a star determines all the other properties of a star: – in particular, determines the lifetime, evolution, and fate of a star – Lifetime: • 10 solar mass star: a few million years • Sun: 10 billion years – Fate of a star: • Low mass (M < a few solar masses) white dwarf • High mass star (>5 – 10 solar masses) supernova explosion and neutron star • Very high mass star (> 20 solar masses) supernova explosion and Black Holes Fate of a low-mass star (Sun) • Main sequence red giant planetary nebula white dwarf; M(white dwarf) < 1.44 solar mass Fate of a High-mass star • Main sequence (short) supergiant supernova neutron star; M(neutron star) < 3 solar mass or black hole Black Holes • After a massive star supernova, if the core has a mass > 3 M, the force of gravity will be too strong for even neutron degeneracy to stop. • The star will collapse into oblivion. – GRAVITY FINALLY WINS!! • This is what we call a black hole. • The star becomes very small. – it creates a “hole” in the Universe – That not even light could escape • Since 3 M or more are compressed into an infinitely small space, the gravity of the star is HUGE! • WARNING!! – Newton’s Law of Gravity is no longer valid !! – Have to use Einstein’s general relativity to calculate the properties of black holes