The Sun - II. Alexei Gilchrist

Transcription

1 The Sun - II Alexei Gilchrist

2 Some resources The Universe: Secrets of the Sun video Search on youtube Secrets of the Sun (references are to clips here parts 1-5) Chp 14 of Voyages (references and links at end) References noted in these slides

3 Why does the sun shine? Long and fascinating history involving - geology - evolution - meteors - E = mc2 - quantum mechanics - particle physics - two Nobel prizes More detail:

4 How much energy does the sun put out? Imagine an ice cube in the sun during summer... takes about 30min to melt say If you put all those ice cubes out there at same time a shell 1cm thick and surrounding the sun it would all melt in about 30 min That ice cube could be anywhere in a sphere with radius 150,000,000 km around the sun... 1 calorie (=4.2 J) -> 1g water by 1 degree oc = 6.3 x1026 W

5 How much do we produce? recommended daily dietary intake ~ 2000 Calories (food Calorie = 1000 calories) NB most of the energy we consume goes in heat

6 Age of the sun? First edition of The Origin of The Species by Natural In 1859 Charles Darwin estimated it Selection at 300 million years based on the erosion of the Weald in south England (time enough for evolution to occur)

7 Problem... no fuel source known Chemical? No source known that would supply that much energy The sun would burn out in only 3,000 years

8 Problem... no fuel source known Meteoric impact? Initially favored by Lord Kelvin That some form of the meteoric theory is certainly the true and complete explanation of solar heat can scarcely be doubted, when the following reasons are considered: (1) No other natural explanation, except by chemical action, can be conceived. (2) The chemical theory is quite insufficient, because the most energetic chemical action we know, taking place between substances amounting to the whole sun's mass, would only generate about 3,000 years heat. (3) There is no difficulty in accounting for 20,000,000 years heat by the meteoric theory.... What then are we to think of such geological estimates as [Darwin's] 300,000,000 years for the denudation of the Weald? but a huge number should be hitting earth too...

9 Problem... no fuel source known Gravitational Contraction? Helmoltz & Kelvin Sun shrinking ~ 7inches / day 7000 miles in 10,000 years (recorded history) not noticeable at that time Estimated age of the sun at 18 million years (Not enough for evolution) Darwin removes mention of timescales in later editions of The Origin of The Species by Natural Selection

10 Fission 1896 Radioactivity discovered (Henri Becquerel) Heavy unstable nuclei split apart The discovery of the radio-active elements, which in their disintegration liberate enormous amounts of energy, thus increases the possible limit of the duration of life on this planet, and allows the time claimed by the geologist and biologist for the process of evolution. Rutherford, 1904 release of energy Problem requires massive unstable elements... inconsistent with sun s density and composition

11 Fusion Nucleus 2 protons 2 neutrons 1905 Albert Einstein: E = mc2 Sun is mostly Hydrogen weight He < weight 2p +2n (Aston 1920 looking for isotopes of Neon) Fusion will form Helium Lots of energy released Difference in weight is released as energy... a lot of energy

12 but like charges repel... protons will repel each other as they both have +ve charge Is the sun hot enough so that they slam into each and get close enough for fusion to occur? Eddington 1925: Sun not collapsing hydrostatic equilibrium (pressure in = pressure out) for a gas (pressure) = (density) (temperature) working from surface down estimated core of sun at 10 million K Not hot enough!

13 Quantum again Since matter has wave-like behaviour we can get quantum mechanical tunneling Even though the protons don t have enough energy to fuse they can sort of leak into each other and fuse some of the time

14 p-p cycle Hans Bethe received the 1967 Nobel Prize in Physics "for his contributions to the theory of nuclear reactions, especially his discoveries concerning the energy production in stars"

15 The p-p cycle

16 neutrinos

17 neutrinos Hard to detect probability of hitting something in earth is < 1/1,000,000,000, ,000,000,000 pass through your thumbnail every second could travel through 100 light years of iron without hitting anything Detection: first: o 100,000 gallons of cleaning fluid (perchloroethylene) o Occasional collision with chlorine -> Argon o Detect the Argon o expect only a few atoms / week e.g. super-kamiokande: o 50,000 tons of ultra pure water o 11,000 tv-sized detectors o detect flashes of light from a neutrino collision

18 super-kamiokande

19 solar neutrino problem too few neutrinos detected (factor of three) Energy correct; T of sun verified independently; rate depends on T25! x3 is particularly good Raymond Davis, Jr. & Masatoshi Koshiba, 2002 Nobel Prize in Phyics "for pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos" Required a modification of our basic understanding of particle physics o neutrinos have mass o can oscillate between other types (electron neutrino, muon neutrino, tau neutrino) o Only electron neutrino detected initially Sudbury Neutrino Observatory

20 Secrets of the Sun (nuclear fusion) (1) 1:14 (5min)

21 Solar Cycle o Solar activity goes on an 11 year cycle o last maximum in

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24 Secrets of the Sun (solar cycle) (4) 4:40 (2min)

25 Magnetic environment o Suns activity is driven by magnetism (originating in movement of charged gasses) o Can measure this through the Zeeman effect Strong magnetic fields split the quantum energy levels and this can be seen in the spectra Sunspots are regions of strong magnetic fields

26 Magnetic Environment o coronal loops align with magnetic field lines

27 Magnetic Environment o Suns magnetic field is enormously complex the differential rotation twists the field lines (~ million poles)

28 Plages... bright clouds: o higher temp and density Sunspots... o usually originate near sunspots (release of energy from twisted magnetic field lines) o relatively cool: 60,000 K o loops can remain stable for hours or days o eruptions faster 1,300 km/s & heights of a million km Prominences...

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30 Solar flares o Violent eruptions (some of the largest explosions in solar system) a billion Mt of TNT o usually 5-10 minutes (largest several hours) o near solar maximum: several smaller flares a day; a large flare every few weeks o 10 million K ; lots of UV and X-rays o can cause solar quakes A solar quake from a flare o can emit large amount of coronal material e.g equivalent to mount everest (coronal mass ejections or CMEs) - bubble of 10s of millions of tons gas blown into space - can reach earch in a few days - can change the orbits of satellites - distorts earths magnetic field - disrupts communication satellites

31 Secrets of the Sun (magnetics; coronal loops; sun spots; solar flares; sun quakes; coronal mass ejections) (2) 2:50 (3) 1:38 (11min)

32 Solar wind o Discovered by observing comets tails o stream of charged particles o coronal gases are so hot particles can escape suns gravity o 400km/s outward o sun losing 10 million tons / year o bright regions of corona gases trapped by magnetic fields o dark regions magnetic field extends outwards o solar wind predominantly from these holes

33 Effect on Earth - auroras o interaction of solar wind with earths magnetic field o complex! aurora_img_ _sm

34 Effect on Earth - auroras

35 Effect on Earth - auroras

36 Effect on Earth - climate o little ice age in europe during maunder minimum (Thames froze at least 11 times) Why? Solar luminance change too small Recent model: UV changes 10x 100x bigger More ozone produced transfer of heat around earth changes wind patterns Warms stratosphere Absorbs sunlight

37 Effect on Earth 14 C production more longer term records of solar activity

38 Effect on Earth solar storms Secrets of the Sun (CME; solar storms; effect on technology; auroras) (3) 1:38 9:55 (4) 0:00 4:42 (~13min)