Published on November 13, 2007
Our nearest star – the Sun: UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SOLAR & STELLAR PHYSICS GROUP Our nearest star – the Sun Dr. Laura Bone UCL - Mullard Space Science Laboratory Solar and stellar physics group Overview: UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SOLAR & STELLAR PHYSICS GROUP Overview How do stars form? Molecular clouds Protostars Important timescales Initiation of nuclear burning Electromagnetic radiation Stars like the sun Stellar structure The solar magnetic field Solar flares and CME’s Interaction with the solar system Aurorae on the Earth, Saturn and Jupiter Consequences for technology and manned spaceflight The birth of a star: UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SOLAR & STELLAR PHYSICS GROUP The birth of a star Stars are formed in molecular clouds. These are cold T<20K, have densities of n~109K and masses of 102 - 105 solar masses. Very cold means difficult to spot but radiate in infra-red and radio wavelengths. A protostar is formed by the gravitational collapse of some part of the star, perhaps induced by; Supernova explosion Spiral density wave Winds from other young stars Collapse occurs on free fall timescale; Molecular cloud collapse: UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SOLAR & STELLAR PHYSICS GROUP Molecular cloud collapse Protostar to star: UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SOLAR & STELLAR PHYSICS GROUP Protostar to star Slow contraction phase last for ~ 106 years. Occurs on thermal timescale ; Until early part of 20th century this was considered the timescale governing stellar evolution but for the Sun tth ~ 107 years. Problem! Problem solved!: UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SOLAR & STELLAR PHYSICS GROUP Problem solved! Nuclear ignition: UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SOLAR & STELLAR PHYSICS GROUP Nuclear ignition In fact gravitational contraction continues for about a million years. Once core temperature exceeds 106K nuclear reactions become self sustaining. For solar like stars this reaction converts hydrogen to helium using the proton – proton (p – p) chain Nuclear fuel: UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SOLAR & STELLAR PHYSICS GROUP Nuclear fuel Definition : 1 atomic mass unit is the equivalent of one twelfth of the mass of an atom of Carbon 12 or 1.67 x 10-27kg. We have four protons of atomic mass 1.008 combining to form one Helium atom of atomic mass 4.004 What happens to the extra mass? ` Nuclear fuel: UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SOLAR & STELLAR PHYSICS GROUP Nuclear fuel Definition : 1 atomic mass unit is the equivalent of one twelfth of the mass of an atom of Carbon 12 or 1.67 x 10-27kg. We have four protons of atomic mass 1.008 combining to form one Helium atom of atomic mass 4.004 What happens to the extra mass? Converted to energy by For each Hydrogen nucleus 0.007 amu is converted to 1.05 x 10-12J Main Sequence: Main Sequence The longest interval of a star’s evolution during which Hydrogen is fused to Helium When H is converted to He in the core, the core temperature is the same in all parts (isothermal) Schonberg and Chadrasekhar showed that the mass of an isothermal region cannot exceed 10-15% of the total stellar mass. Slide11: Thus the total energy available is where X is the fraction of the total mass of the star that is hydrogen (0.73) and qsc(0.1-0.15) The energy E is radiated at a rate L (luminosity) for a time t. For the Sun Ms = 2 x 1030kg, L = 4 x 1026W, tms=7 x 109years What about other stars? Slide12: In fact So a 40 solar mass star has a main sequence lifetime of around a million years Big stars live fast and die young! Blackbody radiation: Blackbody radiation Stars can often be approximated as black bodies – the amount and wavelength (colour) of light they emit is directly related to temperature. The spectrum (amount of light emitted at each wavelength of a blackbody emitter is governed by Planck’s law; Blackbody radiation: Blackbody radiation At short wavelengths this can be approximated to At long wavelengths it can be approximated to The peak wavelength can be found by taking the first derivative of Planck curve (since at the peak ) this can be shown to be Where The Stephan – Boltzman law: The Stephan – Boltzman law Integrating under the curve we can obtain the total flux F emitted by a blackbody Where s is the Stephan – Boltzman constant ( ) Thus the total energy output (or luminosity) of a star is just the flux times the surface area The Herzsprung – Russell diagram: The Herzsprung – Russell diagram Heavier stars are more luminous and live for less time The more luminous a star is the hotter is is. The hotter it is the bluer the radiation. Putting it all together gives us the Herzsprung – Russell diagram. Question time!: Question time! A star is three times the mass of the Sun and has a radius of ~2 x 109 m can you work out; The main sequence lifetime The temperature The peak wavelength of emission What colour would you describe the peak wavelength as? What is the main sequence lifetime of a 0.1 solar mass star. If the universe is 13 billion years old, what does this imply? Question time!: Question time! A star is three times the mass of the Sun and has a radius of (~2 x 109 m) can you work out ; The main sequence lifetime (2.6 x 108 years) The temperature (~10 000K) The peak wavelength of emission (280 nm) What colour would you describe the peak wavelength as? (ultraviolet) What is the main sequence lifetime of a 0.1 solar mass star. If the universe is 13 billion years old, what does this imply? (7 x 1012 years – all the 0.1 solar mass stars that have ever existed still exist.) Why study the Sun?: Why study the Sun? The Sun is a prototype for all stars of it’s type – one of the most common in the Universe. The equations that govern the behaviour of the Sun also govern the behaviour of all magnetised plasma in the Universe. The Sun affects all of the planets in the solar system – most importantly our own There is a lot we still don’t understand! The structure of the Sun - overview: The structure of the Sun - overview The core and radiative zone: The core and radiative zone The core temperature is around 15 million degrees Kelvin. The pressure is around 200 billion atmospheres. The power plant of the star Energy transported outward by radiative diffusion. It takes 8 minutes for a photon to travel from the Sun to the Earth. How long does it take a photon to move from the interior of the Sun to the surface? The core and radiative zone: The core and radiative zone The core temperature is around 15 million degrees Kelvin. The pressure is around 200 billion atmospheres. The power plant of the star Energy transported outward by radiative diffusion. It takes 8 minutes for a photon to travel from the Sun to the Earth. How long does it take a photon to move from the interior of the Sun to the surface? Depending on who you ask, anything up to a million years! The convection zone.: The convection zone. From about 0.7 solar radii outwards the temperature decreases sharply from about 2 million K to about 6000 K. This temperature gradient is steep enough for convection to set in The combination of convection and differential rotation – the fact the Sun rotates faster at the equator than poles generates the Suns magnetic field – a process known as dynamo action The solar magnetic field: The solar magnetic field Most obvious manifestation of magnetic field we can see is the 11 year sunspot cycle Sunspots initially emerge at high latitudes moving closer to the equator as cycle progresses. This is known as Spörers law. A sunspot generally emerges as a magnetic bipole. The leading pole is always the same in each hemisphere but reversed from northern to southern hemisphere. After 11 years the polarity reverses and a full magnetic cycle is 22 years. This is known as Hales law. The Solar dynamo and the 11 year cycle: The Solar dynamo and the 11 year cycle Simplest model is known as the Babcock – Leighton model. Initially solar field is dipolar, like a bar magnet Differential rotation causes magnetic field lines to get wrapped around latitudinally. The solar dynamo and the 11 year cycle: The solar dynamo and the 11 year cycle Combination of rotation and convection causes the magnetic field to get twisted up into a ‘flux tube’ Total pressure is in equillibrium inside and outside magnetic field Temperature inside and outside magnetic field is the same. Thus density inside the field is lower and it rises buoyantly to the surface emerging as a magnetic bipole The Solar dynamo and the 11 year cycle: The Solar dynamo and the 11 year cycle Positive and negative polarities drift apart with the leading polarity moving towards the equator and the trailing polarity moving toward the pole. Trailing polarity is always opposite to polar field so contributes to large scale field cancellation and reversal. Sunspots: Sunspots Sunspots appear darker than the surrounding surface because they are cooler. Concentrated magnetic field inhibits convection of heat flux from solar interior Magnetic field in a sunspot is ~1000x stronger than the global solar magnetic field! The solar atmosphere: The solar atmosphere Above the photosphere the temperature begins to rise rapidly once again. At the top of the chromosphere the temperature is around 80 000K Within a few hundred kilometres (known as the transition region) this rises to around a million degrees in the corona. The solar corona: The solar corona Although the corona is much hotter than the visible Sun it is much less dense. Only emits about 1 millionth of the amount of visible light than the solar surface. Visible light from the corona is a result of solar radiation scattering from electrons in the corona. Coronal heating problem: Coronal heating problem The second law of thermodynamics states; “The entropy of an isolated system not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium.” In other (simpler!) words; “Heat cannot of itself pass from a colder to a hotter body.” So if the surface of the Sun is at a temperature of 6000 K and the corona is at a million K then where does the heat come from? If we look at the Sun and ultraviolet and X-ray wavelengths we begin to see… The coronal heating problem: The coronal heating problem The most intense emission is above sunspots where the magnetic field is strongest. The heating of the corona is caused (at least in active regions – the area above sunspots) by magnetic fields. Magnetic reconnection: Magnetic reconnection Magnetic field above sunspots gets twisted and tangled by motion at solar surface storing up energy. Occasionally the field snaps, releasing energy and altering it’s configuration. This is known as magnetic reconnection and it’s sometimes spectacular! Solar flares: Solar flares Dramatic and dynamic process Energy release can span many orders of magnitude from the smallest microflares to flares that go off the scale, 1017 - > 1025 joules. First flare observed by Carrington in 1859 in white light but much more intense in radio, ultraviolet, X-ray and gamma – rays. Solar flares: Solar flares Need to take a multi-wavelength approach to understanding solar flares. Observations with SOHO/LASCO show impressive coronal mass ejections. A billion tons of mass moving at a million miles an hour. If a flare/ CME is pointed towards earth it can have extreme consequences. The Sun – Earth connection: The Sun – Earth connection Aurorae on other planets: Aurorae on other planets More serious consequences: More serious consequences The best-known example of a space weather event is the collapse of the Quebec power network in1989 due to geomagnetically induced currents. The blackout lasted 9 hours and affected 6 million people. A Coronal Mass Ejection on the 7th of January 1997 hit the Earth's magnetosphere on the 10th of January and caused the loss of the AT&T Telstar 401 communication satellite (a $200 million value) Transpolar routes flown by airplanes are particularly sensitive to space weather. It is estimated to cost about $100,000 each time such a flight is diverted from a polar route. Nine airlines are currently operating polar routes. More serious consequences: More serious consequences No large Solar Energetic Particle event happened during a manned mission. However, such a large event happened in August 1972, between Apollo 16 and Apollo 17. Had this event happened during one of these missions the dose of radiation would probably have been deadly. Nozomi Mars Probe was hit by a large Solar Energetic Particle event in 2002 which caused large-scale failure and the mission to be abandoned. Global positioning systems and compasses can also be affected by solar storms New missions - Stereo: New missions - Stereo The Sun in 3-D Four instrument packages on each spacecraft. SECCHI - Sun Earth Connection and Heliospheric Investigation SWAVES - interplanetary radio burst tracker IMPACT – Measuring CME and particle parameters PLASTIC – Measuring characteristics of protons and ions New missions - Hinode: New missions - Hinode Hinode is the follow-up to the highly successful UK/US/Japanese Yohkoh mission. Launched September 2006 Designed to study interaction between Sun’s magnetic field and the corona to fulfil science aim of understanding solar variability – on many scales. Three instruments work as an observatory Solar Optical Telescope (SOT) X-Ray Telescope (XRT) Extreme Ultraviolet Imaging Spectrometer (EIS) The EIS instrument was built by an MSSL led consortium. Small group exercise: Small group exercise Given the effect that rotation has on the Sun’s magnetic field generation. What do you think the consequences of a more rapid rotation would be (some young solar type stars rotate in as little as nine hours). This year (2007) is solar minimum, the period in the 11 year cycle when the Sun is least active. NASA/ESA plans to send a manned mission to the Moon around 2020 and to Mars around 2030-2050. What years would be good years to go? The eventual aim of manned space-flight is to establish permanent colonies on the Moon and Mars. How would you go about protecting such colonies from solar radiation, given neither has a magnetosphere?