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Transcript
PH507 Astrophysics Dr Dirk Froebrich 1 The Sun: A Model Star Our Sun is the nearest star. The fascinating properties and phenomena of the solar surface layers are easily observed and have been studied intensely. Unfortunately, models for understanding solar phenomena have not kept pace with such detailed data. Because the Sun is a fairly typical star and because it is the only star that spans a large angular diameter as seen from the Earth, the discussion here serves as the physical basis to investigate the other stars. Sun Mass (1024 kg) 1,989,100. 6 3 2 GM (x 10 km /s ) 132,712. 12 3 Volume (10 km ) 1,412,000. Volumetric mean radius (km) 696,000. Mean density (kg/m3) 1408. 2 Surface gravity (eq.) (m/s ) 274. Escape velocity (km/s) 617.7 Ellipticity 0.00005 2 Moment of inertia (I/MR ) 0.059 Visual magnitude V -26.74 Absolute magnitude +4.83 24 Luminosity (10 J/s) 384.6 6 Mass conversion rate (10 kg/s) 4300. Mean energy production (10-3 J/kg) 0.1937 Surface emission (106 J/m2s) 63.29 Spectral type G2 V Earth 5.9736 0.3986 1.083 6371. 5515. 9.78 11.2 0.0034 0.3308 -3.86 (Sun/Earth) 333,000. 333,000. 1,304,000. 109.2 0.255 28.0 55.2 0.015 0.178 PH507 Astrophysics Dr Dirk Froebrich 2 Model values at center of Sun: Central pressure: 2.477 x 1011 bar Central temperature: 1.571 x 107 K Central density: 1.622 x 105 kg/m3 The Structure of the Sun The average density of the Sun is only 1400 kg/m3 - consistent with a composition of mostly gaseous hydrogen and helium. From its angular size of about 0.5° and its distance of almost 150 million kilometres, we determine that its diameter is 1,392,000 kilometres (109 Earth diameters and almost 10 times the size of the largest planet, Jupiter). All of the planets orbit the Sun because of its enormous gravity. It has about 333,000 times the Earth's mass and is over 1,000 times as massive as Jupiter. The Sun is made of 94% Hydrogen, 6% Helium, - the other elements make up just 0.13% (the three most abundant ‘metals’ Oxygen, Carbon, and Nitrogen make up 0.11%). The Sun’s atmosphere has the following layers (from innermost to outermost): o The photosphere is about 300 km thick. Most of the Sun's visible light that we see originates from this region. o The chromosphere is about 2000 km thick. We only see this layer and the other outer layers during an eclipse. o The corona extends outwards for more than a solar radius. The Photosphere An image of the Sun's Photosphere shows: Limb Darkening. Limb darkening is evidence that the temperature of the Sun's photosphere decreases outwards. Sunspots PH507 Astrophysics Dr Dirk Froebrich 3 The Sun's Spectrum is an Absorption Spectrum Since the photosphere is cooler and less dense than the interior region it allows the continuous blackbody spectrum to flow through it. Only at the wavelengths at which atoms in the photosphere can absorb light will photons be impeded in their outward travel. Sunspots: Sunspots are regions with high magnetic fields (1000 x higher magnetic field than average) Typical size of spots is similar to the size of the Earth. These regions are cooler (redder) than average, so they look darker than the surrounding hotter region. Sunspots are related to X-ray flares, mass ejections and the aurora seen on earth . PH507 Astrophysics Dr Dirk Froebrich 4 Close-up Picture of a group of Sunspots The darkest regions (umbra) have the largest magnetic fields and the coolest temperatures. The outer brighter region is the penumbra. Sunspots come in pairs: each member of the pair has opposite polarity. (I.e. one is a north magnetic pole, the other is south.) Each sunspot region lasts for a few days to a few weeks. The filaments in the penumbra are due to the magnetic lines of force. Movement of Sunspots Movements of spots reveal that the Sun rotates with a period close to one month. Equator rotates faster than the higher lattitudes. Differential Rotation You can find photos of the Sun in many different wavelengths (updated daily) at the website: http://umbra.nascom.nasa.gov/images/latest.html http://science.nasa.gov/ssl/pad/solar/surface.htm Granules Close-up Picture of the Photosphere Granules are the cell-like features seen on the Sun's photosphere that cover the entire solar surface, except for the sunspot regions. PH507 Astrophysics Dr Dirk Froebrich 5 The granules are the tops of convective cells which lie in the convective zone just below the photosphere. Each cell ranges in size from 100 km to 1000 km across and may last up to half an hour. The bright regions are zones where hot gas rises. They are the tops of deep gas columns where energy is transported by convection. Spectra of the centers of the granules shows these regions to be a few hundred Kelvin hotter than the surrounding darker lanes. The dark borders are the places where the cool gas sinks. The gas moves outwards or inwards at speeds up to 7 km/s. (Measured through Doppler shifts.). The Sun's Chromosphere A Solar Eclipse The photosphere is much brighter than the outer parts of the Sun's atmosphere (the chromosphere and the corona), so regular photos of the Sun do not show the outer atmosphere. During a solar eclipse the Moon blocks out the light from the photosphere and we can only see the light coming from the chromosphere and corona. The Chromosphere with a close-up of the spicules. The Chromosphere is not exactly a sphere: there are many spicules and prominences which jut outwards. PH507 Astrophysics Dr Dirk Froebrich 6 Magnetic fields help support the spicules and the prominences. The red colour results from the emission of Balmer-alpha photons: electrons jumping from the n=3 level to the n=2 level. The emission lines can only occur if the gas in the chromosphere is very hot and the density is very low. The chromosphere is hotter (but less dense) than the photosphere. In the spicules, which are best observed in H, gas is rising at about 20 to 25 km/s. Although spicules occupy less than 1% of the Sun’s surface area and have lifetimes of 15 minutes or less, they probably play a significant role in the mass balance of the chromosphere, corona, and solar wind, and occur in regions of enhanced magnetic fields Solar spicules, short-lived narrow jets of gas that typically last mere minutes, can be seen sprouting up from the solar chromosphere in this H image of the Sun. The spicules are the thin, dark, spikelike regions. They appear dark against the face of the Sun because they are cooler than the solar photosphere PH507 Astrophysics Dr Dirk Froebrich 7 Prominences Close-up picture of the chromosphere showing a prominence. The prominences are loops of gas which arch over sunspot regions. The quiescent prominences are very stable and can last weeks or months. Eruptive Prominences Some of the prominences will erupt, causing gas to be flung outwards. The gas travels outwards about 70,000 km in the course of a few hours. Prominences are more likely to erupt when the magnetic fields near the sunspots are changing. Variation of Temperature in the Sun's Atmosphere: PH507 Astrophysics Dr Dirk Froebrich 8 Photosphere: Temperature decreases outwards. o At bottom: T = 6400 K o At top: T = 4000 K Chromosphere: Temperature increases outwards. o At top: T = 10,000 K Transition Zone: Temperature shoots up to near 1 million K Corona: Temperatures increase to about 2 million K The source of this heat is not well understood. Current theories suggest that magnetic waves might transport energy from the convective zone to the corona. The Transition Zone The next picture shows the transition zone as seen through a filter which only sees the light coming from an electronic transition of Sulfur VI at temperatures of about 200,000ºC. Instead of hydrogen, the light emitted by the transition region is dominated by such ions as C IV, O IV, and Si IV (carbon, oxygen, and silicon each with three electrons stripped off). These ions emit light in the ultraviolet region of the solar spectrum that is only accessible from space. PH507 Astrophysics Dr Dirk Froebrich 9 These emission lines are Ultra-violet. which are only possible when the gas is very hot, near 100,000 K. The structures seen here are similar to those seen in the chromosphere. The Corona A visible light photograph of the Corona during a solar eclipse. PH507 Astrophysics Dr Dirk Froebrich 10 Photograph of the solar corona during the July, 1991 eclipse, at the peak of the sunspot cycle. At these times, the corona is much less regular and much more extended than at sunspot minimum. Astronomers believe that coronal heating is caused by surface activity on the Sun. The changing shape and size of the corona are the direct result of variations in prominence and flare activity over the course of the solar cycle. The Corona emits X-rays. This image corresponds to an electronic transition of highly ionized iron. (Iron stripped of 11 of its electrons.) PH507 Astrophysics Dr Dirk Froebrich 11 Iron can only lose 11 electrons and emit this X-ray light if the temperature is more than one million K. The dark regions are coronal holes which are lower density than average. The solar wind originates from the coronal holes. Coronal Loops Huge numbers of small, closely intertwined magnetic loops continuously emerge from the Sun's visible surface, clash with one another and dissolve within 40 hours. The loops seem to form a tight pattern that form a magnetic carpet. Their interaction generates electrical and magnetic short-circuits (magnetic reconnection) and releases enough energy to heat the corona to temperatures hundreds of times higher than those of the solar surface. PH507 Astrophysics Dr Dirk Froebrich 12 Coronal loops come in a variety of shapes and sizes, but most are enormous, capable of spanning several Earth's. (Photo: NASA and the TRACE team) PH507 Astrophysics Dr Dirk Froebrich 13 Solar Flares Solar flares are large outbursts similar to eruptive prominences, but larger and more energetic. Solar flares increase the amount of particles which escape into the solar wind. If the particles ejected from the flare hit the Earth, then we get intense auroral displays. A negative effect is that the solar wind particles can disrupt radio transmissions. Coronal Mass Ejection When an eruptive prominence or a solar flare occurs, a coronal mass ejection (CME) can also take place. A CME is a stream of plasma (charged particles) ejected from the corona. PH507 Astrophysics Dr Dirk Froebrich 14 The Solar Wind UV images show the flow of gas from the Sun. The solar wind is a stream of charged particles (protons and electrons) which flow outwards from the coronal holes. The wind speed is high (800 km/s) over coronal holes and low (300 km/s) over streamers. These high and low speed streams interact with each other and alternately pass by the Earth as the Sun rotates. The solar wind particles flow throughout the solar system. The variations buffet the Earth's magnetic field and can produce storms in the Earth's magnetosphere Further Evidence for the Solar Wind A comet's tail always points away from the Sun, no matter in what direction it moves. The particles in the solar wind push outwards on the gas sublimating from the comet so that the tail points away from the Sun. PH507 Astrophysics Dr Dirk Froebrich 15 Aurora When solar wind particles hit atoms in the Earth's atmosphere, they cause atoms excitation. The electrons in the excited atoms then jump down to a lower state and give off radiation. The green and red aurora are usually due to electronic transitions in Oxygen. There is almost always an auroral oval over the Earth's north and south magnetic poles. The size of the auroral oval and the intensity of the emissions depend on the strength of the solar wind. The Solar Cycle The number and location of sunspots and the severity and number of flares, eruptive prominences and coronal mass ejections are not constant in time. The number of sunspots (R=f+10*g) on the Sun varies on an 11-year cycle. The number increases from zero at solar minimum, to over 100 at solar maximum, 5 1/2 years later. Every 11 years the polarity of the Sun's magnetic field flips. Therefore, the Sun's magnetic field varies on a 22-year cycle. As the number of sunspots increases, the number of flares and other forms of activity increase. The luminosity of the Sun also increases when there are lots of spots! (Bright plages also increase when sunspots increase.) The time at which there is a maximum number of spots is called Solar Maximum. The most recent Solar Maximum occurred late in 2000. PH507 Astrophysics Dr Dirk Froebrich 16 The period of time from 1645 to 1715, known as the Maunder minimum, was a time with a very low number of sunspots. During the Maunder minimum Europe had colder than usual weather. PH507 Astrophysics Dr Dirk Froebrich 17 Predictions about the present sunspot cycle from http://science.nasa.gov/ssl/PAD/SOLAR/sunspots.htm. Polarity of Sunspots The most important characteristic of a sunspot is its magnetic field. The magnetic field in a typical sunspot is about 1000 times greater than the field in neighbouring, undisturbed photospheric regions. Typical field strengths are near 0.1 T, but fields as strong as 0.4 T have been measured. Related to the magnetic field is a horizontal flow of gas in the sunspot penumbra: gas moves out along the lower filaments and inward along the higher filaments (at speeds up to 6 km/s). In each sunspot pair, one sunspot is a north pole and the other is a south pole. Note that in the northern hemisphere of the Sun, the north poles are always to the right of the south poles. In the southern hemisphere, the south poles are always to the right of the north poles. Location of Sunspots At the beginning of each 11-year cycle the sunspots appear at middle latitudes above and below the Sun's equator. Very few sunspots are ever found at latitudes greater than ±40. As the cycle progresses, the sunspots appear closer to the equator . Most spots are near ±15 at maximum, and the few spots at the end of the cycle cluster near ±8. The cycles overlap at the time of sunspot cycle minimum with old cycle spots near the equator and new cycle spots at high latitudes. PH507 Astrophysics Dr Dirk Froebrich A sunspot dies at the same latitude where it was born 18 PH507 Astrophysics Dr Dirk Froebrich 19 The sunspots follow active latitude belts during the course of a cycle. Less clear, but just as tantalising, large active regions and spot groups seem to fall into preferred longitudes during a cycle - active longitude belts. Concentrations of magnetic fields appear to persist below the photosphere, so that new spot groups arise from about the same locations as previous ones. The active longitude belts sometimes appear about 180 apart This plot is known as a "butterfly diagram". It shows the number and position of sunspots over time. PH507 Astrophysics Dr Dirk Froebrich 20 At the end of the 11 year cycle the opposite polarity sunspots at the equator "cancel" each other out and the Sun's polarity flips. Flux Tube Model for the Sun's Magnetic Activity The magnetic field lines get "frozen" into the gas of the Sun and get dragged around when the gas moves. The differential rotation of the Sun drags the magnetic field lines into "horizontal" tubes which break out of the surface, causing sunspots with the observed polarity.