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Walking on the Sun by Smash Mouth Donna Kubik Spring, 2006.

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1 Walking on the Sun by Smash Mouth Donna Kubik Spring, 2006

2 Walking on what? What is the surface of the sun? –Although astronomers talk about the surface of the Sun, the Sun is so hot that it has no liquid nor solid material –It is comprised totally of gas that gets denser and denser toward the center –The Sun contains >99.85% of the total mass of the solar system

3 Walking on the photosphere? The Sun appears to have a surface only because most of its visible light comes from one specific layer, called the photosphere. The photosphere is the lowest of 3 layers comprising the Sun’s atmosphere Because the upper 2 layers are transparent to most wavelengths of visible light, we see through them down to the photosphere. We cannot see through the photosphere, so everything below the photosphere is called the Sun’s interior

4 Solar atmosphere –Photosphere –Chromosphere –Corona

5 Photosphere 400 km thick 5800K

6 Sun appears darker near the edges This is called limb darkening

7 At the edges, we are looking through more of the cooler atmosphere, so there is more absorption of the photons from the hottest (innermost) part of the photosphere In the center we can receive more photons from the hotter part of the photosphere 5800 K 4000 K

8 Granulation –Granulation is the fine grain structure of the photosphere. –Individual granules are about 1000 km across. –The granulation is constantly changing, usually over time scales of minutes or less.

9 Granulation Each granule is a convective cell which consists of a bright, roughly-polygonal area of hot rising gas, and a cooler edge of descending gas The rising and descending is determined via Doppler shift of spectral lines Convection in photosphere Darker because coolerBrighter because hotter

10 Granulation The energy (E) and temperature (T) according to Stephan- Boltzman law: E~T 4 So more photons per area emitted from hot regions Convection in photosphere Darker because coolerBrighter because hotter

11 Chromosphere 2000 km thick 4000K

12 The EUV Sun The name, chromosphere “sphere of color” is misleading The name suggests it is the layer we normally see But the chromosphere’s light is swamped by that of the photosphere

13 The EUV Sun The chromosphere is only visible when the photosphere is blocked, as during a total solar eclipse, or when viewed at nonvisible wavelengths that the chromosphere is especially bright (as EUV), or when viewed through a filter (H  ) that blocks most of the photosphere’s light

14 The EUV Sun Image taken at EUV wavelengths by SOHO (Solar and Heliospheric Observatory) operated by ESA and NASA. The UV light originates from the lower regions of the chromosphere These wavelengths also indicate active regions.

15 Supergranulation The dark graininess seen in the image is due to supergranulation. Supergranules contain ~ 900 granules Typical diameter of a supergranule is slightly larger than the earth’s diameter The source of this light is the chromosphere

16 Spicules High resolution images of the chromosphere, taken through an H  filter, reveal numerous spikes, which are jets of gas called spicules Spicules are usually located on the edge of supergranules Spicules rise for several minutes at 45,000mph to a height of ~10,000km SpiculesSupergranules

17 Spicules The image shows spicules on the limb of the Sun as imaged by the Big Bear Solar Observatory. It shows a superposition of 11 limb images taken at different wavelengths

18 Sunspots inhibit formation of supergranules In these photos taken at the same time, there are no supergranules where there are sunspots.

19 Solar observatories The Big Bear Solar Observatory is located in the middle of Big Bear Lake (in CA) to reduce the image distortion which usually occurs when the Sun heats the ground and produces convection in the air just above the ground Turbulent motions in the air near the observatory are also reduced by the smooth flow of the wind across the lake instead of the turbulent flow that occurs over mountain peaks and forests. Big Bear Solar Observatory

20 Solar observatories In addition to the atmospheric effects, solar telescopes suffer from heating by sunlight of the optics and the air within the telescope tube. This causes the image to shiver and become blurred. Modern solar telescopes are either vacuum telescopes, filled with helium or use careful control of the optic's temperature to reduce heating of the air in the telescope. Big Bear Solar Observatory telescopes

21 Solar observatories The 65 cm and 25 cm telescopes are evacuated to avoid air turbulence inside the telescope tubes caused by the solar beam heating air molecules. Special white paint used inside and outside the observatory diffusely reflects sunlight and radiates heat away to reduce turbulence due to solar heating. Big Bear Solar Observatory telescopes

22 Solar observatories Some solar telescopes look very different from other optical telescopes Kitt Peak National Observatory

23 Solar observatories Close to the ground the heating effect of the Sun causes a layer of hot, turbulent air, which makes images formed by mirrors near the ground unsteady, so the first mirror (heliostat) is often placed on a tall tower

24 Solar towers National Solar Observatory Sunspot, New Mexico National Solar Observatory Kitt Peak, Arizona

25 The Radio Sun Radio image taken from Japan’s Nobeyama Radio Observatory The most active regions are the most luminous. The radio image provides information about the transition region between the chromosphere and corona.

26 Corona Several million km thick ~1-2 million K

27 Corona viewed during eclipse The glow of the corona is a million times less bright than that of the photosphere Like the chromosphere, the corona can only be seen when the photosphere is blocked by special filters, at non-visible wavelengths at which the corona is especially bright, or when the disk of the Sun is blocked during a total solar eclipse…….

28 Coronagraph …..or by using a special instrument called a coronagraph that artificially blocks the disk of the Sun so that it can image the region surrounding the photosphere

29 Coronagraph –A very common way to observe the corona is to cover the bright disk of the Sun. –This creates a sort of mini- eclipse and allows us to see the Sun's fainter outer atmosphere

30 What can you see with a coronagraph? Streamers are structures formed by the Sun's magnetic field. They can last for months. Sometimes streamers go unstable and erupt in huge magnetic bubbles of plasma known as coronal mass ejections (or CMEs) that blow out from the Sun's corona and travel through space at high speed.

31 Corona It seems that the temperature should decrease as one rises through the Sun’s atmosphere (moving away from the apparent heat source It does decrease from the photosphere (~5800K) to the chromosphere (~4000K), but then it rises to much higher temps in the corona (1-2 million K)!

32 Corona Unexpected increase in temperature was discovered in about 1940 when Fe XIV (an iron atom stripped of 13 e-) was detected in the spectrum of the corona Takes lots of energy to strip so many electrons from an atom, so the corona must be very hot

33 Corona Astronomers have mounting evidence that the corona is heated by energy carried aloft and released there by the Sun’s complex magnetic fields (more on that later)

34 TRACE TRACE (Transition Region and Coronal Explorer) is a NASA space telescope designed to provide high resolution images and observation of the photosphere and transition region to the corona. The satellite, launched in April 1998

35 TRACE The telescope is designed to take images in a range of wavelengths from visible light, through the Lyman alpha line to far ultraviolet. The different wavelength passbands correspond to plasma emission temperatures from 4,000 to 4,000,000 K. Sun-synchronous (98°) orbit of 600×650 km.

36 Sun-synchronous orbit Sun-synchronous (98°) orbit of 600×650 km. This type of orbit is designed to keep the satellite in full sun light for nine months a year. The orbit moves the satellite to the west at the exact same rate that the sun appears to move across the Earth's surface.

37 Corona If the temperature is to high, why doesn’t the corona outshine the photosphere? According to according to Stephan-Boltzman law: E~T 4 So more photons should be emitted per area emitted from hot regions!? But the density of the corona is very, very, very low, otherwise it would outshine the photosphere!

38 Solar wind The Sun’s gravity keeps most of its atmosphere from escaping to space, but some of the gas in the corona is moving fast enough to escape. This is the solar wind

39 Space weather The solar wind is one aspect of space weather You can view the current space weather conditions and the space weather forecast at SPACE WEATHER Current Conditions Solar Wind speed: km/s density: 2.5 protons/cm 3

40 Solar wind The solar wind is comprised mostly of hydrogen and helium nuclei Hydrogen nuclei are protons SPACE WEATHER Current Conditions Solar Wind speed: km/s density: 2.5 protons/cm 3

41 Solar wind The solar wind particles reach speeds up to 805 km/s The wind achieves these high speeds in part by being accelerated by the Sun’s magnetic field SPACE WEATHER Current Conditions Solar Wind speed: km/s density: 2.5 protons/cm 3

42 Solar wind The Sun ejects a million tons of matter each second Even at this rate of emission, the mass loss due to the solar wind will amount to only a few tenths of a percent of the Sun’s total mass throughout its lifetime SPACE WEATHER Current Conditions Solar Wind speed: km/s density: 2.5 protons/cm 3

43 SOHO –SOHO (Solar and Heliospheric Observatory), operated by NASA and ESA, is designed to study the internal structure of the Sun, its extensive outer atmosphere and the origin of the solar wind, the stream of highly ionized gas that blows continuously outward through the Solar System. SOHO orbit is sunward of Earth Not to scale

44 SOHO –All previous solar observatories have orbited the Earth, from where their observations were periodically interrupted as our planet `eclipsed' the Sun. –A continuous view of the Sun is achieved by operating SOHO from a permanent vantage point 1.5 million kilometers sunward of the Earth SOHO orbit is sunward of Earth Not to scale

45 Lagrange points The Italian-French mathematician Joseph-Louis Lagrange discovered five special points in the vicinity of two orbiting masses where a third, smaller mass can orbit at a fixed distance from the larger masses.

46 Lagrange points Of the five Lagrange points, three are unstable and two are stable. The unstable Lagrange points - labeled L1, L2 and L3 - lie along the line connecting the two large masses. The stable Lagrange points - labeled L4 and L5 - form the apex of two equilateral triangles that have the large masses at their vertices. SOHO

47 Lagrange points The L1 point of the Earth-Sun system provide an uninterrupted view of the sun and is the location of SOHO SOHO

48 Lagrange points The L2 point of the Earth-Sun system is the location of WMAP and (perhaps by the year 2011) the James Webb Space Telescope. The L1 and L2 points are unstable on a time scale of approximately 23 days, which requires satellites parked at these positions to undergo regular course and attitude corrections.

49 The quiet Sun vs. the active Sun Quiet Sun –Granules, supergranules, spicules, and the solar wind occur continuously. They are features of the quiet Sun. Active Sun –But the Sun’s atmosphere is periodically disrupted by magnetic fields that stir things up, creating the active Sun.

50 Solar magnetic field In contrast to the Earth, the Sun has a very weak overall magnetic field (average dipole field). However, the solar surface has very strong and tremendously complicated magnetic fields. Because the surface magnetic fields are so complex, solar astronomers use computers to simulate the Sun's magnetic fields.

51 Solar magnetic field It is the dynamics of the Sun’s magnetic fields that is thought to cause many of the features of the active Sun

52 Discovery of sunspots Sunspots are one of the features of the active Sun Galileo looked at the Sun through his telescope One should NEVER look directly through a telescope at the Sun This caused Galileo to suffer from partial blindness.

53 Discovery of sunspots Galileo did see spots on the Sun These were sunspots This animation shows a sequence of drawings made by Galileo as he observed the Sun from June 2nd to 26th, 1612.

54 Sunspots are cooler spots A typical sunspot is 10,000 km across and lasts between a few hours and a few moths It is comprised of two parts –The dark, central region is called the umbra –The brighter ring around it is called the prenumbra UmbraPrenumbra

55 Sunspots are cooler spots Seen without the surrounding very bright granules that outshine it, the umbra appears red and the penumbra orange From Wein’s Law max =0.0029/T –The orange umbra is ~4300 K –The red penumbra is ~5000 K –Both are cooler than the surrounding 5800 K photosphere UmbraPrenumbra

56 Zeeman effect In 1908, George Ellery Hale discovered that sunspots are directly linked to magnetic fields When he observed the spectra from sunlight coming from a sunspot, he found that each spectral line in the normal solar spectrum was flanked by additional, closely-spaced spectal lines not usually observed

57 Zeeman effect This “splitting” of a single spectral line into two or more lines is called the Zeeman effect Pieter Zeeman first observed such splitting in the laboratory in 1896

58 Zeeman effect Zeeman showed that an intense magnetic field splits the lines of a light source if the source is inside the field The more intense the magnetic field, the more the split lines are separated

59 Sunspots The intense magnetic field below a sunspot strangles the normal up-flow of energy from the hot solar interior, leaving the spot cooler and therefore darker than its surroundings

60 Sunspots The suppression of the bubbling convective motions forms a kind of plug that prevents some of the energy in the interior from reaching the surface. As a result, the material above the plug cools and becomes denser, causing it to plunge downward at up to 3,000 miles per hour, according to new observations from SOHO

61 Sunspots This time-lapse movie shows in five seconds what happens in 20 minutes on the Sun's surface near a sunspot. This sunspot measured about 25,000 kilometers across. Visible is boiling granulation outside the sunspot, inward motion of bright grains in the outer penumbral region toward the sunspot, and the churning of small magnetic elements between solar granules.

62 Sunspots Sunspots themselves are relatively cool regions of the solar surface depressed by magnetic fields. The dark lanes surrounding the sunspot are called penumbral filaments, and recent computer simulations have shown that their behavior is also dominated by magnetic fields. The movie was taken with the Dutch Open Telescope

63 Sunspots Sunspots reveal –The solar cycle –The Sun’s rotation

64 Differential rotation Sun rotates differentially 25 days for one rotation at equator 27 days at latitude 30 deg 33 days at latitude 75 deg 35 days near poles

65 The Solar Cycle Sunspot maximum and minimum occur on 11-year cycle Orientation of the Sun’s magnetic field flips every 11 years Solar cycle is ~22 years

66 Butterfly diagram Sunspots do not appear at random locations over the surface of the sun but are concentrated in two latitude bands on either side of the equator.

67 Butterfly diagram A butterfly diagram showing the positions of the spots for each rotation of the sun since May 1874 shows that these bands first form at mid- latitudes, widen, and then move toward the equator as each cycle progresses

68 The Solar Cycle Prominences, flares, and plages vary with the same 11-year cycle as sunspots Coronal mass ejections, the major source of hazardous particles from the Sun, occur with varying frequency, but never totally cease.

69 Filaments, plages, and prominences Hotter, therefore brighter, regions in chromosphere Created by magnetic fields under the photosphere just before they they emerge Prominences are filaments viewed from the side All associated with sunspots Filaments Plages Prominences

70 Filaments, plages, and prominences This image of 1,000,000K gas in the Sun's thin, outer atmosphere indicates ionized iron at 171 Å The loops of energized particles clearly follow magnetic field lines around an active region.

71 Filaments, plages, and prominences This image of 1-million degrees Kelvin gas in the Sun's thin, outer atmosphere which detects ionized iron here at 171 Å The loops of energized particles clearly follow magnetic field lines around an active region. Compare loops and prominences (left) to models of Solar magnetic fields (right)

72 The x-ray Sun This x-ray image was obtained by the Japanese observatory Yohkoh (Sunbeam), a collaborative effort with the US and UK. The x-rays originate from the Sun’s corona.

73 Yohkoh –The Japanese satellite, known as Yohkoh ("Sunbeam"), a cooperative mission of Japan, the USA, and the UK, was launched in 1991 (ended operation in 2005) –The scientific objective has been to observe the energetic phenomena taking place on the Sun, specifically solar flares in x-ray and gamma-ray emissions

74 The x-ray Sun The brightest regions correspond to violent solar flares which send high energy particles to Earth. Darker regions denote cooler areas which are called coronal holes, because gases can escape this region.

75 Solar flares in the chromosphere Violent eruptive events, solar flares, send out vast quantities of high-energy particles as well as x-rays and UV radiation. Lots of flares at sunspot maximum Can last for hours

76 Solar flares vs. prominences Solar flares are more sudden and violent events than prominences. While they are thought to also be the result of magnetic kinks, flares do not show the arcing or looping pattern characteristic of prominences Flares Prominences (which are filaments viewed from the side)

77 Solar flares vs. prominences Flares are explosions of incredible power, rising local temperatures to 100,000,000 K Prominences release their energy over days or week, while flares release their energy in minutes or hours Flares Prominences (which are filaments viewed from the side)

78 Coronal mass ejections In the foreground of the 15 degree wide field of view, a bubble of hot plasma, called a coronal mass ejection Can alter the Sun’s magnetic field Often associated with solar flares Image from SOHO using coronagraph

79 Coronal mass ejections Another Image of a CME from SOHO using coronagraph

80 Effect on Earth Some coronal mass ejections, solar flares, and prominences head toward Earth Takes 8 minutes for radiation to arrive Takes a few days for particles to arrive Can produce aurora Can disrupt communications

81 Maunder Minimum There are irregularities in the cycles Sometimes one pole reverses before the other Sometimes no sunspots for decades (as from , Maunder Minimum)

82 Solar cycle predictions From SCIENCE VOL MARCH 2006 Researchers at the National Center for Atmospheric Research (NCAR) predict that the next peak in sunspots will come a little late but will be far bigger than the last peak— bigger, in fact, than all but one of the 12 solar maxima since 1880.

83 Solar cycle predictions They found that it takes a good 20 years for the magnetic remnants of past sunspots to recirculate deep into the interior, where the twisting action of the sun’s rotation amplifies them, and to rise back to the surface near the equator as the next cycle’s sunspots.

84 Solar cycle predictions The model did an impressively accurate job “hindcasting” the size and timing of past cycles. That track record made researchers confident that the next solar cycle will be 30% to 50% stronger than the last solar cycle. The next cycle will begin 6 to 12 month later than average

85 Where does the Sun’s energy come from? We see hot gas, intense magnetic fields, and the many features of both the quiet and active sun Where does the energy come from? Can’t come from the hot gas or magnetic fields; they have no mechanism to create energy

86 Where does the Sun’s energy come from? In 1905, Einstein showed that mass can be converted into energy: E=mc 2 In 1920’s, Eddington proposed that temperatures in the core of the sun are high enough to fuse H to He. In this reaction, a tiny amount of mass is lost. This mass is transformed into a very large amount of energy – the energy of the Sun

87 Thermonuclear fusion Mass of 4 H atoms = x kg - Mass of 1 He atom = x kg Mass lost = x kg E = mc 2 E = (0.048 x kg)(3x 10 8 m/s) 2 E = 4.3 x Joules 4H  He + neutrinos + gamma rays

88 Sources of the Sun’s energy The energy generated by hydrogen fusion is the Sun’s core eventually escapers through the photosphere into space That energy makes the sun shine

89 Where does the Sun’s energy come from? There are 2 ways stars convert H to He –Proton-proton chain –CNO cycle Both yield the same results 4H  He + energy

90 Where does the Sun’s energy come from? For stars with masses not greater than the Sun’s, the core temperature does not exceed 16 million K, so the proton- proton chain dominates For stars more massive than the sun, the core temperature is greater than 16 million K, and hydrogen burning occurs mainly via the CNO cycle

91 Sources of the Sun’s energy ~98.5% of the Sun’s energy comes from the p-p chain ~1.5% of the energy comes from the CNO cycle

92 Proton-proton chain Hydrogen fusion - converts hydrogen to helium Possible because of high temperature and pressure in the Sun’s core Mass of 4 H > Mass of 1 He Results in 4H He + neutrinos + gamma rays The gamma rays balance inward force of gravity

93 Proton-proton chain There are 4 branches of the proton-proton chain The one below produces 85% of the Sun’s energy In the other 3 branches, the 3 He nucleus follows a different fate Neutrinos are produced by all branches Physicists want to study these neutrinos

94 CNO cycle The initial reaction involves a carbon nucleus (with 6 protons) and a hydrogen nucleus (1 proton) Because of the large electrical charge of the carbon nucleus, there is a stronger electrical repulsion Therefore a higher temperature is needed in order for the reaction to take place

95 CNO cycle Since the CNO cycle recovers the original C nucleus, the carbon, nitrogen, and oxygen are unaffected, in net, by the reactions So it could start anywhere in the cycle with the addition of one proton to any of the carbon or nitrogen nuclei

96 CNO cycle Consequently, this cycle is often called the CN cycle (as They Might Be Giants call it!)

97 Why Does the Sun Shine? –THEY MIGHT BE GIANTS The sun is a mass of incandescent gas A gigantic nuclear furnace Where hydrogen is built into helium At a temperature of millions of degrees Yo ho, it's hot, the sun is not A place where we could live But here on Earth there'd be no life Without the light it gives We need its light We need its heat We need its energy Without the sun, without a doubt There'd be no you and me The sun is a mass of incandescent gas A gigantic nuclear furnace Where hydrogen is built into helium At a temperature of millions of degrees The sun is hot It is so hot that everything on it is a gas: iron, copper, aluminum, and many others. The sun is large If the sun were hollow, a million Earths could fit inside. And yet, the sun is only a middle- sized star. The sun is far away About 93 million miles away, and that's why it looks so small. And even when it's out of sight The sun shines night and day The sun gives heat The sun gives light The sunlight that we see The sunlight comes from our own sun's Atomic energy Scientists have found that the sun is a huge atom-smashing machine. The heat and light of the sun come from the nuclear reactions of hydrogen, carbon, nitrogen, and helium. The sun is a mass of incandescent gas A gigantic nuclear furnace Where hydrogen is built into helium At a temperature of millions of degrees CN cycle! p-p chain!

98 Direct observation of nuclear processes in the Sun Since the sequence of events, the variety of reactions, and the number of assumptions are so numerous, direct verification of the postulated nuclear reaction is desirable The most promising observations would involve measuring the neutrinos emitted in the nuclear reactions Neutrinos can easily escape the Sun If astronomers could detect these neutrinos, they would have a means of probing the reactions occurring at Sun’s core.

99 Direct observation of nuclear processes in the Sun Early attempt by Ray Davis and colleagues Looked for neutrinos via – 37 Cl +  37 Ar +e- – 37 Ar is radioactive and decays emitting an x-ray which can be recorded

100 Missing neutrinos The 37 Cl neutrino detector is a tank containing 100,000 gallons of perchloroethylene in the cavity 4,850 feet below ground in the Homestake Mine in Lead, S.D

101 Missing neutrinos But only 1/3 of the expected number of neutrinos from the Sun were detected Maybe there are 3 kinds of neutrinos that can change into one another If so, then Ray Davis’ detector would only have detected 1/3 of the expected number of neutrinos emitted by the Sun

102 Neutrino oscillations Changing from on type of neutrino to another is called neutrino oscillation If neutrinos oscillate, it implies neutrinos have mass

103 Super-Kamiokande Neutrinos can interact in water and give rise to Cherenkov light The Cherenkov light provides information about the neutrino energy, direction, and type This light can be detected by phototubes lining the inside of the Super-Kamiokande detector which is filled with water In 1998, Super-K determined that neutrinos produced in the atmosphere by cosmic rays do oscillate Super-K

104 Neutrino detectors In 2001, evidence of oscillation of solar neutrinos was found in the combined data from Super-K and the Sudbury Neutrino Observatory (SNO) Since then, these oscillations have been confirmed using man-made neutrino beams SNO

105 Missing neutrinos get 2002 Nobel Prize Raymond Davis Jr.Masatoshi Koshiba Riccardo Giacconi 1/4 of the prize 1/2 of the prize USA Japan USA University of Pennsylvania Philadelphia, PA, USA University of Tokyo Tokyo, Japan Associated Universities Inc. Washington, DC, USA b. 1914b b (in Genoa, Italy) "for pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos" "for pioneering contributions to astrophysics, which have led to the discovery of cosmic X-ray sources"

106 Missing neutrinos get 2002 Nobel Prize Raymond Davis Jr constructed a completely new detector, a gigantic tank filled with 600 tons of perchloroethylene, which was placed in a mine. When a neutrino hits a Cl atom, it can convert one of its neutrons into a proton, creating a radioactive atom of Ar. By measuring the amount of Ar produced, one can infer how many neutrinos were detected. Over a period of 30 years he succeeded in capturing a total of 2,000 neutrinos from the Sun and was thus able to prove that fusion provided the energy from the Sun. With another gigantic detector, called Kamiokande, a group of researchers led by Masatoshi Koshiba was able to confirm Davis’s results.

107 Solar model How does the energy produced in the core of the Sun get to us? By combining theoretical modeling of the Sun’s interior with observations of the energy that the sun produces, astronomers have created the standard solar model This is a mathematically-based picture of the structure of the Sun. The model seeks to explain both the observable properties of the Sun and the properties of the unobservable interior

108 Solar model There are 3 methods of energy transport –Conduction –Convection –Radiation

109 Solar model Experience tells us that energy always flows from hot regions to cooler ones The efficiency of this method, called conduction, varies significantly from one substance to another Conduction is not an efficient means of energy transport inside stars like the Sun It is important in compact stars like white dwarfs (later lecture)

110 Solar Model In the Sun, energy moves from the center to the surface by convection and radiative diffusion –Convection zone –Radiation zone

111 Solar Model Core –At the very high temperatures of the core, all matter is completely ionized (stripped of its electrons). –Photons move slowly out of the core into the next layer of the sun’s interior, the radiation zone

112 Solar Model Radiative zone –Here the temperature is a bit lower, and the photons emitted from the core of the Sun interact continuously with the charged particles located there, being absorbed and re-emitted –This is called radiative transport –This occurs 80% of the way out to photosphere in the radiative zone

113 Solar Model Convection zone –While the photons remain in the radiative zone, heating it and losing energy, some of the energy escapes into the convection zone.

114 Solar Model Convection zone –Here hot gases rise to the photosphere and cooling gasses sink back into the convection zone –Convection cells become smaller and smaller, eventually becoming visible as granules at the solar surface (photosphere).

115 Solar model –At the Sun’s surface, a variety of processes give rise to the electromagnetic radiation that we detect from Earth. Atoms and molecules in the photosphere absorb some of the photons at particular wavelengths giving rise to the Sun’s absorption-line spectrum –Given the temperature of the Sun, most of the radiation is emitted in the visible part of the spectrum, in agreement with the blackbody curve for a body at that temperature

116 Helioseismology The sun vibrates; discovered in 1960 Can study these vibrations to learn about the interior. Learned that the convective zone is twice as thick as first thought Below the convective zone, the Sun rotates as a rigid body (not differentially)

117 Helioseismology With a technique that uses ripples on the Sun's visible surface to probe its interior, SOHO scientists have, for the first time, imaged solar storm regions on the far side of the Sun, the side facing away from the Earth. The new technique, which uses the Michelson Doppler Imager (MDI) instrument on SOHO, gives a warning of storms by creating a window to the far side of the Sun.

118 The Sun's motion around the Galaxy’s center Using the VLBA, astronomers plotted the motion of the Milky Way and found that the Sun is orbiting the Galaxy at about 135 miles per second. Used the motion of Sagittarius A* relative to distant quasars 135 miles/sec Sgr A*

119 The Sun's motion around the Galaxy’s center The spiral arms extend in a direction opposite to our motion At the moment, the motion of the Sun is toward the constellation of Hercules 135 miles/sec Sgr A*

120 The Sun's motion around the Galaxy’s center It takes the Sun 226 million years to orbit the Galaxy The last time the Sun was at this spot of its Galactic orbit, dinosaurs ruled the world The period of time is called a cosmic year The Sun has orbited the Galaxy about 20 times during its 5 billion year lifetime 135 miles/sec Sgr A*

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