Notes for HCDE Workshop on Sun and Seasons Feb. 4, 2009

Notes for HCDE Workshop on Sun and Seasons Feb. 4, 2009
Welcome! Sun and Seasons Revised for Harris County Dept of Education Workshop, Feb. 3., 2009, by Christine Shupla, Photo from Photo from Created by the Lunar and Planetary Institute For Educational Use Only LPI is not responsible for the ways in which this powerpoint may be used or altered. Lunar and Planetary Institute

What are we going to cover
Notes for HCDE Workshop on Sun and Seasons Feb. 4, 2009 What are we going to cover Properties of the Sun Influence on Earth: Gravity Light Solar wind Life cycle of the Sun Seasons Photo from Photo from Lunar and Planetary Institute

The Sun Is a star Made of gases Is our primary source of energy 70% hydrogen and 28% helium Light (radiation) The Nine Planets website has all of this excellent information: The Sun contains 99.8% of all of the mass in our Solar System. It is a yellow, medium-temperature star– 9,981°F (5527°C) on the outside and 28 million°F (15 million°C) in the center. Image at Lunar and Planetary Institute

How Big is the Sun? Activity: Let’s measure the Sun Our Sun is 865 thousand miles (1,390,000 km) wide. Even a medium-sized sunspot is as big at the Earth! Photo from Lunar and Planetary Institute

How Big is the Sun? About 110 times wider than Earth Or 1.3 million times bigger than Earth Our Sun is 865 thousand miles (1,390,000 km) wide. Even a medium-sized sunspot is as big at the Earth! Photo from Photo from Lunar and Planetary Institute

How does our Sun compare to other Stars?
Notes for HCDE Workshop on Sun and Seasons Feb. 4, 2009 How does our Sun compare to other Stars? Active stars range in size from supergiants to dwarfs Stars range from very bright (supergiants) to very dim (dwarfs) Stars range from very hot blue on the outside (O class) to cool red on the outside (M class) Our Sun is a dwarf—medium mass Our Sun is a medium-bright dwarf Our Sun is in-between--yellow Supergiants can be hundreds to thousands of times bigger than our Sun, and possibly up to 100 times more massive than our Sun. The smallest stars are about 1/100 of the Sun’s mass. Temperature ranges for the outside of a star (spectral classification): O (blue), B (blue-white), A (white), F (white-yellow), G (yellow), K (orange), M (red) Very large stars can be very hot on the outside (O class) or cool on the outside (all the way down to M class)—all large stars are extremely hot in their cores. All small active stars (excludes remaining cores of old stars) are cooler on the outside and in their cores, compared to the most massive stars. Lunar and Planetary Institute

So is our Sun an average star?
Notes for HCDE Workshop on Sun and Seasons Feb. 4, 2009 So is our Sun an average star? No—most stars are smaller and cooler than our Sun BUT Most of the bright stars we see are bigger and hotter For more details and a graph, go to Nick Strobel’s Astronomy Notes: Lunar and Planetary Institute

Rotation High cadence solar rotation, EIT 195Š(Dec , 1999) Movie at High cadence solar rotation, EIT 195Š(Dec , 1999) Movie at At the equator, the Sun rotates once every 25.4 days Near its poles, the Sun rotates once every 36 days Known as “differential rotation” Lunar and Planetary Institute

Sun’s Magnetic Field Winds up due to differential rotation Eventually forms loops and becomes tangled Animation of how the Sun's magnetic field winds up and loops out. Movie at Animation of how the Sun's magnetic field winds up and loops out. Movie at Lunar and Planetary Institute

Inside the Sun Core Radiative Zone Convection zone Image at Core .25 of the Sun’s radius H fusing to form He Radiative Zone .25 to .85 of Sun’s radius Energy radiated from one particle to the next Interface Layer Change in fluid flow generates Magnetic field Convection zone .85 to 1.0 of Sun’s radius Gases rising in circulation cells Radiation is formed in the core from nuclear fusion—hydrogen atoms fusing to form helium The radiation escapes from the core and bounces around in the radiative layer, with atoms absorbing the radiation then re-releasing it in random directions. The radiation moves more quickly through the convection layer, as the atoms are being circulated up to the surface in circulation cells. Finally, when the radiation reaches the bottom of the atmosphere (the photosphere) it is emitted into space, primarily as visible light. The radiation takes thousands to millions of years to travel from the core to the Sun’s atmosphere. It only takes 8 minutes to reach us on Earth. Image at Lunar and Planetary Institute

The Sun’s Atmosphere Photosphere Chromosphere Corona Photosphere image and information: Chromosphere image and information: Corona information: Corona image from Go to for additional information Photosphere image: Chromosphere image: Corona image: Lunar and Planetary Institute

Energy from the Sun Nuclear chain reaction (hydrogen forming helium) Releases radiation (gamma rays) The gamma ray loses energy as it bounces around inside the Sun It is finally released at the photosphere, primarily as visible light Information and images at . Image at Lunar and Planetary Institute

Features in the Photosphere
Notes for HCDE Workshop on Sun and Seasons Feb. 4, 2009 Features in the Photosphere Sunspots Dark and small (but brighter than Full Moon and big as Earth) Cool-- temperatures only 6,200 F (Sun’s surface is 10,000 F) Associated with magnetic fields: one set of spots is positive, other is negative Image at Information at Image at Lunar and Planetary Institute

More on Sunspots Our Sun has an activity cycle of 11 years Sunspots appear at specific latitudes on Sun Bands of latitude move towards equator during cycle Basic information and image at Sunspot cycle and detailed data for activities at Images at and Lunar and Planetary Institute

Solar Events Flares (Explosions of energy on the surface of the Sun) Prominences Coronal Mass Ejections (massive clouds of plasma ejected from the Sun) Movie: Six months with EIT 171 (Aug. 12, Feb. 9, 2004) Information on flares at Movie: Six months with EIT 171 (Aug. 12, Feb. 9, 2004) Lunar and Planetary Institute

Solar Wind Blows charged particles and magnetic fields away from the Sun Charged particles captured by Earth’s magnetic field Create Auroras or Northern and Southern Lights Image and information at Image at Lunar and Planetary Institute

Auroras Electrons from solar wind are captured by the Earth’s magnetic field Interact with atoms in our atmosphere: oxygen and nitrogen make red and green; nitrogen can also make violet Northern lights are Aurora Borealis, while southern are Aurora Australis Animation of solar wind impacting the magnetosphere and creating aurora Animation of solar wind impacting the magnetosphere and creating aurora A booklet is available at Lunar and Planetary Institute

Coronal Mass Ejection This series of images of coronal mass ejections taken with LASCO C3 (May 1-31, 1997) at Information at This series of images of coronal mass ejections taken with LASCO C3 (May 1-31, 1997) is available at The eruption of a huge bubble of hot gas from the Sun Lunar and Planetary Institute

CME’s effects on Earth Can damage satellites Very dangerous to astronauts Power problems Animation of a CME leaving the Sun, slamming into our magnetosphere. Animation of a CME leaving the Sun, slamming into our magnetosphere. Lunar and Planetary Institute

Activities Let’s go observe the Sun Sunspot graphing

Influences on Earth Gravity Light (Radiation)
Solar Wind (already discussed)

Gravity Orbits The Sun’s powerful gravity keeps the planets in orbit From Lunar and Planetary Institute

Radiation Our Sun (and all active stars) emits radiation Radio, infrared, visible, ultraviolet, x-ray and even some gamma rays Most of the sunlight is yellow-green visible light or close to it Some information at Image and info at or The Sun at X-ray wavelengths Image and info at and . Lunar and Planetary Institute

Activities on Sunlight
UV Man (or woman, or dog, bug, etc.) Observations of infrared light using filters and cell phones

Sun’s Radiation at Earth
Notes for HCDE Workshop on Sun and Seasons Feb. 4, 2009 Sun’s Radiation at Earth The Earth’s atmosphere filters out some frequencies Ozone layer protects us from some ultra-violet, and most x-rays and gamma rays Water and oxygen absorb some radio waves Water vapor, carbon dioxide, and ozone absorbs some infrared Electromagnetic spectrum . Lunar and Planetary Institute

Sunlight is absorbed by Earth
Let’s test what happens to the light. Activity Time!!

Sunlight is absorbed by Earth
The Sun does NOT send “heat rays” into space. Some of its light is infrared, but that is not the same thing as heat. The Sun’s light is absorbed by Earth (clouds, plants, oceans, rock…) By absorbing the light, we are transforming it into heat energy

Sun as a Source of Energy
Notes for HCDE Workshop on Sun and Seasons Feb. 4, 2009 Sun as a Source of Energy Light from the Sun is absorbed by the Earth, unevenly to: drive wind bands – which drive surface currents drive deep ocean currents drive water cycle drive weather Left image caption Every day the Moderate-resolution Imaging Spectroradiometer (MODIS) measures sea surface temperature over the entire globe with high accuracy. This false-color image shows a one-month composite for May Red and yellow indicates warmer temperatures, green is an intermediate value, while blues and then purples are progressively colder values. Information about the water cycle at NASA image at Credit: NASA GSFC Water and Energy Cycle Lunar and Planetary Institute

Sun as a Source of Energy
Notes for HCDE Workshop on Sun and Seasons Feb. 4, 2009 Sun as a Source of Energy Plants need light for photosynthesis Without its heat, the only inhabitable areas on Earth would be near volcanic vents Images from and Lunar and Planetary Institute

Young stars form in nebulae from Small Magellanic Cloud
Notes for HCDE Workshop on Sun and Seasons Feb. 4, 2009 Young stars form in nebulae from Small Magellanic Cloud This new image taken with NASA's Hubble Space Telescope depicts bright, blue, newly formed stars that are blowing a cavity in the center of a star-forming region in the Small Magellanic Cloud. At the heart of the star-forming region, lies star cluster NGC 602. The high-energy radiation blazing out from the hot young stars is sculpting the inner edge of the outer portions of the nebula, slowly eroding it away and eating into the material beyond. The diffuse outer reaches of the nebula prevent the energetic outflows from streaming away from the cluster. Ridges of dust and gaseous filaments are seen towards the northwest (in the upper-left part of the image) and towards the southeast (in the lower right-hand corner). Elephant trunk-like dust pillars point towards the hot blue stars and are tell-tale signs of their eroding effect. In this region it is possible with Hubble to trace how the star formation started at the center of the cluster and propagated outward, with the youngest stars still forming today along the dust ridges. The Small Magellanic Cloud, in the constellation Tucana, is roughly 200,000 light-years from the Earth. Its proximity to us makes it an exceptional laboratory to perform in-depth studies of star formation processes and their evolution in an environment slightly different from our own Milky Way. Dwarf galaxies such as the Small Magellanic Cloud, with significantly fewer stars compared to our own galaxy, are considered to be the primitive building blocks of larger galaxies. The study of star formation within this dwarf galaxy is particularly interesting to astronomers because its primitive nature means that it lacks a large percentage of the heavier elements that are forged in successive generations of stars through nuclear fusion. Image at Lunar and Planetary Institute

In commemoration of NASA's Hubble Space Telescope completing its 100,000th orbit in its 18th year of exploration and discovery, scientists at the Space Telescope Science Institute in Baltimore, Md., have aimed Hubble to take a snapshot of a dazzling region of celestial birth and renewal. Hubble peered into a small portion of the nebula near the star cluster NGC 2074 (upper, left). The region is a firestorm of raw stellar creation, perhaps triggered by a nearby supernova explosion. It lies about 170,000 light-years away near the Tarantula nebula, one of the most active star-forming regions in our Local Group of galaxies. The three-dimensional-looking image reveals dramatic ridges and valleys of dust, serpent-head "pillars of creation," and gaseous filaments glowing fiercely under torrential ultraviolet radiation. The region is on the edge of a dark molecular cloud that is an incubator for the birth of new stars. The high-energy radiation blazing out from clusters of hot young stars already born in NGC 2074 is sculpting the wall of the nebula by slowly eroding it away. Another young cluster may be hidden beneath a circle of brilliant blue gas at center, bottom. In this approximately 100-light-year-wide fantasy-like landscape, dark towers of dust rise above a glowing wall of gases on the surface of the molecular cloud. The seahorse-shaped pillar at lower, right is approximately 20 light-years long, roughly four times the distance between our Sun and the nearest star, Alpha Centauri. The region is in the Large Magellanic Cloud (LMC), a satellite of our Milky Way galaxy. It is a fascinating laboratory for observing star-formation regions and their evolution. Dwarf galaxies like the LMC are considered to be the primitive building blocks of larger galaxies. Star-forming region in the Large Magellanic Cloud: Lunar and Planetary Institute

This dramatic image offers a peek inside a cavern of roiling dust and gas where thousands of stars are forming. The image, taken by the Advanced Camera for Surveys (ACS) aboard NASA's Hubble Space Telescope, represents the sharpest view ever taken of this region, called the Orion Nebula. More than 3,000 stars of various sizes appear in this image. Some of them have never been seen in visible light. These stars reside in a dramatic dust-and-gas landscape of plateaus, mountains, and valleys that are reminiscent of the Grand Canyon. The Orion Nebula is a picture book of star formation, from the massive, young stars that are shaping the nebula to the pillars of dense gas that may be the homes of budding stars. The bright central region is the home of the four heftiest stars in the nebula. The stars are called the Trapezium because they are arranged in a trapezoid pattern. Ultraviolet light unleashed by these stars is carving a cavity in the nebula and disrupting the growth of hundreds of smaller stars. Located near the Trapezium stars are stars still young enough to have disks of material encircling them. These disks are called protoplanetary disks or "proplyds" and are too small to see clearly in this image. The disks are the building blocks of solar systems. The bright glow at upper left is from M43, a small region being shaped by a massive, young star's ultraviolet light. Astronomers call the region a miniature Orion Nebula because only one star is sculpting the landscape. The Orion Nebula has four such stars. Next to M43 are dense, dark pillars of dust and gas that point toward the Trapezium. These pillars are resisting erosion from the Trapezium's intense ultraviolet light. The glowing region on the right reveals arcs and bubbles formed when stellar winds - streams of charged particles ejected from the Trapezium stars — collide with material. The faint red stars near the bottom are the myriad brown dwarfs that Hubble spied for the first time in the nebula in visible light. Sometimes called "failed stars," brown dwarfs are cool objects that are too small to be ordinary stars because they cannot sustain nuclear fusion in their cores the way our Sun does. The dark red column, below, left, shows an illuminated edge of the cavity wall. The Orion Nebula is 1,500 light-years away, the nearest star-forming region to Earth. Astronomers used 520 Hubble images, taken in five colors, to make this picture. They also added ground-based photos to fill out the nebula. The ACS mosaic covers approximately the apparent angular size of the full moon. The Orion observations were taken between 2004 and 2005. Orion image at Lunar and Planetary Institute

Our Sun is a Regular/ Small Star
Image at

In a few Billion years… Red Giant
Image at

Our Sun’s Habitable Zone
Notes for HCDE Workshop on Sun and Seasons Feb. 4, 2009 Our Sun’s Habitable Zone Billions of years ago, things may have been different The Sun was cooler (by up to 30%!) Earth’s atmosphere was different (thicker, carbon dioxide) Conditions will be different in the future By many accounts, increases in the Sun’s temperature will make Earth uninhabitable in 1 billion years or less These changes will also affect other planets… Mars? Animation at Information on Venus’ history at Information on Mars’ history at Details and animations at A sun-like star grows into its red giant phase, increasing in size and luminosity. Energy in the form of heat can now reach a once-frozen and dead moon. The icy surface quickly melts into liquid water, filling in old craters with warmer seas. The stage is now set for the possible formation of new life. Details on solar constant at Lunar and Planetary Institute

By 5 billion years… White Dwarf
Notes for HCDE Workshop on Sun and Seasons Feb. 4, 2009 By 5 billion years… White Dwarf NOVEMBER 5, 1998: NGC 3132 is a striking example of a planetary nebula. This expanding cloud of gas surrounding a dying star is known to amateur astronomers in the Southern Hemisphere as the "Eight-Burst" or the "Southern Ring" Nebula. The name "planetary nebula" refers only to the round shape that many of these objects show when examined through a small telescope. In reality, these nebulae have little or nothing to do with planets, but are instead huge shells of gas ejected by stars as they near the ends of their lifetimes. NGC 3132 is nearly half a light year in diameter, and at a distance of about 2,000 light-years is one of the nearest known planetary nebulae. The gases are expanding away from the central star at a speed of 9 miles per second. Image at Lunar and Planetary Institute

Glowing like a multi-faceted jewel, the planetary nebula IC 418 lies about 2,000 light-years from Earth in the direction of the constellation Lepus. This photograph is one of the latest from NASA's Hubble Space Telescope, obtained with the Wide Field Planetary Camera 2. A planetary nebula represents the final stage in the evolution of a star similar to our Sun. The star at the center of IC 418 was a red giant a few thousand years ago, but then ejected its outer layers into space to form the nebula, which has now expanded to a diameter of about 0.1 light-year. The stellar remnant at the center is the hot core of the red giant, from which ultraviolet radiation floods out into the surrounding gas, causing it to fluoresce. Over the next several thousand years, the nebula will gradually disperse into space, and then the star will cool and fade away for billions of years as a white dwarf. Our own Sun is expected to undergo a similar fate, but fortunately this will not occur until some 5 billion years from now. The Hubble image of IC 418 is shown in a false-color representation, based on Wide Field Planetary Camera 2 exposures taken in February and September, 1999 through filters that isolate light from various chemical elements. Red shows emission from ionized nitrogen (the coolest gas in the nebula, located furthest from the hot nucleus), green shows emission from hydrogen, and blue traces the emission from ionized oxygen (the hottest gas, closest to the central star). The remarkable textures seen in the nebula are newly revealed by the Hubble telescope, and their origin is still uncertain. Image at Lunar and Planetary Institute

In this detailed view from NASA's Hubble Space Telescope, the so-called Cat's Eye Nebula looks like the penetrating eye of the disembodied sorcerer Sauron from the film adaptation of "The Lord of the Rings." The nebula, formally cataloged NGC 6543, is every bit as inscrutable as the J.R.R. Tolkien phantom character. Though the Cat's Eye Nebula was one of the first planetary nebulae to be discovered, it is one of the most complex such nebulae seen in space. A planetary nebula forms when Sun-like stars gently eject their outer gaseous layers that form bright nebulae with amazing and confounding shapes. In 1994, Hubble first revealed NGC 6543's surprisingly intricate structures, including concentric gas shells, jets of high-speed gas, and unusual shock-induced knots of gas. As if the Cat's Eye itself isn't spectacular enough, this new image taken with Hubble's Advanced Camera for Surveys (ACS) reveals the full beauty of a bull's eye pattern of eleven or even more concentric rings, or shells, around the Cat's Eye. Each 'ring' is actually the edge of a spherical bubble seen projected onto the sky — that's why it appears bright along its outer edge. Observations suggest the star ejected its mass in a series of pulses at 1,500-year intervals. These convulsions created dust shells, each of which contain as much mass as all of the planets in our solar system combined (still only one percent of the Sun's mass). These concentric shells make a layered, onion-skin structure around the dying star. The view from Hubble is like seeing an onion cut in half, where each skin layer is discernible. Until recently, it was thought that such shells around planetary nebulae were a rare phenomenon. However, Romano Corradi (Isaac Newton Group of Telescopes, Spain) and collaborators, in a paper published in the European journal Astronomy and Astrophysics in April 2004, have instead shown that the formation of these rings is likely to be the rule rather than the exception. The bull's-eye patterns seen around planetary nebulae come as a surprise to astronomers because they had no expectation that episodes of mass loss at the end of stellar lives would repeat every 1,500 years. Several explanations have been proposed, including cycles of magnetic activity somewhat similar to our own Sun's sunspot cycle, the action of companion stars orbiting around the dying star, and stellar pulsations. Another school of thought is that the material is ejected smoothly from the star, and the rings are created later on due to formation of waves in the outflowing material. It will take further observations and more theoretical studies to decide between these and other possible explanations. Approximately 1,000 years ago the pattern of mass loss suddenly changed, and the Cat's Eye Nebula started forming inside the dusty shells. It has been expanding ever since, as discernible in comparing Hubble images taken in 1994, 1997, 2000, and The puzzle is what caused this dramatic change? Many aspects of the process that leads a star to lose its gaseous envelope are still poorly known, and the study of planetary nebulae is one of the few ways to recover information about these last few thousand years in the life of a Sun-like star. Image at Lunar and Planetary Institute

Massive Stars are different
Notes for HCDE Workshop on Sun and Seasons Feb. 4, 2009 Massive Stars are different From Astronomers using the Hubble telescope have identified what may be the most luminous star known ? a celestial mammoth that releases up to 10 million times the power of the Sun and is big enough to fill the diameter of Earth's orbit. The star [center of image] unleashes as much energy in six seconds as our Sun does in one year. The image, taken in infrared light, also reveals a bright nebula [magenta-colored material], created by extremely massive stellar eruptions. The nebula is so big (4 light-years) that it would nearly span the distance from the Sun to Alpha Centauri, the nearest star to Earth's solar system. Image from Lunar and Planetary Institute

Betelgeuse This is the first direct image of a star other than the Sun, made with NASA's Hubble Space Telescope. Called Alpha Orionis, or Betelgeuse, it is a red supergiant star marking the shoulder of the winter constellation Orion the Hunter (diagram at right). The Hubble image reveals a huge ultraviolet atmosphere with a mysterious hot spot on the stellar behemoth's surface. The enormous bright spot, twice the diameter of the Earth's orbit, is at least 2,000 Kelvin degrees hotter than the surface of the star. The image suggests that a totally new physical phenomenon may be affecting the atmospheres of some stars. Follow-up observations will be needed to help astronomers understand whether the spot is linked to oscillations previously detected in the giant star, or whether it moves systematically across the star's surface under the grip of powerful magnetic fields. The observations were made by Andrea Dupree of the Harvard- Smithsonian Center for Astrophysics in Cambridge, MA, and Ronald Gilliland of the Space Telescope Science Institute in Baltimore, MD, who announced their discovery today at the 187th meeting of the American Astronomical Society in San Antonio, Texas. The image was taken in ultraviolet light with the Faint Object Camera on March 3, 1995. Hubble can resolve the star even though the apparent size is 20,000 times smaller than the width of the full Moon — roughly equivalent to being able to resolve a car's headlights at a distance of 6,000 miles. Betelgeuse is so huge that, if it replaced the Sun at the center of our Solar System, its outer atmosphere would extend past the orbit of Jupiter (scale at lower left). Image from Lunar and Planetary Institute

Supernova—Massive Star Explodes
Notes for HCDE Workshop on Sun and Seasons Feb. 4, 2009 Supernova—Massive Star Explodes (top right) FEBRUARY 19, 2004: Seventeen years ago, astronomers spotted the brightest stellar explosion ever seen since the one observed by Johannes Kepler 400 years ago. Called SN 1987A, the titanic supernova explosion blazed with the power of 100,000,000 suns for several months following its discovery on Feb. 23, Although the supernova itself is a million times fainter than 17 years ago, a new light show in the space surrounding it is just beginning. This image, taken Nov. 28, 2003 by the Advanced Camera for Surveys aboard NASA's Hubble Space Telescope, shows many bright spots along a ring of gas, like pearls on a necklace. These cosmic "pearls" are being produced as a supersonic shock wave unleashed during the explosion slams into the ring at more than a million miles per hour. The collision is heating the gas ring, causing its innermost regions to glow. Curiously, one of the bright spots on the ring [at 4 o'clock] is a star that happens to lie along the telescope's line of sight. Left: This new movie of X-ray data from Chandra of the supernova remnant Cassiopeia A (Cas A) was made by combining observations taken in January 2000, February 2002, February 2004, and December In these images, the lowest-energy X-rays Chandra detects are shown in red, intermediate energies in green, and the highest energies in blue. Scientists have used the movie to measure the expansion velocity of the leading edge of the explosion's outer blast wave (shown in blue). The researchers find that the velocity is 11 million miles per hour, which is significantly slower than expected for an explosion with the energy estimated to have been released in Cas A. This slower velocity is explained by a special type of energy loss by the blast wave. Electrons are accelerated to high energies as they travel backwards and forwards across the shock front produced by the blast wave. As the electrons travel around magnetic fields in the shock they lose energy by producing synchrotron emission and glowing in X-rays. Scientists think heavier particles like protons and ions are accelerated in the same way. The energy lost by these heavier particles can amount to a large fraction of the energy from the supernova explosion, resulting in a slower shock velocity. The accelerated protons and ions which escape from the remnant are known as "cosmic rays", and continually bombard the Earth's atmosphere. Supernova remnants are believed to be one of the main sources of cosmic rays. The authors have constructed a model that combined the measured expansion velocity, as well as its observed size, with estimates of the explosion energy, the mass of the ejected material in Cas A and efficient particle acceleration. For everything to agree, about 35% of the energy of the Cas A supernova went into accelerating cosmic rays. Another new feature seen in the Cas A movie is "flickering" of the blue synchrotron emission seen on timescales of about a year. This flickering is thought to be a direct result of acceleration of particles to high energies, causing the emission to become brighter, followed by rapid cooling, causing the emission to fade. These variations provide important clues about the location of the acceleration, a topic of some controversy. For the first time, this flaring is seen in the outer blast wave. This casts doubt upon the possibility, suggested previously, that cosmic ray acceleration occurs in the so-called "reverse shock". This is a shock that travels backwards into the expanding remnant and is therefore located inside the outer blast wave. Previous claims that flaring occurs in the reverse shock may simply have been caused by regions in the outer blast wave that are projected onto the middle of the two-dimensional image. The rapid flickering not only gives information about acceleration of particles to high energies, but it also shows that relatively strong magnetic fields have been generated in the shock front. The Crab Nebula (bottom right) is a six-light-year-wide expanding remnant of a star's supernova explosion. Japanese and Chinese astronomers recorded this violent event nearly 1,000 years ago in 1054, as did, almost certainly, Native Americans. This composite image was assembled from 24 individual exposures taken with the NASA Hubble Space Telescope’s Wide Field and Planetary Camera 2 in October 1999, January 2000, and December It is one of the largest images taken by Hubble and is the highest resolution image ever made of the entire Crab Nebula. Images at Lunar and Planetary Institute

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Notes for HCDE Workshop on Sun and Seasons Feb. 4, 2009

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