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Chapter 7 The Sun—Our Star The sun is a normal star The Sun—Our Star The Sun—Our Star General Properties of Stars Stars are very simple--balls of hot gas held together by their own gravity their gravity would collapse them if they were not so hot the hot gas inside stars has very high pressure but their gravity confines them and keeps them from exploding A star is a simple structure like a soap bubble. It is balanced between opposing pressures . We study the sun because it is a close-up example of a star Life on earth is depends critically on the sun observations and phenomena found on the sun are also found on other more distant stars by our study of our sun, we can make some assumptions for studying other stars 7-1 The Solar Atmosphere The Solar Atmosphere Sun is hot gas from highest layers down to its center More dense than water but less dense than rock Atmosphere is divided into three layers Photosphere Chromosphere Corona The Solar Atmosphere Below the Surface Stars make their energy near their centers so all are dominated by the outward flow of energy. Evidence of this outflow is found in the form of hot and cool regions, gas motions, and magnetic fields. The Solar Atmosphere The Photosphere (visible surface) Not solid surface Thin layer of gas from which we receive most of the sun’s light Dense enough to emit plenty of light—not so dense that the light cannot escape Less than 500km deep and has an average temperature of about 6000K. Image from: Horizons, 6th Edition; Seeds, Michael A.; Brooks/Cole The Solar Atmosphere Visible-light photo of sun's surface showing granulation The Photosphere (visible surface) Photosphere in good photographs has a mottled appearance because it is made up of dark-edged regions. Regions are called granules. Each granule is about the size of Texas and lasts between 10 and twenty minutes. Centers are a few hundred degrees hotter than edges. Granules are thought to be the tops of rising convection currents just below the photosphere. Images from: Horizons, 6th Edition; Seeds, Michael A.; Brooks/Cole The Solar Atmosphere The Photosphere (visible surface) What is convection? Convection is what happens when hot fluid rises and cool fluid sinks. Rising currents of hot gas heat small regions of the photosphere which then emit more black body radiation and look brighter. Cool sinking gas on the edges emits less light and looks darker. The Solar Atmosphere The Chromosphere Produces an emission spectrum Kirchoff's laws says it is an excited, low-density gas Temperature ranges from 10,000 K to 1,000,000 K or more Image from: Horizons, 6 th Edition; Seeds, Michael A.; Brooks/Cole The Solar Atmosphere The Chromosphere The chromosphere is roughly 1000 times fainter than the photosphere Can only be seen with specialized telescopes except during a total solar eclipse During an eclipse (moon completely covers the bright photosphere), the chromosphere appears as a narrow layer of pink gas The Solar Atmosphere The Chromosphere Source of Balmer lines from the sun filtergrams reveal spicules – extensions of relatively cool chromspheric gas up into the much hotter corona spicules spring up around the edges of supergranules twice the diameter of Earth An H filtergram showing H filtergram spicules and supergranules Image from: Horizons, 6 th Edition; Seeds, Michael A.; Brooks/Cole The Solar Atmosphere The Corona Located above the chromosphere Extends 12 solar radii or more From the Latin word for crown Best studies of the corona have been made from spacecraft above Earth’s atmosphere. Temperature of the corona rises with altitude ranges from 50,000 K to 3,000,000 K image from: Foundations of Astronomy, 7th Edition; Seeds, Michael A.; Brooks/Cole The Solar Atmosphere The Corona Density is 1 to 10 atoms/cm3 High temperature of corona has been a long standing mystery, but it has been partially solved by SOHO (Solar and Heliospheric Observatory). The Solar Atmosphere The Corona The Solar Atmosphere The Corona Astronomers have mapped small loops of magnetic field scattered all over the solar surface. The loops of this magnetic carpet constantly thrash around breaking and reconnecting. This agitates the atoms above the photosphere and deposits heat in the upper chromosphere and corona. The upper corona is so hot that the sun cannot contain it. High-velocity gas atoms stream away from the sun in a continuous breeze called the Solar Wind. Solar Wind blows past Earth at a speed of 300 to 800 km/sec continuing out past the planets to mix with the gases between the stars. The corona is heated by energy flowing from the sun’s interior not as heat but as magnetic energy. SOHO Images Photosphere with sunspots Image from: Horizons, 6 th Edition; Seeds, Michael A.; Brooks/Cole Far ultraviolet images from: Foundations of Astronomy, 7 th Edition; Seeds, Michael A.; Brooks/Cole Corona The Solar Atmosphere Shorter wavelengths do not penetrate as deeply as longer ones. Waves reflecting off underside of surface can cause a pattern of rising and falling regions. Helioseismology Helioseismology is the study of the modes of vibration of the sun. Solar astronomers explore the sun’s interior by studying how sound waves travel through the sun. Solar astronomers can determine temperature, density, pressure, composition and motion of the inner layers. image from: Foundations of Astronomy, 7th Edition; Seeds, Michael A.; Brooks/Cole One of 10 million possible patterns of oscillation WARNING! DO NOT LOOK DIRECTLY AT SUN WITH EITHER AIDED OR UNAIDED EYE THE INFRARED RAYS COULD COOK YOUR RETINA Image from: Horizons, 6 th Edition; Seeds, Michael A.; Brooks/Cole 7-2 Solar Activity Sunspots Dark spots on the photosphere Dark center called the umbra Lighter region around center called penumbra Usually occur in pairs and last up to two months Image from: Horizons, 6 th Edition; Seeds, Michael A.; Brooks/Cole image from: Foundations of Astronomy, 7th Edition; Seeds, Michael A.; Brooks/Cole Image from: Horizons, 6th Edition; Seeds, Michael A.; Brooks/Cole image from: Foundations of Astronomy, 7th Edition; Seeds, Michael A.; Brooks/Cole Solar Activity Sunspots Temperature of umbra about 4240 K Number of sunspots visible is not constant and varies with a period of about 11 years This is known as the sunspot cycle. Another maximum cycle is expected in the year 2012. image from: Foundations of Astronomy, 7th Edition; Seeds, Michael A.; Brooks/Cole Solar Activity Sunspots images from: Foundations of Astronomy, 7 th Edition; Seeds, Michael A.; Brooks/Cole Solar Activity Active Regions and Sunspots Maunder Butterfly Diagrams Plots of sunspot activity during sunspot cycles Sunspots appear at about 35˚ latitude at the beginning of the cycle and at about 5˚ latitude at the end of the cycle. Plots of latitude over time image from: Foundations of Astronomy, 7th Edition; Seeds, Michael A.; Brooks/Cole What are sunspots? Visible traces of great magnetic storms on the solar surface This is backed up by the Zeeman Effect The splitting of single spectral lines into multiple components through the influence of magnetic fields Images from: Horizons, 6th Edition; Seeds, Michael A.; Brooks/Cole Solar Activity Active Regions and Sunspots The magnetic field in a typical sunspot is 1000 times stronger than the sun’s average field. Apparently,the strength of the magnetic field inhibits gas motion below the photosphere and the rising gas cannot deliver its heat to the surface. In the cooler area we see a sunspot. Infrared observations show the area outside a sunspot is brighter than the surrounding photosphere, suggesting the heat rising from the interior is deflected and emerges around the sunspot Solar Activity The Sun's Magnetic Cycle Helioseismology tells us that deeper layers of gas rotate slower than the surface layers, and the gases near the poles rotate slowest of all This phenomenon is called differential rotation and is linked with the magnetic cycle image from: Foundations of Astronomy, 7th Edition; Seeds, Michael A.; Brooks/Cole image from: Foundations of Astronomy, 7th Edition; Seeds, Michael A.; Brooks/Cole Solar Activity The Sun's Magnetic Cycle The sun's gases are highly ionized and are, therefore, good conductors of electricity. When an electrical conductor rotates rapidly and is stirred by convection, it converts some of the energy flowing outward as convection into a magnetic field. This process is called the dynamo effect. The Babcock Model Solar Activity The Sun's Magnetic Cycle The sun's magnetic cycle is not fully understood, but the Babcock model begins to explain it. The theory behind the Babcock model is: The differential rotation of the sun winds up the magnetic field. Tangles in the field rise to the surface and cause active regions, visible as sunspot pairs. When the field becomes too tangled, it reorders itself into a simpler but reversed field and the cycle starts over. Solar Activity Prominences and Flares Prominences occur in the chromosphere Have arched shapes Shows they are formed of highly ionized gas trapped in a magnetic field Produce an emission spectrum Two types: eruptive – burst out of active regions and in a few hours extend into the lower corona only to fall back quiescent – sometimes hang in the lower corona for many days, sometimes with gas streaming downward into the active region along magnetic fields images from: Foundations of Astronomy, 7th Edition; Seeds, Michael A.; Brooks/Cole Solar Activity Prominences and Flares Flares are also related to the magnetic field. Sudden eruptions of X-ray, ultraviolet, and visible radiation and high-energy particles from the twisted magnetic fields around sunspot groups Flares can have dramatic effects on Earth, causing communication blackouts and auroras. Images from: Horizons, 6th Edition; Seeds, Michael A.; Brooks/Cole Solar Activity Coronal Activity Magnetic activity also impacts the corona. Corona seems to be composed of streamers of thin hot gas escaping from the magnetic field. In some parts of the corona, the magnetic field does not loop back and the gases escape. These areas are called coronal holes and are thought to be the source of the solar wind. Images from: Horizons, 6th Edition; Seeds, Michael A.; Brooks/Cole Images from: Horizons, 6th Edition; Seeds, Michael A.; Brooks/Cole