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Date Created: 02/13/15
More Sun Stats Composition 912 H and 87 He by number of atoms 710 H and 271 He by mass plus tiny amounts of 65 other identi ed elements Hydrostatic and Thermal Equilibrium Equilibrium is maintained by a balance among three forces 1 The downward pressure of the layers of solar material 2 The upward pressure generated by hot gases 3 The slab s weight gravitational pull it feels from the rest of the sun The Sun is in hydrostatic equilibrium which means that inward gravitational force is balanced by outward radiation pressure Thermal Equilibrium The temperature structure inside the Sun remains constant in time All the energy generated in the core of the Sun must be transported to the surface from where it s radiated into space Energy increases 9 temperature increases 9 pressure increases 9 surface expands 9 more energy radiated out Transporting energy through the Sun The core of the Sun is where its energy is generated This energy is transported by radiation photons in the Radioactive Zone This energy is transported by convection mass motions of gas in the Convective Zone Modeling the Sun To construct a model of a star like the Sun we express the ideas of hydrostatic equilibrium thermal equilibrium and energy transport as a set of equations We then calculate conditions layer by layer in toward the star s center The result is a model of how temperature pressure and density increase with increasing depth below the star s surface Probing the Sun s Interior Heliosiesmology a powerful technique to infer what s going on beneath the Sun s surface involves measuring vibrations of the Sun as a whole The Solar Atmosphere The Photosphere o The visible surface of the Sun that we re most familiar with 0 Since the Sun is a ball of gas this isn t a solid surface it s a layer about 400 km thick 0 The photosphere is the layer in the solar atmosphere from which the Sun s visible light is emitted o The Sun appears darker around its limb or edge bc we see the upper photosphere which is relatively cool and glows less brightly o The dark sunspots are also relatively cool regions Granulation Highresolution photographs of the Sun s surface reveal a blotchy pattern called granulation Granules are convection cells about 1000 km 600 mi wide in the Sun s photosphere Rising hot gas produces bright granules Cooler gas sinks downward along the boundaries between granules this gas glows less brightly giving the boundaries their dark appearance This convective motion transports heat from the Sun s interior outward to the solar atmosphere Supergranules and LargeScale Convection Supergranules display little contrast between their center and edges so they re hard to observe in ordinary images In a falsecolor Doppler image light from gas that s approaching us rising is shifted toward shorter wavelengths whereas light from receding gas descending is shifted toward longer wavelengths Observing Sunspots A sunspot is a region in the photosphere where the temperature is relatively low which makes it appear darker than its surroundings Although sunspots vary greatly in size typical ones measure few tens of thousands of kilometers across comparable to the diameter of the Earth Sunspots are not permanent features of the photosphere but last between a few hours and months Sunspots are associated with magnetic elds 0 Outside the sunspot the magnetic field is low 0 Within the sunspot the magnetic field is strong The Sunspot Cycle The average number of sunspots on the Sun isn t constant but varies in a predictable sunspot cycle Sunspots appear and disappear with time their numbers and distribution across the face of the Sun also change in regular fashion The average number of spots reaches a max every 11 years then falls off to almost 0 before the cycle begins again the Sun s magnetic field changes polarity every 11 years 212 What causes sunspots The Sun rotates faster at its equator than its poles differential rotation 25 rotation period at the equator and 36 days at the poles The strong magnetic field gets distorted as it wraps around the equator The N S magnetic field eventually becomes an EW field Convection causes the magnetized gas to leave the surface and loop through the atmosphere taking the magnetic field with it it creates a sunspot pair Sunspots come and go typically in a few days They re linked by pairs of magnetic field lines Magnetic Arches Plasma charged particles tends to follow the Sun s magnetic field with streamers of electrically charged particles moving along each field line When the magnetic fields of 2 arches come into proximity they can rearrange and combine The tremendous amount of energy stored in the magnetic eld is then released into the solar atmosphere Prominences Magnetic elds can also push upward from the Sun s interior compressing and heating a portion of the chromosphere that appears as bright arching columns of gas called prominences These can extend for tens of thousands of kilometers above the photosphere Some prominences last for only a few hours while others persist for many months The most energetic prominences break free of the magnetic elds that con ned them and burst into space The Sun s Chromosphere During a total solar eclipse the Sun s glowing chromosphere can be seen around the edge of the moon It appears pinkish because its hot gases emit light at certain wavelengths principally the Ha emission of hydrogen at a red wavelength of 6563 nm Spicules are jets of chromospheric gas that surge upward into the Sun s outer atmosphere The temperature increases as you go higher in the chromosphere The Corona The outer layer of the Sun s atmosphere extending out to a distance of several million kilometers Only about onemillionth as bright as the photosphere and can be viewed only when the light from the photosphere is blocked out Looks like numerous streamers extending in different directions far above the solar surface changing over days and weeks Has temperatures far greater than the temperatures in the chromosphere Temperatures in the Sun s Upper Atmosphere The corona is actually not very hot containing very little thermal energy The corona is nearly a vacuum but the atoms there are moving at very high speeds But because there are so few atoms in the corona the total amount of energy in these moving atoms measure of how hot the gas is is rather low Air 10A25 particles corona 10All photosphere 10A23 Corona is much hotter than layers below it must have a heat source It s not clear what heats the corona but apparently the magnetic eld of the Sun acts like a pump that increases the speed of particles in the corona Solar Wind Solar wind is the out ow of coronal gases traveling at 1 million kilometers per hour Each second the Sun ejects about a million tons of material into the solar wind composed almost entirely of electrons and nuclei of hydrogen and helium The aurorae seen at far northern or southern latitudes on Earth are produced when electrons and ions from the solar wind enter our upper atmosphere Solar wind escapes Sun mostly through coronal holes which can be seen in Xray images The Sun is evaporating The wind carries away about 2 million tons of solar matter every second But less than 01 of the Sun s mass has been lost this way since the Solar System formed 48 billion years ago Solar corona changes along with sunspot cycle it s much larger and more irregular at sunspot peak Coronal Mass Ejections Huge blasts of high energy particles are followed by vast amounts of solar plasma traveling outward at hundreds of kms In a day or two they reach the orbit of the Earth The Sun s magnetic elds and releases of plasma directly affect Earth and the rest of the solar system Solar wind shapes the Earth s magnetosphere and magnetic storms can disrupt communications and navigational equipment damage satellites and even cause blackouts Producing the Aurorae Some of the high energy particles can get into the Earth s magnetosphere which otherwise protects us They impact our upper atmosphere and it glows The Earth s magnetic field causes particles to stream preferentially down the magnetic poles so normally you have to be at high latitudes to see them The Aurorae Some solar wind particles are able to leak through Earth s magnetic field at its weaker points and cascade down into the Earth s upper atmosphere to where Earth s magnetic field connects with the Earth near the planet s north and south poles As these highspeed charged particles collide with atoms in the upper atmosphere they excite the atoms to high energy levels The atoms then emit visible light as they drop down to their ground states like the excited gas atoms in a neon light Chapter 10 Observing Properties of Distant Stars Fundamental Information about Stars Distance how do we determine distances to stars Brightness how do we measure stellar brightness Temperature Wien s Law Size how do we know how large stars are Mass how massive are they How do we know Chemical composition spectroscopy Age how old are stars How do we know Vast Emptiness of Space Nearest star to the sun Proxima Centauri which is a member of the 3star system Alpha Centauri complex is 43 ly away Scale of distances 0 Sun is a marble earth is a grain of sand orbiting l m away 0 Nearest star is another marble 270 km away 0 Solar system extends about 50 m from Sun rest of space to nearest star is basically empty Parallax Parallax uses the change in an object s apparent position compared to the background Imagine looking at some nearby object tree against a distant background mountains when you move from one location to another the nearby object appears to shift with respect to the distant background scenery Stellar Parallax As the Earth orbits the Sun a nearby star appears to shift its position against the background of distant stars The parallax of the star is equal to the angular radius of the Earth s orbit as seen from the star The closer the star is to us the greater the parallax angle Parallax and the distances to stars d 1 p d distance to a star in parsecs p parallax angle of that star in arcseconds This simple relationship between parallax and distance reveals that the closest stars have the greatest parallax Astronomers often describe distances in parsecs rather than lightyears because they use the observed parallax to measure distance 1 arcsecond l60th arcminute A Parsec The parsec a unit of length commonly used by astronomers is equal to 326 lightyears o 1 pc 326 ly The parsec is defined as the distance at which 1 AU perpendicular to the observer s line of sight subtends an angle of l arcsec o l arsec l l60 13600 degrees 1 parsec 206265 AU
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