Astronomy - The Sun a Representative Star - Quick Guides

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Study GuideAstronomy–The Sun a RepresentaƟve Star1. The ChromosphereThechromosphereis a layer of the Sun just above thephotosphere(the Sun’s visible surface). It’sabout4,000 kilometers thick.Its name comes from the Greek wordchroma, meaningcolor, because it looks pinkish during a solareclipse. This pink color comes mainly from hydrogen gas glowing in a specific red wavelength calledtheH-alpha line.1.1When Can We See the Chromosphere?Normally, the bright light of the photosphere hides the chromosphere. But during atotal solareclipse, the Moon blocks the photosphere’s light, so we can see the chromosphere shining aroundthe Sun’s edge.Because of this, the chromosphere is sometimes called thereversing layer. That’s because it causesmany of theabsorption lineswe see in the Sun’s light spectrum. This is different from thephotosphere below, which mostlyemits light.1.2Temperature Changes in the ChromosphereAt the bottom of the chromosphere, near the photosphere, the temperature is about4,500 K(very hot,but cooler compared to other parts of the Sun).As you go higher, the temperaturerises to about 10,000 Kat the top of the chromosphere.This temperature increase is unusual because, from the Sun’s center out to the photosphere,temperatures usually get cooler. The chromosphere reverses this pattern and gets hotter again.1.3Why Does the Chromosphere Get HoƩer?The heating happens because ofshock wavesmoving upwards from the photosphere. These shockwaves travel faster than the speed of sound and are created byturbulent convection—which meansboiling-like motion—in the photosphere.When these shock waves reach the thinner gas in the chromosphere, they heat it up.

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Study Guide1.4InteresƟng Features: SpiculesOne cool feature of the chromosphere is calledspicules. These are tiny, narrow jets of gas that shootupward from the chromosphere, reaching high into the Sun’s atmosphere.Scientists think spicules might be connected to thegranulesin the photosphere—these granules arepatterns created by the boiling motion of hot gases on the Sun’s surface.2. The Corona2.1What is the Corona?Thecoronais the Sun’souter atmosphere. It’s very thin and extremely hot, with temperaturessoaring up to1 to 2 million degrees Kelvin(that’s millions of degrees!).Even though it’s so hot, the corona’s light isvery faint—only about as bright as the full moon, or justone millionth as bright as the Sun’s main visible surface (called the solar disk). This faintness is whyyou can only see the corona during asolar eclipse, when the bright disk of the Sun is blocked.2.2Why is the Corona So Hot?Scientists believe the Sun’smagnetic activityplays a big role in heating the corona.•The Sun’s surface, called thephotosphere, is turbulent and full of energy.•Magnetic forces carry this energy upward into the corona.•Because there are very few atoms in the corona, the energy heats them to incredibly hightemperatures.

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Study Guide2.3Bright Features in the CoronaSometimes, parts of the corona look extra bright. Two important features are:•Prominences: Bright loops or arches seen near the Sun’s edge (the “solar limb”).•Plages: Bright patches seen when looking directly at the Sun’s surface.These features often look likestreamersorfilamentsand are shaped by magnetic fields. Althoughthey shine brightly, they don’t always mean that material is flowing or moving in those areas—they’remostly bright because of the way energy is emitted.2.4The Corona in X-raysAt the extremely high temperatures of the corona, the Sun also emitsX-rays, which are very shortwavelength rays.•When viewed in X-rays, the corona’s brightness isn’t the same everywhere.•Some areas calledcoronal loopsshine very brightly.•Other parts, calledcoronal holes, appear dark.2.5The Solar Wind: A Flow from the CoronaThe corona gradually blends into a stream of charged particles called thesolar wind.•The solar wind is a flow ofionized gas(charged particles) that moves away from the Sun fastenough to escape its gravity.•It travels at speeds of around400 kilometers per second(that’s really fast!).When the solar wind reaches Earth, it interacts with our planet’smagnetic field:•It creates abow shock—like the wave formed in front of a boat.•Sometimes, when the solar wind is stronger (for example, after a solar flare), it can disturbEarth’s magnetic field.•These disturbances can affectlong-distance radio communicationson Earth.

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Study Guide2.6Effects of the Solar Wind on EarthThe solar wind particles trapped by Earth’s magnetic field get funneled toward thepolar regions.•When these particles collide with molecules in Earth’s atmosphere, they create beautifulauroras—the Northern and Southern Lights.•They also cause something calledairglow, where atmospheric molecules emit light, makingthe night sky glow faintly.•In fact, the night sky shines with abouttwice as much lightas the stars themselves produce.SummaryThe Sun’s corona is a fascinating, super-hot outer layer that we can only see during an eclipse. Itconnects directly to the powerful solar wind, which has important effects here on Earth—especially increating auroras and sometimes disrupting communications. The mysterious heating of the coronaand its complex magnetic features are still active areas of scientific research.3. The Sunspot Cycle3.1What Are Sunspots?Sunspots are dark spots that appear on the surface of the Sun. They are caused by intense magneticactivity and are cooler than the surrounding areas.3.2The Sunspot CycleThe number of sunspots on the Sun’s surface doesn’t stay the same. Instead, itgoes up and downin a cycle. This cycle lasts about11 years. Here’s what happens during the cycle:•At thesolar maximum, there aremany sunspots.•At thesolar minimum, there arevery few sunspots.3.3Other Solar AcƟvity Follows This CycleNot just sunspots, but other kinds of solar activity likesolar flaresand changes in thesolar windalso follow this 11-year pattern. So the Sun goes through times when it’s very active and times whenit’s much quieter.

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Study Guide3.4How Does the Sun’s MagneƟc Field Work?The Sun creates its magnetic field through a process called thesolar dynamo. Here’s how it works:1.Inside the Sun,electric currentsare generated.2.The Sun doesn’t spin evenly—different parts rotate at different speeds, a process calleddifferential rotation.3.This twisting motion wraps the magnetic field lines around the Sun.4.At the same time, big currents of hot plasma calledconvectionpull the magnetic field linesup and down through the Sun’s surface layer, called thephotosphere.3.5How Sunspots Are MadeSunspots form where magnetic field lines are squeezed together and poke through the photosphere.These compressed field lines stop hot plasma from rising, so the spots look darker and cooler thanthe surrounding areas.3.6The MagneƟc Field ReversesAfter about 11 years, the Sun’s magnetic field becomes so tangled that it disappears. But it doesn’tstay gone—it is rebuilt again with themagnetic poles flipped. This means the magnetic north andsouth switch places.Because of this flip,two sunspot cycles (each 11 years)actually make up a longer22-yearmagnetic cycle.3.7How the Sun’s Atmosphere ChangesThe Sun’s outer atmosphere, called thecorona, also changes during the sunspot cycle:•Atsolar maximum, the corona looks almost round.•Atsolar minimum, it becomes very stretched and distorted.

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Study Guide4. Internal Structure; Standard Solar Model4.1Why Can’t We See Inside the Sun?We cannot directly observe the inside of the Sun because light from its deep interior cannot escape tothe surface.So, scientists usetheory, mathematics, and computer modelsto understand what is happeninginside.This detailed scientific description of the Sun’s interior is called theStandard Solar Model.4.2How ScienƟsts Describe the Sun’s InteriorThe Sun is studied from itscenter (r = 0 km)all the way to itsvisible surface, called thephotosphere (r≈700,000 km).Scientists describe how different physical properties change as we move outward from the center.These properties include:•Mass (M(r))–how much mass is inside a given radius•Density (ρ(r))–how tightly packed the material is•Pressure (P(r))–the force exerted by the hot gas•Temperature (T(r))–how hot each layer is•Luminosity (L(r))–the amount of energy flowing outward•Energy generation rate–how fast energy is produced•Opacity (κ(r))–how easily light can pass through the material•Chemical composition:oX(r)–fraction of hydrogenoY(r)–fraction of heliumoZ(r)–fraction of all heavier elements•Mean molecular weight (μ(r))–average mass of particles in the gas

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