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Lecture 7

PHYS 1600 Lecture 7: F11_PHYS-1600 - Week 7

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University of Guelph
PHYS 1600
Elizabeth Nicol

PHYS 1600 Week 7 Physics Week 7- The Sun The Mass of the Sun 30  The mass of the sun is 2x10 kg  This can be found using Kepler’s 3 law, as the planets rotating around the sun are satellites The Sun’s Atmosphere  10.1 The photosphere (visible layer of the sun) o The apparent surface of the sun o A layer of gas about 400 km thick o Low density, about 0.01% as thick as the air on Earth o Has a blackbody spectrum that corresponds to a temperature of about 5800 K o Innermost of 3 layers that comprise the Sun’s atmosphere o Photosphere is visible because the upper two layers are transparent to visible light o Everything past the photosphere is considered the interior of the sun o Appears darkest towards the edge (limb) of solar disk (called limb darkening)  Occurs because we can see different temperatures at different depths of the photosphere  Happens when we view the Sun as it is spherical, so the light we see leaving the sun comes from different places o Hottest and brightest at the base of the Sun’s disk (center) o Blotchy pattern on the surface called granulation o The bright granules are about 1000 km across and surrounded by dark boundaries o By measuring Doppler shifts of spectral lines in various parts of solar granules, it was determined that the hot gases in each granule move up in the center of the granule and the cooler gases cascade down the sides, caused by convection  Once the gases arrive at the surface, they radiate energy into space (seen as visible light and other electromagnetic radiation) Downloaded free at PHYS 1600 Week 7  Upon radiating, the gases cool, spill over the edges of the granules and dive back down into the Sun along the boundaries of the granule o Centers of granules are 100K hotter than the edges, as determined by Stefan-Boltzmann’s law (hotter regions emit more photons per square meter than cooler regions do  10.2 The chromosphere “sphere of colour” o Immediately above the photosphere o Dim layer of less dense stellar gas o For centuries, was only visible during an eclipse o Today, astronomers can study the chromosphere with specific wavelengths that allow the wavelengths of it to pass through—but not the photosphere’s, or by using telescopes that are sensitive to non- visible wave lengths that the chromosphere emits intensely o During an eclipse, the chromosphere was visible as a pink strip about 2000 km thick around the edge of the dark moon o Characterized by spikes of gas called spicules  Typical spicule rises for several minutes at the rate of 72,000 km/h to a height of nearly 10,000 km before it collapses and fades away  At any one time, roughly 300,000 (1/3 of a million) spicules cover a few percent of the Sun’s chromosphere  Generally located on the boundaries of enormous regions of rising and falling chromospheric gas called supergranules (typical diameter is slightly larger than the Earth’s and contains about 900 granules)  10.3 Temperatures increase higher in the Sun’s atmosphere o 5800 K in the photosphere, drops to 4000 K in the lower chromosphere, then spikes to 10,000 K at the top of the chromosphere o Outermost region of the Sun’s atmosphere is called the corona  Extends several million km from the top of the chromosphere Downloaded free at PHYS 1600 Week 7 o Between the top of the chromosphere and the corona is a transition zone, where the temperature rockets to 1 million K  Discovered in 1940 as a result of the high temperature’s effect on the spectrum of the Sun’s corona  Scientists discovered the spectrum of the corona contained the emission lines of several highly ionized elements (ionized= heated so much the electrons were stripped off the elements) o Temperatures in the corona are typically between 1 and 2 million K, although sometimes hotter o Density of gas in the corona is very low (about 10 trillion times less than sea-level Earth air)  Low density accounts for the dimness of the corona, which would otherwise out-shine the photosphere o Corona is heated by energy carries aloft and released there by the Sun’s complex magnetic fields o Brightness received from the corona is about one-millionth the brightness of the photosphere—about the brightness of the full moon o Like the chromosphere, the corona is only visible when using filters or special wavelength sensitive equipment that can block out the light of the photosphere, or during an eclipse, or while using a special telescope called a coronagraph o Gravity of the sun prevents most of its atmosphere from escaping, however some gases (travelling at around 1 million km/h) can escape and race into space  A portion of the gas comes from the corona, while some comes from below it, however both are funneled out by the Sun’s magnetic field—these particles are called solar wind o The heliosphere is a bubble in space created by the solar wind that contains the sun and surrounding planets, and prevents most of the gases expelled in our direction (called galactic cosmic rays)  Heliosphere is not spherical o Mass of the Sun lost to solar winds will barely amount to anything throughout its lifetime Downloaded free at PHYS 1600 Week 7 o Particles in the solar winds reach speeds up to 2.9 x 10 km/h o It is believed that the chemical composition of the Sun’s surface is nearly identical to the composition of the solar nebula o 99.9% is electrons, Hydrogen and Helium, but Sulfur, Silicon, Calcium, Chromium, Nickel, Neon, and Argon has been detected as well o The Sun’s atmosphere is periodically disrupted by magnetic fields that stir things up, resulting in a group phenomena known as the active Sun o The sun’s most obvious transient features are sunspots—regions of the photosphere that appear dark because they are cooler than the rest of the Suns lower atmosphere  10.4 Sunspots reveal the solar cycle and the Sun’s rotation o The average number and location of sunspots varies in fairly predictable cycles because of the changes in the magnetic field that produce them o Sunspot cycle was discovered in 1843 by Samuel Schwabe o Average sunspot cycle lasts about 11 years, within which is a sunspot maximum and a sunspot minimum (where the sun is nearly devoid of sunspots) o Typical sunspot is about 10,00 km across and lasts between a few hours and a few months o Each sunspot has a dark, central region called the umbra and a brighter ring around that called the penumbra  The umbra appears red and its penumbra bright orange when seen without the bright granules surrounding it  According to Wein’s law, the colours indicate that the umbra is about 4300 K and the penumbra is 5000 K o In a large enough group, sunspots can be visible to the naked eye o By seeing how sunspots move across the solar disk, Galileo determined that the Sun rotates about once every 4 weeks o Sunspot activity also revealed different latitudes revolve around the sun at different rates, called differential rotation Downloaded free at PHYS 1600 Week 7  Equatorial regions rotate more rapidly than polar regions o At the beginning of the sunspot cycle, sunspots typically appear around latitude 30 degrees, where as later in the cycle they first appear closer to the equator  Can be seen in a “butterfly diagram” o Sun emits 0.1% more energy at the peak of the sunspot cycle than usual  10.5 The Sun’s magnetic fields create sunspots o George Ellery Hale in 1908 discovered sunspots are directly linked to intense magnetic fields on the Sun  When he focused on a spectrum of light from one sunspot, he found that in addition to the normal line, it was flanked by closely spaced spectral lines (splitting of lines called Zeeman effect) o Pieter Zeeman, in 1896, discovered that intense magnetic fields would split spectral lines o Sunspots are created as the magnetic field protruding through the photosphere prevents hot, ionized gases inside the Sun from convecting, therefore leaving some regions relatively devoid of hot gas and are therefore cooler/darker o Scientists study solar vibrations (first discovered in 1960) by using a study called helioseismology to learn more about the Sun’s interior o Solar vibrations can be detected using Doppler shift measurements o Inside, the sun rotates as one rigid body (unlike it’s surf
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