reading unit 52, 53, 55. homework 7 unit 52 problems 4, 6, 7, 8 unit 53 problems 5, 6, 9, 11,12 unit...
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Reading
Unit 52, 53, 55
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Homework 7
Unit 52 problems 4, 6, 7, 8Unit 53 problems 5, 6, 9, 11,12
Unit 55 problems 5, 6
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The Aurora
• When CME material reaches the Earth, it interacts with the Earth’s magnetic field and collides with ionospheric particles
• The collision excites ionospheric oxygen, which causes it to emit a photon
• We see these emitted photons as the aurora, or Northern Lights
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The Solar Cycle
• The number of sunspots seen increases and decreases periodically.
• Every 11 years or so, the sunspot number peaks. This is called Solar Maximum
• Around 5.5 years after Solar Maximum, the sunspot number is at its lowest level. This is called Solar Minimum
• Solar activity (CMEs, flares, etc.) peaks with the sunspot number
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The Babcock Cycle
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Differential Rotation
• Different parts of the sun rotate at different speeds– Equator rotates faster than the poles– Solar magnetic fields get twisted as time goes on
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The Maunder Minimum
• Very few sunspots were recorded between 1645 and 1725• This is called the Maunder Minimum• Corresponds to relatively lower temperatures here on Earth, a
“little ice age”• The reason for the Maunder Minimum and its effect on
climate are still unknown
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Temperature and Pressure Are the Key
• In the core of the Sun, the temperature exceeds 15 million K, and the pressure is very high
• High temperatures imply that the nuclei in the core are moving very fast, and the high pressure is pushing them together
• The high speeds of the nuclei allow them to collide and fuse via the proton-proton chain
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The Proton-Proton Chain
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Neutrinos
• One product (aside from energy) of the proton-proton chain is a neutrino– Very low mass, very
high energy particle
– Passes through matter very easily, and so is hard to detect
– Neutrino measurements on Earth confirm our models of fusion in the Sun’s core
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Suggestions to the course by you
• More time per slide
• Make course more challenging
• Make course easier
• Another Zeroth Exam
• More review questions to prepare for the exam
• More questions in the exam
• Make homework easier
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To what do the words "hydrostatic equilibrium" in the Sun refer?
• a. The balance of gravity inward and gas pressure outward.
• b. The balance of gas pressure inward and heat outward.
• c. the balance of gas pressure outward and magnetic forces inward.
• d. the creation of one helium nucleus for the "destruction" of every four helium nuclei.
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The time taken for neutrinos generated in the thermonuclear reactions at the center of the Sun to
escape from its surface is
• a. about 1 million years
• b. about 100,000 years
• c. instantaneous, since neutrinos travel faster than the speed of light
• d. very short, around few seconds
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What problems have observers of solar neutrino run into?
• a. The neutrino are of the wrong type (mostly muon neutrinos and no electron neutrinos)
• b. The neutrinos are about twice as energetic on average than is predicted by theoretical models of the Sun.
• c. Only about 1/3 of the expected number of neutrinos is observed, compared to theoretical models of the Sun.
• d. About six times as many neutrinos are observed than expected from theoretical models of the Sun.
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Solar activity reflected by the number of sunspots is believed to influence the climate on Earth as
• A. when Sun has more sunspots it radiates less heat
• B. sunspots inhibit nuclear reactions in the Sun
• C. the number of energetic particles interacting with Earth atmosphere changes
• D. sunspots affect the Earth orbit
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Sun’s magnetism is due to
• A. iron core of the Sun
• B. heating of Corona by energetic particles generated during Solar Flares
• C. generation of magnetic fields via fluid+magnetic field motions
• D. neutrino flows coming from the Sun’s core
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Why was adaptive optics developed?
• a. To compensate for chromatic aberration
• b. To prevent distortion of mirrors by the vacuum of space
• c. To compensate for the image distortion caused by the Earth atmosphere
• d. To prevent fractures of the main mirror.
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The PRIMARY reason for spreading many radio telescopes across a large area and combining the
signals at a central station (i.e. combining radio telescopes to form an interferometer) is
• a. to produce a much sharper images of radio sources
• b. to avoid interference between signals from separate telescopes
• c. to be able to send a more powerful signal to space
• d. ensure that cloudy weather only affects a few of telescopes, leaving the others to continue observing
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The main absorber in the atmosphere for infrared radiation, which impedes observations of
astronomical infrared objects, is
• a. electrons in the Earth's atmosphere
• b. dust in the Earth atmosphere
• c. oxygen and nitrogen, the major constituents of the atmosphere
• d. water vapor
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Pieces of metal are heated by varying amount in a flame. The hottest of these will be the one that shows
which color most prominently?
• a. blue
• b. yellow
• c. red
• d. black
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To a physicist a blackbody is defined as an object which
• a. absorbs all radiation which falls upon it
• b. always appears to be black, whatever its temperature
• c. always emits the same spectrum of light, whatever its temperature
• d. reflects all radiation which falls upon it, never heating up and always appearing black.
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The specific colors of light emitted by an atom in a hot, thin gas are caused by
• a. protons jumping from level to level
• b. an electron dropping into the nucleus, producing small nuclear changes
• c. electrons jumping to lower energy levels, losing energy as they do so
• d. the vibrations of the nucleus
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When electromagnetic radiation is Doppler-shifted by motion of the source away from the detector
• a. the measured wavelength is longer than the emitted wavelength
• b. the measured frequency of the radiation remains the same, but its wavelength is shortened, compared to the emitted radiation
• c. the speed of the radiation is less than the emitted speed
• d. the measured frequency is higher than the emitted frequency.
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You see this every day!
• More distant streetlights appear dimmer than ones closer to us.
• It works the same with stars!• If we know the total energy output of a
star (luminosity), and we can count the number of photons we receive from that star (brightness), we can calculate its distance
• Some types of stars have a known luminosity, and we can use this standard candle to calculate the distance to the neighborhoods these stars live in.
BL
dπ4
=
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Photons in Stellar Atmospheres
• Photons have a difficult time moving through a star’s atmosphere• If the photon has the right energy, it will be absorbed by an atom and raise an
electron to a higher energy level• Creates absorption spectra, a unique “fingerprint” for the star’s composition.
The strength of this spectra is determined by the star’s temperature.
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Stellar Surface Temperatures
• Remember from Unit 23 that the peak wavelength emitted by stars shifts with the star’s surface temperatures– Hotter stars look blue
– Cooler stars look red
• We can use the star’s color to estimate its surface temperature– If a star emits most strongly in a wavelength
(in nm), then its surface temperature (T) is:
• This is Wien’s Law
nmK109.2 6 ⋅×
=T
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Measuring Temperature using Wein’s Law
nmK109.2 6 ⋅×
=T
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Spectral Classification
• Around 1901, Annie Jump Cannon developed the spectral classification system– Arranges star
classifications by temperature
• Hotter stars are O type• Cooler stars are M
type
• New Types: L and T– Cooler than M
• From hottest to coldest, they are O-B-A-F-G-K-M– Mnemonics: “Oh, Be A Fine Girl/Guy,
Kiss Me– Or: Only Bad Astronomers Forget
Generally Known Mnemonics