Reading

Unit 52, 53, 55

Homework 7

Unit 52 problems 4, 6, 7, 8Unit 53 problems 5, 6, 9, 11,12

Unit 55 problems 5, 6

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

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

The Babcock Cycle

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

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

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

The Proton-Proton Chain

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

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

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.

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

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.

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

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

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.

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

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

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

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.

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

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.

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

=

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.

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

Measuring Temperature using Wein’s Law

nmK109.2 6 ⋅×

=T

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