lec5_1020_sjuh

7/27/2019 lec5_1020_sjuh

1/9

1

Lec 5 Prof Sarah Higdon 1

ASTR1020


http://cheller.phy.georgiasouthern.edu/~shigdon/astr1020/

Reminder - Website:

Not the National

Geographicphoto of the year

- remember tobe skeptical!


Attend the astronomy seminar

at 12:30 tomorrow Weds Jan

30th in this room (MP 3001)Sign up on the day and give me

a hand written 1 paragraph

summary on Thursday in class

Bonus Quiz


Previously Telescopes - Part I1) Telescopes have two primary functions: 1)To collect and concentrate weak signals from

space. (i.e., optical photons, X-ray photons, radio photons etc.)2) Allow us to see structure in astronomical objects

3) Refracting telescopes use lenses to bring light to a focus.

4) Reflecting telescopes use mirrors to bring light to a focus.

5) Chromatic aberration is the inability of a lens to bring light of different colors to a singlefocus.

6) Nearly all modern telescopes used by astronomers are reflectors.Advantages of Reflectors:

Very large mirrors can be built and supported with no distortion. Segmented Mirrors Build big mirrors out of many smaller mirrors fewer light losses no chromatic aberration


Telescopes - part II7. Lightgathering power is proportional to the collecting area (mirror/lens diameter)2

8. The Angular Resolution (AR) is the smallest feature that can be distinguished on an image,

measured in arcseconds. AR = 0.00025 !nm / dm

9. The earths atmosphere limits AR to ~1 in the optical for most observatories.

10. Adaptive optics (AO) is a technique that tries to correct for the loss of AR due to turbulence in

the atmosphere by rapidly deforming a flexible mirror.

11. AO works best in the infrared (easier) and generally works over small regions of sky.

12. Better yet is to put your telescope above the atmosphere, but it is very expensive!

13. The eye is of very limited use in astronomy:

- not designed for faint light levels - cannot integrate a signal

- limited wavelength coverage - the eye can be fooled!

14. Photographic plates were a big improvement over the eye:

- can integrate for hours to build up a faint image - image large region of sky

- not very efficient (only ~2% of photons are captured). - narrow wavelength range.

15. Photographic plates have been largely replaced by CCDs

- very efficient (~70% of photons are captured) - wide wavelength range

- easy to put into computers - limit to small regions of sky


Table 5-1

Astronomy at Many Wavelengths

7/27/2019 lec5_1020_sjuh

2/9

2


Hubble before and after

http://starchild.gsfc.nasa.gov/docs/StarChild/

space_level2/hubble_improvement.html

Hubble Mirror: A Successful Failure

http://news.bbc.co.uk/1/hi/sci/tech/638187.stm


1) What powers the Sun?

2) A Model of The Constant Sun

3) A Dynamic & Active Star

Today - Our Nearest Star: The Sun

Lec 5 Prof Sarah Higdon 9 Lec 5 Prof Sarah Higdon 10

Jupiter 11 times Earth diameter


Sun 109 x Earth diameter


Earth vs. the Sun

Radius: 6400 km 696,000 km ( 110x Earths)

Mass: 6.0 x 1024 kg 1.9 x 1030 kg ( 330,000x Earths)

Density: 5.5 g/cm3 1.4 g/cm3 ( 26% Earths)

Vesc 11.1 km/s 618 km/s

Axis Tilt: 23.4 degrees 7.3 degrees

Temp: 300 K 5780 K (at surface)

Luminosity: 2.0 x 1017 W 3.9 x 1026 W (2.0x109 x Earths)

Rotation: 24-hours 25-36 days

7/27/2019 lec5_1020_sjuh

3/9

3


What Powers The Sun?

The Sun generates a large amount of energy: L! = 3.9 x 1026 Watts.

At a distance of 150-million km from the Sun (i.e., 1 A.U.), each square meter

of space receives 1400 J of energy each second

equivalent to 14 100-Watt bulbs.

Fact: Studies of the fossil record of life on Earth indicates that the Suns

energy output has remained nearly constant for at least 3-billion yrs.

- if the Sun were grew significantly hotter/colder, Earths climate

would be drastically different, leading to large scale

permanent extermination.

Note food 1 calorie = 1kC = 4 kJAmount of energy required to raise the temperature of 1 kg of water by 1 degree


Gravitational Collapse?

Kelvin-Helmholtz Contraction

- huge weight of Suns outer layers

causes Sun to contract and heat up

i.e compress gas -> Temperature

rises

e.g. pumping up a bicycle tire

Problem: only last for 25 million

years!


Chemical Burning?Ordinary burning involves chemical reactions that re-

arrange the outer electrons of the atom, but do not effect

the atoms nuclei.

Not much energy released per atom ~ 10-19 J per atom. Is

this sufficient to power the Sun?

Luminosity of Sun = 3.9 x 1026 J/s

How many atoms per second need to be burned?

The mass of the sun is 2 x 1030 kg assume it is made

entirely of Hydrogen (mass Hydrogen atom =1.7x10-27kg)

How many atoms are there?

How long would this energy supply last ?Lec 5 Prof Sarah Higdon 16

Chemical Burning?How many atoms per second need to be burned?

Luminosity of Sun = 3.9 x 1026 J/s

Burn Hydrogen: energy release ~ 10-19 J /atom

How many atoms are there?

The mass of the sun is 2 x 1030 kg assume it is made entirely of

Hydrogen (mass Hydrogen atom =1.7x10-27kg)

How long would the energy supply last?

= 3.9 x 1026 J/s = 3.9 x1045atoms/s10-19 J/atom

Number of atoms = 2 x 1030kg = 1057 atoms

1.7x10-27 kg/atom

1057 atoms = 3 x 1011 seconds = 104 years !

3.9 x1045atoms/s

.


Energy Generation in StarsThe high pressure & temperature at the Suns center mean that particles

will be colliding very violently (i.e., high speed).

This realization provided the main clue to what powers the Stars.

Like-charges (2 protons) repel each other.

At low speeds (i.e., low temperature) they

will never get very close to each other.

The temperature in the Suns core is such

that a sizable fraction of protons are moving

fast enough to get close enough for the

Strong Nuclear Force to come into play.

The Strong Nuclear Force is the force that

binds atomic nuclei together. It is a very

short range force.


Energy Generation in the SunIn the centers of stars, protons collide

with sufficient speed to get close enough

for the Strong Nuclear Force to fuse

them into a single nucleus.

This fusion reaction can be written:

1H + 1H ! 2H + ! + e+

1H = proton2H = proton + neutron

"! = neutrinoe+= positron (i.e., anti-electron)

This is the first step in a series of fusion

reactions that take place in stars like

the Sun. It is called the

Proton-Proton Chain

7/27/2019 lec5_1020_sjuh

4/9

4


Fusing Hydrogen to Helium:

The Proton-Proton Chain

Step 1 (twice)1H + 1H " 2H + " + e+; e+ + e-" 2 high-energy photonsStep 2 (twice) 2H(deuteron) + 1H " 3He + high-energy photon

Step 3 3He + 3He " 4He + 1H + 1HNet Reaction: 4( 1H)"4He + photons (energy) +2 neutrinos(escape)


Nuclear FusionAlbert Einstein 1905 Special Relativity (see

later in course)

E = mc2

E = energy in joules

m = mass in kg

c = The speed of light = 3 x 108 m/s

Coupled with Arthur Eddingtons theory that

the center of the sun is very hot and Robert

Atkinson suggestion that under these

extremely hot and dense conditions

Hydrogen could fuse to Helium.


Hydrogen Fusion

4 x1H -> 4He + neutrinos + gamma-rays

Difference in Mass:

4 hydrogen atoms = 6.693 x 10-27 kg

-1 Helium atom = 6.645 x 10-27 kg

Mass lost = 0.048 x 10-27 kg = 0.7%

E = mc2 = 4.8x10-29kg x (3x108 m/s)2

= 4.3 x 10-12 J


How Many Tonnes of Hydrogen

are fused to Helium per second?

For every kg of Hydrogen 0.7% of this

mass is converted into energy during

the fusion to Helium.

E = mc2 =0.007 kg x (3 x 108 m/s)2

= 6.3 x 1014 J

Amount of H = 3.9 x 1026 J/s

6.3 x 1014 J/kg

= 6 x 1011 kg/s = 600 million metric tonnes of

hydrogen fused to helium every second!


How Long could The Sun

Fuse Hydrogen at this rate?

Fuses 6 x 1011 kg/s

Mass Sun = 2 x 1030 kg

Could fuse hydrogen for ?? yrs


Applying The Scientific

Method To The Sun

Can not send a probe so need toconstruct a model

7/27/2019 lec5_1020_sjuh

5/9

5


The Sun is ConstantThe Suns diameter has not changed appreciably over the ~400 years

weve been observing it with telescopes.

Records of solar eclipses goes back ~4,000 years (Egypt, Sumer,China, Greece, Meso-America). This implies that the Sun has had the

same angular size as the Moon (1/2 degree) during this time.

The fossil record shows that over very longer timescales the solar

radiation has been roughly constant

The Sun is in a state of

Hydrostatic Equilibrium -

at every point gravity is

balanced by the outward

pressure of the hot gases.


Hydrostatic Equilibrium

Fish float.

How do you

sink?

Sun is not undergoing any drastic

changes (Fossil Record).

This means the sun is in both

hydrostatic and thermal equilbrium


Thermal Equilibrium

Sun is hot and gaseous.

Gas more compressed at greater depth soDensity & temperature increases with

depth

Thermal Equilibrium: Temperature at eachdepth approx constant

Energy generated by fusion at core must

be transported to the surface to maintain

equilibrium e.g too little and core

temperature will rise, too much and core

will cool - both bad news for us!Lec 5 Prof Sarah Higdon 28

The Sun Is A controlled fusion reactor - built in thermostat.

If the core temperature suddenly

drops. The Suns pressure drops as the rate of fusion

reactions decreases. The Sun contracts.

As the Sun contracts it gets hotter again. This increases

the fusion reactions until pressure and gravity are in

balance again. "Hydrostatic Equilibrium.


Energy Transport

Given the basic properties of the Suns composition, Hydrostatic Equilibrium

produces detailed temperature distribution within the Sun.

In all stars, temperature increases with

depth, peaking in the very center.

For stars like the Sun, the peak temp. is

T ~ 15-million degrees.

Temperature falls off gradually with

radius, reaching T ~ 6000 K at the

surface for stars like the Sun.


Radiative Zone: Core - 0.7 Rsun

Radiative diffusion

Convective Zone T ~ 2 x 106K

hydrogen recombination i.e

hydrogen atoms - efficient

absorbers of photons so

medium becomes opaque.

Energy transport now viaconvection

Slow progress: radiative zone 6.96x105km takes 170,000 Yrs

50cm/hr

Sun-> Earth 150 million km takes 8 minutes! 1014cm/hr

Journey of a photon to Earth

7/27/2019 lec5_1020_sjuh

6/9

6


Solar Model

Using the equations from hydrostatic and thermal equilibrium and

energy transport we construct a scientific model.

Core density 160,000kg/m3 (14 x Lead)

Core Temperature 10 million KCore Pressure 3.4 x 1011 atm (~1 atm in the class room)


Solar Interior: Helioseismology1960 Robert Leighton

Caltech high-precision

Doppler shift measurementsof solar surface: patches rise/fall 10 m every 5 mins

Sun can oscillate in millionsof ways.

Strongest tone .003hertz13 octaves below our audible

range.

Observations used to setlimits on amount of He in

the Sun and determine thethickness of the transition

region between radiativeand convective zones

Computer simulation of sound wave

resonating in Sun. red inward, blue outwardmotion

Note: It is thought that the Suns magnetic

field originates in thin layer between theconvective and radiative zones


Solar Neutrinos - direct evidence

of fusion1038 neutrinos per second leave the Sun

~ 1012 pass through your head every second!

Hard to detect no charge and very low mass - can pass

through the Earth without interacting with matter, but

occasionally they do interact and can be detected.

Three types of neutrinos (electron, muon and tau) - Sun

only produces one type (electron) but the neutrinos can

undergo a neutrino oscillation and change type before

arriving at Earth!

For more information see:

http://www.hep.upenn.edu/SNO/intro.htmlLec 5 Prof Sarah Higdon 34

The Sun is very Active

Weve shown that the Suns luminosity has been fairly

constant over the last few billion years.

Now we will look at the outer layers of the Sun, which

are far from constant!

http://video.google.com/videoplay?

docid=-2362494916765406787&q=coronal+mass+ejection&total=

16&start=0&num=10&so=0&type=search&plindex=4


The Photosphere - sphere of lightSuns visible light originates in this hot, thin & opaque layer of gas

~ 400 km. Temperature profile - hot at bottom, cooler at top.Evidence:

Absorption lines: Lower layer ~ 5800 K cooler upper layer 4400 K

Limb Darkening: line of sight through limb only sees cooler

(dimmer) upper layer

Granules: convection cells the size of Texas & Oklahoma (1000

km) - again due to hotter lower layer


SupergranulesLower contrast than

granules - hard to see

Doppler Image showssupergranules

giant convection cells

~ 35,000 km (few hundred

granules)

Churns at 0.4 km/s (1/10

speed in granules) lasts

~day

They carry magnetic field

bundles to cell edges

where they form the

chromospheric network

http://solarscience.msfc.nasa.gov/feature1.shtml

7/27/2019 lec5_1020_sjuh

7/9

7


The Chromosphere- sphere of color

Normally invisible - seen hereduring an eclipse which blocks

the light from the photosphere.

The red gas is Halpha emission

from the tenuous gas (density is10-4 that of photosphere; 10-8

that of our atmosphere.

2000 km thick and temperature

RISES bottom is 4400 K

top is 25,000 K !

Note can see chromosphere at

any time, not just eclipse, if usea narrow Halpha filter.


Spicules

Jets of rising gas

lasts 15mins

rises few 103 km

Spicules found above edgesof Supergranules

Spicules rising gas - at

supergranule boundary gas

is cool and falling.

Not thermal motion - gases

pulled by Suns magnetic

field


Solar Corona

-``crownHot thin gas

10-6 x as bright as photosphereextends for few 106 km

T ~ 2 x 106 K (see [FeXIV] emission line)

But compare densities

corona 1011 atoms/m3

photosphere 1023

Our atmosphere 1025

Energy density in photosphere muchhigher than in the Corona


Solar WindCoronas high temperature translates to high speeds

~ 106 km/hr

Some of the gas can escape the Suns gravitational

pull. Composed mainly of electrons, hydrogen and

helium nuclei some heavier ions. Winds stream outthrough coronal holes(gas thinner)

Million tonnes (109 kg) every second

is lost as wind. Is this a lot?

Given the Suns mass = 2 x 1030 kg

And it will fuse hydrogen for 1010 yrs

What percentage of its mass will be lost as wind?

False color UV image


AuroraElectrons and ions

from solar wind

enter Earths Upperatmosphere - spiral

down magnetic field

lines near poles.

Collisionally exciteatoms in ouratmosphere

(remember the photonfiring range and the

emission line spectra!)


SunspotsLower temperature region in

photosphere - huge Earth and

Jupiter sized spots!

Appear darker as lower flux

Stefan-Boltzmann

Flux from umbra = (4300 K)4 =0.3

Flux from photosphere (5800 K)4

30% of the light comparedto same size patch of

photosphere

Groups of spots like bar magnets.

leading group have

SAME magnetic polarity

to that of the nearest pole

i.e. N if closest to N pole, following group have

OPPOSITE magnetic polarity as nearest pole

7/27/2019 lec5_1020_sjuh

8/9

8


11 Year Sunspot Cycle

Butterfly diagram - at beginning of 11 year cycle spots found

near latitudes 30 N & S end of cycle nearer to equator

Remember leading spots in a group have the same polarity (N

or S) as the suns magnetic pole in that hemisphere.


Magnetic Dynamo model

Differential rotation (measured by observing sun spots) causes magnetic field

lines to be wrapped and concentrated near equator. Convection creates tangles& kinks. Sunspots appear where kinks protrude through surface of photosphere

Differential rotation eventually undoes the kinks. The leading/proceeding spotsmigrate to the equator - polarity cancels as meets another proceeding group

from the other hemisphere The following spots in the group migrate to the poles.They have the opposite polarity of the pole and first cancel andeventually

reverse the polarity.


Sunspots Produced by 22 Year

Cycle in Suns magnetic FieldIt is thought that the Suns magnetic field originates in thin layer

between the convective and radiative zones. The magnetic dynamomodel successfully predicts

! Polarity of preceding and following spots. Preceding spots in a grouphave the same polarity (N or S) as the Suns magnetic pole in that

hemisphere.

! Reversal of polarity of Suns magnetic field! Formation of greater numbers of sunspots initially at high latitudesand at the end of the cycle in greater numbers closer to the equator

Suns magnetic poles reverse every 11 years so whole cycle repeats

every 22 years


Filaments appear dark - cooler

parts of chromosphere pulled

upwards along magnetic field

lines. Seen side-on they arecalled prominences can last for

mere hours or months - most

energetic erupt as flares

Left:H-alpha image of chromosphere

during sunspot maximum. Bright plages

(beaches) appear just prior to newgroups of sunspots.

Prominences, Flares and Coronal Mass

Ejections

SOHO UV image

[HeII] filter


Solar Flares

1030 J = 1014 one megaton nuclear weapons

Brief eruption of hot ionized gas from a sunspot group

Hazardous to astronauts and satellites


Coronal Mass Ejection:

huge magnetic bubble of plasma ejected from the SunCoronal Mass Ejection is much

much larger than a solar flare!

1012 kg - a billion tonnes of high

temperature coronal gas ejected

into space at 100s km/s in the

space of a few hrs

Caused by magnetic reconnection.

See Fig 16 25b in book

Left - TRACE false-color UV image.

Showing glowing gas trapped alongmagnetic field lines.

Above SOHO X-ray image of coronal

mass ejection (Suns image is UV) Takesa few days to reach Earth - thank

goodness for our magnetosphere!

7/27/2019 lec5_1020_sjuh

9/9

9


The active Sun:

http://sdo.gsfc.nasa.gov/data/aiahmi/browse.php

http://stereo.gsfc.nasa.gov/http://video.google.com/videoplay?docid=-2362494916765406787&q=coronal+mass+ejection&total=16&start=0&num=10&so=0&type=search&plindex=4

http://www.youtube.com/watch?v=3za7Y-aIKb0

Video Of Solar Activity

http://www.youtube.com/watch?v=QdwGb-iJOeI

http://sohowww.nascom.nasa.gov/gallery/movies.htmlHubble before and after

http://starchild.gsfc.nasa.gov/docs/StarChild/space_level2/

hubble_improvement.html

Hubble Mirror: A Successful Failure

http://news.bbc.co.uk/1/hi/sci/tech/638187.stm


http://science.nasa.gov/science-

news/science-at-nasa/

2003/29dec_magneticfield/

Can Earths Magnetic Poles Reverse?


Summary: Our Nearest Star

Write your summary here -

lec5_1020_sjuh

Documents

Transcript of lec5_1020_sjuh