ASTR 1120 ASTR 1120 General Astronomy: General Astronomy: Stars - - PowerPoint PPT Presentation

ASTR 1120 ASTR 1120 General Astronomy: General Astronomy: Stars & Galaxies Stars & Galaxies

NNOUNCEMENTS FIRST MIDTERM NEXT THURSDAY FIRST MIDTERM NEXT THURSDAY

irst homework on masring asonom due day by 5p econd homework on masring asonom available om day 11am, and due on TUESDAY, 09/22, by 5pm.

On to Our Nearest Star On to Our Nearest Star Chap 14 Chap 14

De-Mystifying science De-Mystifying science The case of the Sun The case of the Sun

• Ancient philosophers/scientists thought

that the sun was some kind of fire.

• What things would we want to ask

about the Sun in this course?

• 1. How far is the Sun?
• 2. What is the Sun made of?
• 3. How does the Sun produce its energy?
• 4. How old is the Sun?
• 5. Why is the Sun stable?
• 6. What’s inside the Sun?
• 7. What will be the end of the Sun?
• 8. How massive is the Sun?

• 1. How far is the Sun?
• 1. How far is the Sun?
• How can we

determine the distance of the Sun?

1. Find the distance to an inner planet in Astronomical Astronomical Units (AU). Units (AU). 2. Find distance to distance to Sun at position of Sun at position of greatest elongation greatest elongation

• a=152

iion k -

i=146

iion k

• 2. What is the Sun made of?
• 2. What is the Sun made of?
• How can we determine the composition
• f the Sun?

Absorption Line Spectra Absorption Line Spectra

• 3. How does the Sun produce
• 3. How does the Sun produce

its energy? its energy?

• 4. How old is the Sun?
• 4. How old is the Sun?

Could it be chemical burning? Could it be chemical burning?

• 1. Energy released by burning oil

~107 joules/kg

• 2. Solar energy output (Luminosity) =

3 x 1026 joules/sec (mostly in visible light.)

• 3. Need (3 x 1026 joules/sec) / (107 joules/kg) = 3 x 1019 kg of

burning oil per second

• 4. How long will the Sun last? MSun/(amount burning per

second) = (2 x 1030 kg)/ 3 x 1019 kg/s = 6.6 x 1010 seconds = 2,000 2,000 years!! years!!

Maybe it Maybe it’ ’s gravitational s gravitational contraction? contraction?

• Gravitational contraction (Kelvin-Helmholtz

contraction) can give ~10,000 times more energy than chemical burning. How old can the Sun be if its source of energy is gravitational contraction? ~25,000,000 years. ~25,000,000 years. But geological record showed age > 150,000,000 years!

Energy generated by Energy generated by FUSION! FUSION!

Proton-Proton chain =mc =mc2

2

Fission Fission Big nucleus splits into smaller pieces (Nuclear power plants) Fusion Fusion Small nuclei stick together to make a bigger one (Sun, stars)

Fusion Fusion NOT NOT fission fission as as energy source energy source

Sun Sun’ ’s energy budget s energy budget

• Helium has atomic mass 3.97 times that of

Helium has atomic mass 3.97 times that of hydrogen, NOT exactly 4 times hydrogen, NOT exactly 4 times

• Tiny amount of the protons

Tiny amount of the protons’ ’ mass is lost to mass is lost to energy energy

• E = mc

E = mc2

2 (a little mass makes a lot of energy!)

(a little mass makes a lot of energy!)

• 600 million tons of H every second is converted

600 million tons of H every second is converted to 596 million tons of He to 596 million tons of He… …4 million tons of mass 4 million tons of mass are converted into energy each second! are converted into energy each second!

Need high temperatures to make Need high temperatures to make fusion happen fusion happen

High temperature High temperature gives high speeds gives high speeds

The Sun is made up of (mostly) The Sun is made up of (mostly)

• hydrogen. Yet the
• hydrogen. Yet the

P-P chain starts P-P chain starts with two protons. Why are they not with two protons. Why are they not with their electrons? with their electrons?

A. The material is a plasma so the nuclei and electrons are all free. B. The electrons have all moved to the outer layers of the Sun.

• C. The Sun is electrically positive (thus the magnetic

fields) so all that exists are hydrogen ions.

• D. Neutral hydrogen only consists of one proton and one

neutron in the first place. E. The electrons have been all annihilated with positrons.

Clicker Question Clicker Question

The Sun is made up of (mostly) The Sun is made up of (mostly)

• hydrogen. Yet the
• hydrogen. Yet the

P-P chain starts P-P chain starts with two protons. Why are they not with two protons. Why are they not with their electrons? with their electrons?

A. The material is a plasma so the nuclei and electrons are all free. B. The electrons have all moved to the outer layers of the Sun.

• C. The Sun is electrically positive (thus the magnetic

fields) so all that exists are hydrogen ions.

• D. Neutral hydrogen only consists of one proton and one

neutron in the first place. E. The electrons have been all annihilated with positrons.

Clicker Question Clicker Question

Do we have direct evidence for Do we have direct evidence for fusion in the Sun? fusion in the Sun?

• YES! Neutrinos

Those Mysterious Neutrinos Those Mysterious Neutrinos

• Mass-less or (likely) with very small

Mass-less or (likely) with very small masses, masses, travel close to speed of light travel close to speed of light

• Don

Don’ ’t interact (almost) with other matter: t interact (almost) with other matter: requires lead wall 1 light year thick to stop requires lead wall 1 light year thick to stop a neutrino! a neutrino!

• Lots of them:

Lots of them: 10 1038

38 neutrinos/sec from the

neutrinos/sec from the Sun, 10 Sun, 1015

15 coming through YOU each sec

coming through YOU each sec ! !

• But we can still catch some, using massive

But we can still catch some, using massive underground underground “ “detectors detectors” ”: : BIG PUZZLE BIG PUZZLE

MADE BY MADE BY HYDROGEN FUSION IN CORE HYDROGEN FUSION IN CORE

Big Puzzle: First Neutrino Detector Big Puzzle: First Neutrino Detector

• Located deep underground,

Located deep underground, ground blocked other ground blocked other particles particles

• Huge underground vat of

Huge underground vat of dry-cleaning fluid (chlorine) dry-cleaning fluid (chlorine)

– – Chlorine captures neutrino, Chlorine captures neutrino, becomes radioactive argon becomes radioactive argon

• Only collects 1 neutrino

Only collects 1 neutrino about every 3 days about every 3 days -- even

• - even

with 100,000 gallons with 100,000 gallons

• Solar theory predicted

Solar theory predicted THREE TIMES THREE TIMES more! more!

• Big hunt started, called

Big hunt started, called SOLAR NEUTRINO SOLAR NEUTRINO PROBLEM PROBLEM

Homestake Homestake Gold Mine, South Dakota Gold Mine, South Dakota

Sudbury Neutrino Observatory (SNO) Sudbury Neutrino Observatory (SNO)

• Uses

Uses “ “heavy water heavy water” ”

• some H in H

some H in H2

20 replaced

0 replaced by its stable isotope by its stable isotope deuterium (P+N) deuterium (P+N)

• SNO captures

SNO captures all three all three types of neutrinos types of neutrinos (electron, (electron, muon muon, , tao tao) )

– – Detects the Detects the RIGHT RIGHT numbers!! numbers!!

• “

“Solar neutrino problem Solar neutrino problem” ” leads to big physics leads to big physics advance advance (2002 Nobel Prize;

(2002 Nobel Prize; Davis & Davis & Koshiba Koshiba) )

Could the neutrinos flowing through Could the neutrinos flowing through

• ur bodies be a cause of cancer or
• ur bodies be a cause of cancer or
• ther cellular damage?
• ther cellular damage?

A.

• A. YES

YES, because there are so many and , because there are so many and they carry a lot of energy they carry a lot of energy B.

• B. NO

NO, because they don , because they don’ ’t interact with t interact with anything and just flow through anything and just flow through C.

• C. MAYBE

MAYBE, it depends on if they are , it depends on if they are electron, electron, muon muon or

• r tau

tau neutrinos. neutrinos. Clicker Question Clicker Question

Could the neutrinos flowing through Could the neutrinos flowing through

• ur bodies be a cause of cancer or
• ur bodies be a cause of cancer or
• ther cellular damage?
• ther cellular damage?

A.

• A. YES

YES, because there are so many and , because there are so many and they carry a lot of energy they carry a lot of energy B.

• B. NO

NO, because they don , because they don’ ’t interact with t interact with anything and just flow through anything and just flow through C.

• C. MAYBE

MAYBE, it depends on if they are , it depends on if they are electron, electron, muon muon or

• r tau

tau neutrinos. neutrinos. Clicker Question Clicker Question

How does the number of neutrinos How does the number of neutrinos passing through your body at night passing through your body at night compare with the number passing compare with the number passing through during the day? through during the day?

A.

• A. About the same.

About the same. B.

• B. Much smaller during the night.

Much smaller during the night. C.

• C. Much larger during the night.

Much larger during the night. D.

• D. Neutrinos don

Neutrinos don’ ’t pass through our t pass through our body. body. Clicker Question Clicker Question

How does the number of neutrinos How does the number of neutrinos passing through your body at night passing through your body at night compare with the number passing compare with the number passing through during the day? through during the day?

A.

• A. About the same.

About the same. B.

• B. Much smaller during the night.

Much smaller during the night. C.

• C. Much larger during the night.

Much larger during the night. D.

• D. Neutrinos don

Neutrinos don’ ’t pass through our t pass through our body. body. Clicker Question Clicker Question

• 5. What makes the Sun
• 5. What makes the Sun

stable? stable?

• To understand

this, we have to look into the forces at work on the Sun.

Pull Pull

• f gravity
• f gravity =

= Push Push

• f pressure
• f pressure

High PRESSURE High PRESSURE at at CENTER CENTER

Hydrostatic Equilibrium Hydrostatic Equilibrium

PRESSURE PRESSURE = = DENSITY DENSITY x x TEMPERATURE TEMPERATURE In gases, we have, roughly: In gases, we have, roughly: 1.

• 1. A high pressure in the center results in a high

A high pressure in the center results in a high temperature. temperature. 2.

• 2. If really hot,

If really hot, NUCLEAR BURNING NUCLEAR BURNING can supply can supply more energy more energy

• Why don

Why don’ ’t we get a runaway reaction? t we get a runaway reaction?

The Solar Thermostat The Solar Thermostat

Nuclear fusion rate very sensitive to temperature.

• 6. What
• 6. What’

’s inside the Sun? s inside the Sun? From the From the Center Outwards Center Outwards

• Core

Core: Hydrogen

fusing into helium, releasing energy in the form of gamma rays, neutrinos, and positrons

• Core temp = 15

15 million K million K, hot & dense from the gravitational weight of all that mass

Radiation Zone Radiation Zone

• Gamma ray photons

leave the core and move into an area known as the Radiation Zone Radiation Zone

– Neutrinos? They leave right away, no interaction – Positrons? Quickly find electrons in the core to annihilate with.

• Photons only travel about

1 mm before being redirected in another direction

T=10 million K.

Meanderings of outbound photons Meanderings of outbound photons

Our gamma-ray photons Our gamma-ray photons “ “random walk random walk” ” outwards (getting

• utwards (getting

redirected with every step), gradually cooling redirected with every step), gradually cooling Takes Takes hundreds of hundreds of thousands thousands to a million to a million years to get out!! years to get out!!

Convection Zone Convection Zone

• Eventually, gas is cool

enough (2 million K at the boundary) and becomes turbulent

– No longer just redirects photons, now absorbs them

• Convection

Convection: hotter regions rise, cooler regions sink

• Energy continues to work

its way out - nearly 1 million years to get out

Photosphere Photosphere

• At the top of the convection

zone, the densities are now low enough that our photons can zoom away. – Now downgraded all the way to visible energies

• Photosphere is the “visible

surface” of the Sun

• T = only 5800 K

5800 K

• Photons free - seen at Earth

8 min 8 min later

• Blackbody spectrum (T=

5800 K) + absorption from cooler gasses just on top

Granulation Granulation: : turbulent convection turbulent convection

Size: ~ 1 Mm across Size: ~ 1 Mm across (that (that’ ’s a s a Megameter Megameter!) !)

Appearance of the photosphere

Granulation Movie Granulation Movie

• Typical

granulations last only 8 - 15 8 - 15 minutes minutes

• Movie covers

35 min

27,000 27,000 km km Diameter of Diameter of Earth Earth

Taken by G. Scharmer &

• G. Simon at the Swedish

Solar Telescope

The Sun suddenly stops fusing The Sun suddenly stops fusing hydrogen and loses its energy source. hydrogen and loses its energy source. Which of the following is true? Which of the following is true?

A. A. The core will start to collapse. The core will start to collapse. B. B. The core will become cooler. The core will become cooler. C.

• C. Both A & B

Both A & B D.

• D. The Sun will appear fainter to us after 8

The Sun will appear fainter to us after 8 minutes. minutes. E. E. The Sun will not change in brightness, but we The Sun will not change in brightness, but we will see the granulation stop (after 8 minutes). will see the granulation stop (after 8 minutes).

Clicker Question Clicker Question

The Sun suddenly stops fusing The Sun suddenly stops fusing hydrogen and loses its energy source. hydrogen and loses its energy source. Which of the following is true? Which of the following is true?

A. A. The core will start to collapse. The core will start to collapse. B. B. The core will become cooler. The core will become cooler. C.

• C. Both A & B

Both A & B D.

• D. The Sun will appear fainter to us after 8

The Sun will appear fainter to us after 8 minutes. minutes. E. E. The Sun will not change in brightness, but we The Sun will not change in brightness, but we will see the granulation stop (after 8 minutes). will see the granulation stop (after 8 minutes).

Clicker Question Clicker Question

Reading/assignment Reading/assignment

Homework #2 on mastering astronomy, Due by 5pm on TUESDAY, 09/22. Reading: Ch. 14, sec. 14.1, 14.2 NEXT class: we will complete Ch.14