Astro 10-Lecture 10:Stellar Structure and Evolution

Now that we know the properties of stars, lets talk about how stars work.

Important Dates!

• April 8: Spring Break

• April 15: Exam #2. Homework #4 due

• April 16: Chabot field trip.

• April 22: Project #1 due

Chabot Trip

• Friday, April 16

• Meet at a Ashby BART station 6:30 pm.

– Need volunteers to drive from there.

• Admission $11+$6 for planetarium show

• Planetarium show (7:30) and telescope viewing

Star Structure and FormationThe Big Picture

Stars are big balls of hot gas Stars exist because of gravity They shine because they are hot They continuously generate energy through nuclear

fusion They balance gravity with pressure They form because gravity makes interstellar clouds

of gas contract They die when they have no more fuel, and gravity

takes over

Stellar Structure – MS Stars

Main Sequence STARS Gravitational (hydrostatic) equilibrium:

Gravity and Pressure balance DEMO

Temperature and Pressure related When Temp increases, so does P (DEMO)

Temperature-Pressure relation

Pressure inside balances Pressure outside

Balloon cools, molecules inside slow down, pressure inside decreases

Balloon shrinks until inside and outside pressures again balance

Hydrostatic Equilibrium

Stellar Structure (2)

Stellar structure analogy Star is emitting light/heat, so losing energy!

It should cool => P decrease => collapse BUT Sun has been stable for 5 billion years!

What’s the energy source to maintain equilibrium? FUSION

Energy Generation in the Sun

Energy is generated through nuclear fusion in the CENTER of the Sun!

What are the conditions there?

How hot are stars? Sun: 5800 K But this was a SURFACE TEMPERATURE

Central Temperature of Sun

What’s the “surface”? Stars densest and hottest in the centers

This is why we see dark absorption lines in stellar spectra Energy generated in the center, and flows outward Light emitted blocked by material sitting on top “Surface” = layer where star still dense enough to emit lots

of light, but not so dense that light is blocked

Sun’s surface Temp = 5800 K Sun’s central Temp = 1.5 x 107 K

All atoms are fully ionized, electrons stripped away

ConcepTest

Would a star that is more massive than the Sun have a (A) higher (B) lower or (C) same central pressure? Or (D) can you not tell from the information given?

What about central temperature?

FUSION

Fusion: transforms elements themselves Unlike chemical reaction

Center of sun: have just nuclei and electrons, separated

Nucleus made up of protons/neutrons: why don’t positive protons repel one another by electrostatic repulsion? Strong nuclear force binds them together

Atoms/Nuclei

Fusion (2)

“Strong nuclear force” – only acts when protons/neutrons are VERY close together

“Electrostatic repulsion” – positive charges repel

IF you can overcome the repulsion and get 2 protons close enough, then they will STAY together (details deferred)

H ion = 1 proton, 4H = 4p He ion = 2 p + 2 n

FUSION: 4H He + ENERGY! Why is energy released?

FUSION – Releasing Energy

Due to the strong nuclear force, it takes ENERGY to pull apart a nucleus: This is a BINDING ENERGY

It takes more energy to pull apart a Helium nucleus (2p + 2n) than a Deuterium nucleus (1p + 1n).

Most tightly bound element is Iron (Fe)

2p + 2n in a He nucleus He nucleus in lower energy state than 4 protons in 4 H nuclei 4H He + energy! He more tightly bound than 4H, so energy is emitted

Energy and mass are really the same thing. (Einstein) A He nucleus weighs 0.7% less than 4 P (because of the binding energy

difference) E=mc2 (Einstein!) Compare 4 x proton mass to mass of He nucleus, and convert the difference to

Energy Energy released = 0.43 x 10-11 J

Fusion and the Sun

Sun transforms 5 TONS of matter into energy EVERY SECOND! Over lifetime of sun, 1/10 of Sun’s H is converted to He, so

0.07% of Sun’s mass will keep it shining 10 billion years

Why isn’t fusion happening all the time? ONLY happens when Temp high enough to overcome

electrostatic repulsion between +ve ions DEMO

NOT a chemical reaction! TRANSFORMS the elements.

Stellar Structure

• How do we figure out the structure of a star? We can only see the surface.

• Scientific Models:• BUILD a model based on our “rulebook”• COMPARE calculated properties with observed

properties• IF they DIFFER, adjust the “rulebook”

• What’s the “rulebook”?• Imagine a star is divided into a series of shells• Apply conservation of mass, conservation of energy,

physical laws of energy transport

Stellar Structure - Ingredients

1) Total Mass = sum of masses in each layer2) Amount of Energy flowing out of a shell =

energy coming in + energy generated there3) Hydrostatic Equilibrium4) Energy Travels from hot to cool regions by

Conduction, Radiation, and Convection

Stellar Structure – cont’d

• The devil is in the details:• How much energy is generated in each layer?

• Depends on the conditions in the layer

• How is the energy transported? How quickly? By what mechanism? (More later on the mechanisms)

ConcepTest

In nuclear fusion, energy is released:A) when an electron changes energy levels within an

atom

B) when an atomic nucleus splits

C) when 2 atomic nuclei combine

D) in a chemical reaction

Stellar Structure:Testing our Model

Models like this correctly “predict” the radius, age, and other surface properties of the sun (luminosity, surface Temp…) Not every detail is exactly right, but these indicate

we’re on the right track The results of the computer model are only as

good as the assumptions that went in! We must test Predictions of the Model with

Observations

Stellar Structure: Testing the Model in the Sun’s Interior

Comparing surface predictions with surface properties doesn’t test the model at it’s heart – the center of the Sun where energy generation occurs!

Key to testing the model’s description of the conditions INSIDE the Sun is in the BYPRODUCTS of FUSION

Byproducts of Fusion

4H He + Energy Details are complicated (conservation of charge,

energy, number of leptons) One possible first step:

H1 + H1 1H2 + e+ + ν p + p pn + e+ + ν e+ = positron, ν = NEUTRINO

Net Result of Chain of Steps 4H + He + gamma ray photons + neutrinos

Proton-Proton Chain

CNO Cycle

Neutrinos Neutrinos – what are they like?

ALMOST NEVER interact with matter 100s of trillions of neutrinos pass through you every

second! An inch of lead stops an X-ray, but you need a slab of

lead more than a light-year thick to stop a neutrino Pass almost DIRECTLY out of Sun! IF we could detect them, they would tell us about

the conditions at the CENTER of the SUN!

Detecting Neutrinos: RARELY: Chlorine + neutrino Argon

Solar Neutrino Experiment

Original Experiment

100,000 gallons of dry-cleaning fluid (C2Cl4) buried deep in a mine in South Dakota

Solar Neutrino Experiment:Good News

Do we detect Do we detect neutrinos from neutrinos from the Sun? the Sun?

YES! YES!

Neutrino image Neutrino image of Sun from a of Sun from a more modern more modern experiment experiment (Super (Super Kamiokande -500 Kamiokande -500 days of data)days of data)

Solar Neutrino Experiment: Bad News

Model predicts: ~1 Argon atom per day Original Experiment Observes: ~1 Argon atom

every 3 days!

What’s Wrong?

Solar Neutrino “Problem”

Solar Models Predicted ~3x more neutrinos than have been detected

3 options: Something wrong with the experiment Something wrong with the solar model Something wrong with basic particle physics (our

understanding of neutrinos)

Solar Neutrino “Problem” (2)

Something wrong with the experiment? Over 20 years of repetition and careful testing say the

experiment is fine!

Something wrong with the solar model? For over 30 years, the particle physicists said it was the

experiment or the solar modelling, but now we believe...

Something wrong with particle physics! Currently, we believe that the predicted behaviour of

neutrinos was in error

Solar Neutrino “Solution”

Neutrinos: Actually come in 3 “flavours” Only 1 interacts with Chlorine

IF neutrinos have mass (in earlier models, they had none, like photons), neutrinos can “oscillate” between flavours. So we wouldn’t have detected all of the neutrinos

generated by the sun! Newer experiments TEST this

Testing the Solar Neutrino Solution:Super Kamiokande

Testing the Solar Neutrino Solution:Sudbury Neutrino Observatory

Solar Neutrinos: Some Current Experiments

Super Kamiokande uses water

Sudbury Neutrino Observatory uses Heavy Water (HDO, D

2O) more neutrons

+n → p+ + e- or p- + e+

GALLEX/GNO uses Gallium: Solar neutrino flux varies with a period related to rotation

rate of earth (GALLEX/GNO)

AMANDA places detectors deep in Antarctic ice

Solar Model Testing: Conclusions

Current experiments that use different methods Current experiments that use different methods to detect neutrinos indicate thatto detect neutrinos indicate that Neutrinos have mass, and oscillate between Neutrinos have mass, and oscillate between

flavoursflavours NOTE: Neutrinos have MASS will be important NOTE: Neutrinos have MASS will be important

to Cosmology laterto Cosmology later

HELIOSEISMOLOGY:HELIOSEISMOLOGY: Studying the way the Sun vibrates to study the Studying the way the Sun vibrates to study the

Sun’s interior (depends on T, P, density)Sun’s interior (depends on T, P, density) Indicates the Solar Models are correct!Indicates the Solar Models are correct!

Pressure-Temperature Thermostat

In a star, inward pull of gravity balanced by the internal pressure

As the star loses energy, the T and P would drop, except nuclear fusion is generating just enough energy to maintain the balance If reactions begin to produce too much energy, this extra

energy raises T, which raises P, so star expands, which cools it slightly. This slows the nuclear reactions.

If reactions slow, then inner T drops, lowering P. Gravity compresses the star slightly. Compression of gas raises T & P increasing nuclear fusion rate.

ConcepTest

The “solar neutrino problem” is that

a) Neutrinos are impossible to detect

b) Early experiments detected fewer neutrinos than the models predicted

c) Experiments are detecting more neutrinos than expected

d) Neutrinos cannot escape the sun

M-L Relation explained

Remember that most massive MS stars are also the most luminous?

Explained by GRAVITATIONAL EQUILIBRIUM

M-L Relation Explained (2)

More MASSIVE star => More weight pressing down on center => higher Pressure at center => higher Temp at center => higher T + higher P = higher rate of fusion => more energy generation (ie more LUMINOUS)

Energy Transport Mechanisms

How does the energy of fusion get to us?

Conduction: like sticking one end of a metal stick into a fire

Radiation: photons transport energy Convection: like currents above a candle flame

Material somewhat opaque to radiation, energy can’t flow, backs up as if behind a dam, PUSHES the material out of the way

Energy Transport in the Sun (1)

Center of Sun: flows by RADIATION BUT I told you sun’s interior was OPAQUE to light! AND I told you fusion produces GAMMA RAYS! How do we see visible photons from surface?

Photons travel SHORT distance, then absorbed by atoms. Then re-emitted. This process breaks gamma-rays down into many lower-energy photons It takes ~ 1 million years for a gamma-ray photon to reach

the surface By this time, it has been converted to ~ 1600 visible

photons

Random Walk

Energy Transport in the Sun (2)

Center of Sun: Radiation

As photons move outward, encounter layers of star which are even MORE opaque. Energy gets dammed up behind these layers, and CONVECTION takes over.

Eventually, we get to the surface, where RADIATION takes over and visible photons travel directly to Earth at the speed of light!

Energy Transport in the Sun (3)

Stellar Structure Summary• The structure of a star is determined by the

interplay between • Gravitational Equilibrium• Energy generated through fusion• Energy transport

• Mass-Luminosity for MAIN-SEQUENCE STARS is explained by Gravitational Equilibrium and the Pressure-Temperature Relationship

• EVIDENCE for this model provided by • Solar neutrinos• Helioseismology

Stellar Births (1)

• Stars form from material between the stars – the Interstellar Medium (ISM)

• ISM is ~ 75% H, 25% He, <1% other– This is ~same as the Sun!

• Any cloud of dust/gas is a “nebula”

Interstellar Medium

• How do we know there’s stuff between the stars?

• We see it!– Emiision from hydrogen in the radio (21 cm)– Dark nebulae (block starlight)– Emission nebulae (excited by starlight, but emits

its own light)– Reflection nebulae (reflects starlight)

Interstellar Medium

• 21 cm emission– “spin flip” transition of hydrogen

Interstellar Medium

Dark NebulaeDark Cloud

Dark Cloud / Cluster

Interstellar Medium - Gas

• Narrow absorption lines in stellar spectra– Line from the atmosphere of the star are broad due to

“doppler broadening.” (Remember temperature is motion of atoms).

– Cool interstellar gas (not much motion) results in narrow lines.

• Emission nebulae• Usually pink/red because of energies of electrons transitions

Emission Nebulae

Interstellar Medium - Dust

• Evidence for Dust:– Reddening of starlight

– Dust radiates in Infrared

– Reflection Nebulae (appear blue!)

DEMO

Reflection Nebulae

• Look Blue!

Interstellar Medium vs Sky Colour

Daytime Sky is Blue!

Evening Sky is Red!

ConcepTest

• You see emission at a wavelength of 21cm

This is evidence for Interstellar Dust (A=yes, B=no)

Infrared observations penetrate dust clouds more easily than visible observations (A=yes, B=no)