Download - Lecture 21 Stellar Evolution. Announcements Homework 11 due now Homework 11 due now Homework 12 – Due Monday April 30 Homework 12 – Due Monday April 30.

Lecture 21Lecture 21

Stellar EvolutionStellar Evolution

AnnouncementsAnnouncements

Homework 11 due nowHomework 11 due now

Homework 12 – Due Monday April Homework 12 – Due Monday April 3030– Unit 62: RQ1, P3, TY1Unit 62: RQ1, P3, TY1– Unit 64: RQ1, 3, TY1Unit 64: RQ1, 3, TY1– Unit 65: RQ3, TY1, 2Unit 65: RQ3, TY1, 2– Unit 67: RQ5, P2, TY1Unit 67: RQ5, P2, TY1

Pressure vs. GravityPressure vs. Gravity

The Pressure-The Pressure-Temperature Temperature ThermostatThermostat Hotter temperatures:Hotter temperatures:

– Atomic nuclei move faster.Atomic nuclei move faster.– They “touch” more frequently.They “touch” more frequently.– More fusion reactions each second.More fusion reactions each second.– More energy released.More energy released.– HIGHER PRESSURES PUSH OUTWARD.HIGHER PRESSURES PUSH OUTWARD.

Cooler temperatures:Cooler temperatures:– Atomic nuclei move more slowly.Atomic nuclei move more slowly.– Less fusion.Less fusion.– PRESSURE GOES DOWN.PRESSURE GOES DOWN.

The Main SequenceThe Main Sequence

More Mass means:More Mass means:– More gravity, so the star weighs more.More gravity, so the star weighs more.– The star needs to create more internal The star needs to create more internal

pressure to support its weight.pressure to support its weight. What Creates More Internal Pressure?What Creates More Internal Pressure?

– More nuclear fusion!More nuclear fusion!– More fusion = more heat and light!More fusion = more heat and light!

So more massive stars are So more massive stars are brighterbrighter and and hotterhotter..

Recall the H-R DiagramRecall the H-R Diagram

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Relates temperature vs. luminosity

Stars are found only in certain regions of the diagram.


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Most (90%) of stars are found in this band that runs diagonally from the upper left to the lower right. This band is called the main sequence.


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These stars are brighter than main sequence stars of the same temperature, so they must be larger. They are called giants (or red giants, because many are cool, red stars).


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These stars are brighter (and therefore larger) than even giants of the same temperature. They’re called supergiants.


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Finally, these stars are hot (white and blue-white), but very dim, so they must be very small. They are called white dwarfs.

But What Are These But What Are These Types of Stars?Types of Stars?

The The Main SequenceMain Sequence are normal stars. are normal stars. They burn hydrogen to helium in their cores.They burn hydrogen to helium in their cores. They have a relationship between mass, temperature, They have a relationship between mass, temperature,

and luminosity:and luminosity:– More massive = bigger (higher R)More massive = bigger (higher R)– More massive = hotter (higher T)More massive = hotter (higher T)– More massive = brighter (higher L)More massive = brighter (higher L)

But What Are These But What Are These Types of Stars?Types of Stars?

The The (Red) Giants(Red) Giants are very old stars nearing the end of are very old stars nearing the end of their life.their life.

They have run out of hydrogen fuel in their cores.They have run out of hydrogen fuel in their cores. They are low and intermediate mass (typically less than 8 They are low and intermediate mass (typically less than 8

times the sun’s mass).times the sun’s mass). They are large (R = 10 to 100) and bright (L = 100 to They are large (R = 10 to 100) and bright (L = 100 to

10,000)10,000)

One Day The Sun Will One Day The Sun Will Become A Red Giant!Become A Red Giant!

SupergiantsSupergiants

The The supergiantssupergiants are also stars near the end of their are also stars near the end of their lives.lives.

They have run out of hydrogen fuel in their cores.They have run out of hydrogen fuel in their cores. They are high mass (more than 8 solar masses).They are high mass (more than 8 solar masses). They are very large (R = 100 to 1,000) and extremely They are very large (R = 100 to 1,000) and extremely

bright (L = 10,000 to 1,000,000).bright (L = 10,000 to 1,000,000).

The White DwarfsThe White Dwarfs

White dwarfsWhite dwarfs are what low and are what low and intermediate mass stars become when they intermediate mass stars become when they die.die.

They are tiny, hot, collapsed stars that have They are tiny, hot, collapsed stars that have totally run out of nuclear fuel.totally run out of nuclear fuel.

What Makes The Stars What Makes The Stars Shine?Shine? Stars shine because of NUCLEAR Stars shine because of NUCLEAR

FUSION.FUSION. Atomic nuclei all have positive Atomic nuclei all have positive

electric charge.electric charge. Two nuclei repel each other with Two nuclei repel each other with

a very powerful force.a very powerful force. The bigger the nuclei, the more The bigger the nuclei, the more

powerful the repulsive force.powerful the repulsive force.

What Makes The Stars What Makes The Stars Shine?Shine? Normally this force keeps nuclei Normally this force keeps nuclei

from getting close together.from getting close together. At very high temperatures, the At very high temperatures, the

nuclei move so quickly that they nuclei move so quickly that they can “touch” (sort of) before the can “touch” (sort of) before the repulsive force can push them repulsive force can push them apart.apart.

What Makes The Stars What Makes The Stars Shine?Shine? The nuclei then “fuse” into a single The nuclei then “fuse” into a single

nucleus.nucleus. IF the two nuclei were lighter than iron:IF the two nuclei were lighter than iron:

– The new nucleus is just a bit lighter than both The new nucleus is just a bit lighter than both old nuclei together.old nuclei together.

– The “missing” mass is converted into energy: The “missing” mass is converted into energy: heat and light.heat and light.

IF the two nuclei were as heavy/heavier IF the two nuclei were as heavy/heavier than iron:than iron:– The new nucleus is just a bit heavier than both The new nucleus is just a bit heavier than both

old nuclei together.old nuclei together.– Heat energy is converted into some extra Heat energy is converted into some extra

mass.mass.

Nuclear FusionNuclear Fusion

So fusing elements lighter than So fusing elements lighter than iron gives off energy.iron gives off energy.

Fusing elements heavier than iron Fusing elements heavier than iron takes in energy.takes in energy.

Some Like It Hot…Some Like It Hot…

Fusion produces a LOT of energy, but things Fusion produces a LOT of energy, but things need to be VERY hot and under very high need to be VERY hot and under very high pressure to get it started.pressure to get it started.

Heavier nuclei than hydrogen require even Heavier nuclei than hydrogen require even HIGHER temperatures and pressures to start HIGHER temperatures and pressures to start fusion!fusion!

The Fuel Of The StarsThe Fuel Of The Stars

Nearly all stars fuse Nearly all stars fuse hydrogen hydrogen into into heliumhelium..

BUT at high enough BUT at high enough temperatures, a star temperatures, a star can fuse heavier can fuse heavier elements as fuel:elements as fuel:– Helium into Carbon, Helium into Carbon,

Nitrogen, and Nitrogen, and Oxygen.Oxygen.

– Oxygen into Oxygen into Magnesium and Magnesium and Neon.Neon.

– And others….And others….

The Mass-Luminosity The Mass-Luminosity RelationRelation There is a There is a

definite definite relationship relationship between a between a main main sequence sequence star’s star’s mass and it’s mass and it’s luminosity:luminosity:

L = ML = M3.53.5

M=0.2

L=0.0036

M=1

L=1

M=3.2

L=58.6

The Mass-Luminosity The Mass-Luminosity RelationRelation The Mass-Luminosity relation can be The Mass-Luminosity relation can be

used to figure out how long before a used to figure out how long before a star begins to run out of hydrogen fuel.star begins to run out of hydrogen fuel.– A star’s mass determines how much A star’s mass determines how much

hydrogen fuel it has to burn (more matter hydrogen fuel it has to burn (more matter = more fuel)= more fuel)

– A star’s luminosity determines how fast A star’s luminosity determines how fast the hydrogen fuel is burned (more the hydrogen fuel is burned (more luminous = burning fuel faster)luminous = burning fuel faster)

Stars = Cars?Stars = Cars?

A low-mass star is A low-mass star is like an economy car:like an economy car:– Small fuel tankSmall fuel tank– Poor performance Poor performance

(low energy output)(low energy output)– Excellent “gas Excellent “gas

mileage.”mileage.” Low mass stars burn Low mass stars burn

their fuel very, very their fuel very, very slowly.slowly.

So they last a very So they last a very long time.long time.

Stars = Cars?Stars = Cars?

A high-mass star is A high-mass star is like a sports car:like a sports car:– Larger fuel tankLarger fuel tank– High performance High performance

(high energy output)(high energy output)– Poor “gas mileage.”Poor “gas mileage.”

High mass stars High mass stars burn their fuel very, burn their fuel very, very quickly.very quickly.

So they run out of So they run out of fuel very quickly.fuel very quickly.

Live Fast, Die YoungLive Fast, Die Young

Lifetime of a main-Lifetime of a main-sequence star:sequence star:

t = 1/Mt = 1/M2.52.5 × 10 billion × 10 billion (years)(years)

……gives the length gives the length of time before the of time before the star runs out of star runs out of hydrogen fuel.hydrogen fuel.

Something’s Wrong…Something’s Wrong…

But let’s do a quick But let’s do a quick test…test…

The sun’s mass is The sun’s mass is 22×10×103030 kg. About kg. About 75% of that mass is 75% of that mass is hydrogen.hydrogen.

The sun converts The sun converts about 6×10about 6×101111 kg of kg of hydrogen into hydrogen into helium every helium every second.second.

Something’s Wrong…Something’s Wrong…

So the sun has 0.75 So the sun has 0.75 × (2 × 10× (2 × 103030) = ) = 1.5×101.5×103030 kg of hydrogen available. kg of hydrogen available.

Every second it uses 6×10Every second it uses 6×101111 kg of kg of that hydrogen.that hydrogen.

So the sun should be able to fuse So the sun should be able to fuse hydrogen into helium for:hydrogen into helium for:1.5×101.5×103030 / 6×10 / 6×101111 = 2.5×10 = 2.5×101818 s s

That’s equal to That’s equal to 80 billion years80 billion years..

What Gives?What Gives?

So why will the So why will the sun run out of sun run out of fuel in fuel in onlyonly 10 10 billion years?billion years?

Has to do with Has to do with the structure of the structure of the sun!the sun!

The Sun’s Fuel TankThe Sun’s Fuel Tank

Remember the sun’s inside is divided into layers…

Convection Zone

Radiative Zone

Core

The Sun’s Fuel TankThe Sun’s Fuel Tank

Only the CORE has temperatures and pressures high enough for hydrogen to helium fusion! So once the core runs out of hydrogen, the sun has effectively run out of fuel!

Different On The InsideDifferent On The Inside

Here’s the inside of the sun again. Other main sequence stars look different on the inside.

Convection Zone – the gas here is “churning” or “boiling”

Radiative Zone – the gas doesn’t “flow” here – light and heat radiate through this layer.

Core – where fusion is happening


High mass stars have radiative outsides and convective insides.


Radiative Zone – the gas doesn’t “flow” here – light and heat radiate through this layer.



Very low mass stars are convective all the way through.



The Life And Times Of The Life And Times Of A StarA Star

Here’s a diagram of a main sequence star near the beginning of its life.

Hydrogen is being fused into helium in the star’s core.


Helium is heavier than hydrogen, so it sinks down to the very center of the star and collects.


The star’s core isn’t hot enough to fuse helium.

So the tiny little knot of helium starts to collapse under its own weight (no fusion to provide internal pressure)


The little knot of helium heats up a little as it collapses.


And this extra heat flows out into the core, heating up the core (just a little bit).


Because the core is just a little bit hotter, it fuses hydrogen to helium just a little bit faster. Two things happen.

1. Helium “ash” is produced a little faster than before.

2. The extra heat from the faster fusion flows out into the rest of the star.


The extra internal pressure from the extra heat makes the star swell up (just a tiny little bit) and cool off (just a tiny little bit).

Why cool off? Remember when you expand a gas (swell up), you cause it to cool off a bit.


This process is very slow.

Over time, the helium core gets bigger.

The star also gets bigger and cooler, BUT:

The core is hotter, so there is more fusion, so the star is also getting brighter.


For a very, very long time this process is very, very slow.

Over the last 5 BILLION years, the sun has doubled in brightness and increased in size by perhaps 10%.


Over the next 5 BILLION years the sun will again double in brightness, and continue to slowly grow in size (and get cooler).

Main Sequence Main Sequence EvolutionEvolution So as they age, main sequence stars:So as they age, main sequence stars:

– Very slowly get brighter.Very slowly get brighter.– Very slowly get bigger.Very slowly get bigger.– Very slowly get cooler (a bit redder).Very slowly get cooler (a bit redder).

Eventually (after about 10 billion Eventually (after about 10 billion years for our sun), the helium core years for our sun), the helium core will begin to take up most of the will begin to take up most of the sun’s core.sun’s core.


Eventually (after about Eventually (after about 10 billion years for our 10 billion years for our sun), the helium core will sun), the helium core will begin to take up most of begin to take up most of the sun’s core.the sun’s core.

The sun’s core will be The sun’s core will be much hotter than when it much hotter than when it was first born.was first born.

Hydrogen is now burning Hydrogen is now burning in a thick shell around a in a thick shell around a “dead” helium core. “dead” helium core.

And it’s burning very, And it’s burning very, very fastvery fast!!

The Life And Times Of The Life And Times Of A StarA Star The hydrogen fusion is happening The hydrogen fusion is happening

very, very fast now, so a LOT of waste very, very fast now, so a LOT of waste helium is getting produced and helium is getting produced and dumped onto the dead helium core.dumped onto the dead helium core.

The dead helium core is now very big The dead helium core is now very big and massive, but still isn’t producing and massive, but still isn’t producing energy using nuclear fusion, so it energy using nuclear fusion, so it continues to collapse under its own continues to collapse under its own weight.weight.


As the helium core As the helium core collapses it produces collapses it produces enormous amounts of enormous amounts of heat.heat.

The fusion becomes The fusion becomes even faster in the even faster in the hydrogen shell.hydrogen shell.

Also areas once too Also areas once too cool for fusion are now cool for fusion are now hot enough to begin to hot enough to begin to convert hydrogen into convert hydrogen into helium.helium.


The tremendous heat The tremendous heat produced floods into produced floods into the star’s outer layers.the star’s outer layers.

The outer layers The outer layers continue to swell up continue to swell up and cool off.and cool off.

This is like what was This is like what was happening when the happening when the star was main star was main sequence, but it’s sequence, but it’s happening MUCH, happening MUCH, MUCH faster.MUCH faster.

The star is swelling The star is swelling into a into a RED GIANTRED GIANT!!

The End Of The Main The End Of The Main SequenceSequence The star’s core is now full of helium ash The star’s core is now full of helium ash

collapsing under it’s own weight.collapsing under it’s own weight. The star has run out of hydrogen fuel in its The star has run out of hydrogen fuel in its

core.core. The hydrogen fusion is now happening in a The hydrogen fusion is now happening in a

thin shell surrounding the “dead” core.thin shell surrounding the “dead” core. All the extra energy has swelled the stars All the extra energy has swelled the stars

outer layers a great deal, and caused the star outer layers a great deal, and caused the star to cool off.to cool off.

It has left the main sequence and become a It has left the main sequence and become a red giant.red giant.

The Red Giant PhaseThe Red Giant Phase

The Power Source:

Hydrogen-burning shell slowing burning out from the core.

Waste helium is dumped onto core.


The Helium Core:

Getting heavier (more massive) with time.

Slowly shrinking.

Heating up as it shrinks.

Extra heat is speeding up fusion in the surrounding hydrogen shell.


Outer Layers (Envelope):

Slowly expanding away from the core.

Star is getting larger with time.

Getting cooler as they expand.

Star is getting redder.


So as the red So as the red giant ages:giant ages:– It gets bigger.It gets bigger.– It gets brighter.It gets brighter.– It gets cooler and It gets cooler and

redder.redder. Similar to a main Similar to a main

sequence star, sequence star, but it’s happening but it’s happening much, much much, much fasterfaster!!

About How Big Will About How Big Will Our Sun Get?Our Sun Get? Here’s the Here’s the sunsun today … today …

About How Big Will About How Big Will Our Sun Get?Our Sun Get? A few million years after leaving A few million years after leaving

the main sequence, the sun will the main sequence, the sun will be much bigger and brighter…be much bigger and brighter…

L = 50

T = 3,700 K

R = 17

About How Big Will About How Big Will Our Sun Get?Our Sun Get? For the next few million years, the For the next few million years, the

sun continues to swell and sun continues to swell and become brighter…become brighter…

L = 2,100

T = 2,900 K

R = 183

How Big Will The Sun How Big Will The Sun Get?Get?

This is large enough to swallow This is large enough to swallow Mercury and Venus, but the Earth is Mercury and Venus, but the Earth is spared.spared.

Red Giants Lose MassRed Giants Lose Mass

The sun’s mass The sun’s mass hasn’thasn’t gone up. So gone up. So sun’s surface gravity is now over sun’s surface gravity is now over 10,000 times weaker (because the 10,000 times weaker (because the surface is over 100 times farther from surface is over 100 times farther from the center).the center).

The The escape velocityescape velocity from the sun’s from the sun’s surface is reduced by 10 times.surface is reduced by 10 times.

The fastest moving gas atoms at the The fastest moving gas atoms at the sun’s surface can now easily escape sun’s surface can now easily escape into space.into space.

Red Giants Lose MassRed Giants Lose Mass

THE RESULT: The solar wind becomes much THE RESULT: The solar wind becomes much more intense.more intense.

The sun begins to quickly lose several The sun begins to quickly lose several percent of its mass every few million years.percent of its mass every few million years.– The sun is “recycling” hydrogen back into The sun is “recycling” hydrogen back into

space.space.– As it becomes lighter, it’s gravity As it becomes lighter, it’s gravity

weakens.weakens.– Mass lossMass loss speeds up with time. speeds up with time.– The remaining planets drift outward a bit.The remaining planets drift outward a bit.

A Second Birth…A Second Birth…

In the core:

The temperatures eventually reach 100 million K.

Helium begins to fuse into Carbon, Nitrogen, and Oxygen.

Helium Ignition Has Begun!

The Helium FlashThe Helium Flash

When the sun was first born When the sun was first born hydrogen to helium fusion began hydrogen to helium fusion began slowly.slowly.

Helium fusion begins very quickly:Helium fusion begins very quickly:– Called the Called the Helium FlashHelium Flash!!– Core Temperature rises to over 350 Core Temperature rises to over 350

million K!million K!– Energy output spikes dramatically (goes Energy output spikes dramatically (goes

up by millions of times).up by millions of times).

Reset The Thermostat!Reset The Thermostat!

The Pressure-Temperature The Pressure-Temperature Thermostat kicks back in:Thermostat kicks back in:– The expansion cools the star’s core off.The expansion cools the star’s core off.– Puts out the hydrogen burning shell.Puts out the hydrogen burning shell.– Star’s luminosity and size go down, Star’s luminosity and size go down,

and the star shrinks in size a little.and the star shrinks in size a little. The star becomes a stable helium-The star becomes a stable helium-

burning giant.burning giant.

The Sun’s Second LifeThe Sun’s Second Life

The sun is now a The sun is now a horizontal horizontal branchbranch star (we’ll see why next star (we’ll see why next time).time).

L = 40

T = 4,500 K

R = 10.5

For Next TimeFor Next Time

Look over Units 64, 65 and 66 Look over Units 64, 65 and 66

We will discuss the final fate of We will discuss the final fate of stars including the Sun (and the stars including the Sun (and the Earth).Earth).– aka, the kaboom lectureaka, the kaboom lecture