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Stellar EvolutionSolar Mass Stars

Overview

How and Why stars evolve off the MS?

Life on the Main Sequence

Evolutions depends on Mass

Will focus on Sun Type Stars today

Sun is a fairly average star A Low Mass Star

Overview

Stages of evolution of a Low Mass Star

Main Sequence Star

Red Giants Processes in the Core & on the Surface

Helium Flash to Horizontal Branch (HB)

HB to Asymptotic Giant Branch

Planetary Nebula & White Dwarfs

Main Sequence Stars are Stable

Energy Balance :

Energy lost at the surface (Luminosity)

Energy of fusing H into He at the stars core.

Pressure (push) counters Gravity (pull):

Reaction rates are Temperature dependent

Too high - core heats up and then expands

Expansion cools the core, so reaction rates get slower.

Too small - core cools off and then shrinks

Shrinking heats the core, so reaction rates get faster.

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Main Idea Life on the Main Sequence

A nice Pressure-Temperaturefeedback mechanism, keeps the

stars Luminosity constant.

Main Sequence Stars are Stable

What happens to the He created by H fusion

Core is too cool to ignite He fusion

Slowly builds up an inert He core

Main Sequence (H-burning) Lifetime

10 Gyr for 1Msun star (eg. Sun)

10 Tyr for 0.1Msun star (red dwarf)

Stellar Evolution

Main sequence stars are just a part of a bigger

picture

Stellar evolution depends of the stars mass

Massive stars burn more quickly,

Therefore, evolve more quickly

Four final states: White Dwarfs, Neutron Stars,

Black Holes and Nothing!

Main-Sequence Evolution 41

1 24 during the MS lifetime

Number density of the particles drop with time

Pressure drops Core contracts (Why?)

Core Contracts core Temperature rises Fusion rates go up

Stars Luminosity will go up with time

Star will expand somewhat (Why?)

Surface opacity keeps Temperature same

L 24

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Stars evolve, even on the Main-Sequence

Lum

inosity (

Lsu

n)

O B A F G K M

Zero Age

Main-Sequence

TemperatureRef: Seeds

30 M

3 M

Initial Sun

Present Sun

1010 yrs

6x108 yrs

5x106 yrs

Stage 1: Life on the Main Sequence

5 billion years ago:

Beginning of its life on main-sequence

Sun had 70% luminosity it has now.

5 billion years from now:

End of its life on main-sequence

Sun will have twice the luminosity it has now.

At some point, the oceans will boil

Whats happening Inside

He created by H-fusion in the core

Slowly builds up He core

But, core is too cool to ignite He fusion

Eventually, Hydrogen in the core is exhausted

Gravity starts winning He core collapses & heats up

H-burning in a Thin Shell surrounding the core

MS (H He in core): 90% of lifetime

Whats happening Outside

H-fusion in the shell surrounding core

Collapsing core heats up this shell

Hotter shell faster fusion

H-burning shell drives expansion of the Envelope

Pressure starts winning Envelope expands & cools

Star gets Brighter & Redder

Creates a Red Giant/Supergiant

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Inert He

Core

H-Burning Shell

Cool, Extended Envelope

Some Real Stars

Stage 2: Red Giants

Sun like stars expand to 10-100 r

e.g. Arcturus (26 r)

O Stars expand upto 1000 r

~ 5 AU = Jupiters Orbit!

e.g. Mira (Omicron Ceti)

Takes about 1Gyr to climb up RGB

He core contracting & Heating but no fusion

Whats happening Outside

At the surface very low density of material.

Surface cools to 3000-3500K by ion opacity feedback mechanism

Opacity slows the heat flow, regulates temperature

Too cool metals are not ionized: +

breaks down Opacity goes down Temperature rises

Too hot metals ionize: +

Too much Opacity goes up Temperature goes down

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Whats happening Inside

Heat source is not the core

Gravity winning in the core Contraction

Shell gets hotter Increases H-burning in shell Luminosity

But, surface temperature maintained constant

L 24

This stage marked by variability and L~ 103

Mass loss from surface possible as gravity is weak

Whats happening Inside

Near the end of the red-giant stage, the core is very small

compared with the envelope

Like comparing a Baseball to the Stadium

But, the core contains most of the Mass

~ 12% but

Density is large ~ 1000 g/cm3

1 Coffee cup ~1300 lbs, thats one strong cup!

At the tip of the RGB: reaches 100 Million K Helium

Flash

Degenerate Matter

At high densities, quantum effects become important

Electrons are only allowed in discrete states, and no two electrons can

occupy the same state

High density lots of e-/cm3 degenerate matter

Electrons are trapped in certain states

Forming an energy level stack: low to high

Degenerate Matter resists Compression

Pressure becomes independent of Temperature

This Degeneracy Pressure can now support the Core

Degenerate Matter

What if we add energy to degenerate matter?

Cant change Pressure! Cant compress/expand!

All the Energy absorbed by particles large velocities,

Here, the electrons become degenerate

Electron Degeneracy Pressure, important in:

Helium cores of Red Giant Stars

Stellar end states of White Dwarfs

Will later see neutron degeneracy pressure in Neutron Stars

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Cool, Extended EnvelopePossible Mass Loss

Main Sequence Star

Red Giant Star

He core supported by degeneracy Pressure

Stage 3: The Helium Flash Core supported by reaches 10

8

New Fusion Source takes over the Triple- Process ( 4 12)

Starts He fusion! Fusion Rate 40

For Massive stars (> 3)

Core contracts quickly, Degeneracy plays small role

He fusion starts smoothly, No Helium flash

For Less Massive stars (0.4 3)

Core contracts slowly, Degeneracy is important

Density goes to 105g/cm3. He fusion starts at the Flash!

Stars < 0.4 DO NOT burn Helium at all

Whats happening

Star generates 1011 for a few seconds - More than the Galaxy!

Huge Energy from He-flash, relieves degeneracy pressure

Expands the Core, restores Pressure-Gravity Balance

Inside:

He burning in the core via Triple- Process

Additional H burning in the shell surrounding the core

Outside:

Star gets Hotter & Bluer, also Shrinks in Radius

Luminosity Drops, Leaves RGB Moves to Horizontal Branch

He Burning

Core

H-Burning Shell

Envelope

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Stage 4: The Horizontal Branch

Can be thought as the He-burning Main Sequence

Energy generated in core = Energy lost at surface through Luminosity

P-T feedback mechanism star is stable

On the HB, Energy is derived from:

He-burning in the Core and H-burning in the shell

L ~ 30 100

Whats happening Inside

Carbon created by He-fusion in the core

Triple- Process is comparatively inefficient, Slowly builds up C-core

But, core is too cool to ignite Carbon fusion

Eventually, Helium in the core is exhausted

Gravity starts winning, C core collapses & heats up

He-burning in a Thin Shell surrounding the C core

H-burning in a Shell outside the He burning shell

Horizontal Branch (He-burning) Lifetime

~ few 100 Million Years

Whats happening Outside

H & He-fusion in shells surrounding Carbon core

Collapsing core heats up these shells

Hotter shell sustain fusion

Pressure starts winning Envelope expands & cools

Star gets Brighter & Redder

But, remember He fusion requires higher temperature

This time the star has a higher effective Temperature

Climbs up the Asymptotic Giant Branch

Inert Carbon

Core

He-Burning Shell

Cool, Extended Envelope

H-Burning Shell

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Stage 5: The Asymptotic Giant Branch

Core Envelope Separation

H & He-shell burning drives Luminosity

Eventually, Radiation Pressure drives away the outer envelope

Rapid Process, takes ~ 105yrs

Expanding envelope forms a nebula around the contracting Core

Planetary Nebula

Whats happening

For a star like Sun, the core continues to contract

But, now mass of the envelope is gone

Less gravity to push, ,

Until supported by electron degeneracy pressure

White Dwarfs

It never gets hot enough to fuse Carbon (600 Million K)

More massive stars will use these elements

More catastrophic endings

White Dwarf

Planetary Nebula

Stage 6: Planetary Nebula & White Dwarfs The hot core remains and becomes a White Dwarf

Mass ~ 1 & Radius ~

Supported by degeneracy pressure

Stars with 0.4 3end up producing Planetary Nebula & White

Dwarfs

This is our Suns future

Stars upto 8 can gracefully exit this way due to Mass Loss

Even less massive stars (Red Dwarfs) can live on the Main sequence

for longer than the age of the Universe

Eventually become Black Dwarfs, as H-shell burning never starts

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Stage 6: Planetary Nebula & White Dwarfs

Planetary Nebula: Look like Planets

But they are not - Ofcourse, you knew that!

Many examples Pretty Pictures following

~ 10 30 km/s & Radius ~ 0.3 pc

Ages ~ 10,000 , vanish in ~ 50,000

Mass ~ 0.5

About 1500 known, ~ 50,000 estimated in Galaxy

Planetary Nebula: WD at Center of NGC 2440 (HST)

Planetary Nebula: Ring Nebula (HST) Planetary Nebula: NGC 3132 (HST)

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Planetary Nebula: Helix Nebula Planetary Nebula: IC 418 (HST)

Planetary Nebula A39: Optical Planetary Nebula: IC 3568 Lemon Slice Nebula (HST)

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Planetary Nebula: Owl Nebula (HST) Planetary Nebula: M27

Planetary Nebula: Dumbbell (VLT) Planetary Nebula: NGC 6751 (HST)

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Planetary Nebula: NGC 6751 Zoomed in (HST) Planetary Nebula: Red Rectangle

Planetary Nebula: NGC 7027 (HST) Planetary Nebula: Stingray (HST)

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Planetary Nebula: M2 Butterfly Nebula in the Visible : VLT

Planetary Nebula: CRL 2688 Egg Nebula Planetary Nebula: Egg Nebula in NIR

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Planetary Nebula Planetary Nebula: NGC 7009 Saturn Nebula (HST)

Planetary Nebula: Cats Eye Nebula (HST) Planetary Nebula: Etched Hourglass (HST)

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Sirius A and B

Optical X-rays