Stellar Evolutionafter the Main

SequenceLow Mass Stars

The Path to the Main Sequence

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Life on the Main Sequence• Once a low mass star, such as the Sun,

settles down on the Main Sequence, it is in balance

• In the core, Hydrogen is being converted to Helium, the resulting energy, in the form of heat and radiation, works its way to the surface

• While the force of gravity has not been stopped, the gravitational collapse has been halted

• The star will remain in this state for several billion years or more, depending on its mass

Nuclear Fusion• In order to supply today's Solar

Luminosity, the Sun must convert 600 million tons of Hydrogen to Helium every second.

• Simultaneously about 4 million tons of matter is being converted to energy.

• Energy in the form of radiation makes its way out of the sun.

• But what happens to the Helium?

Running Out…

• nuclear fuel runs out at the center of the star first… where reactions happen fastest (and where it is hottest)

Forming a He Core• Helium is 4 times heavier than

hydrogen, so the inert helium will begin to slowly collect at the center of the star

• At first there is no problem, but as the amount of the He becomes substantial, an inert core forms

Helium Core• The Helium "ash" continues to grow.• The Hydrogen is burning in a shell

surrounding the He• Once there is enough He, gravity starts

to compress the helium core – It begins to get hot from the compression,

the heat causes the H He reaction to increase causing more He, increasing the core mass which increases the gravitational force

– Things start getting out of hand

Becoming a Giant H fusion in core ends

and core is crushed and heated…

Fusion continues in a “shell” surrounding the

He core …

Large energy release makes envelope

expand

He

mostly H

CROSS-SECTION

Degeneracy• The He has had a lot of time to pack together; The electrons

have formed what is called a degenerate electron gas• At usual stellar densities, the electrons in a gas act as though

they were ordinary molecules and obey the usual gas laws• As the electrons are squeezed into tighter and tighter spaces,

they begin to encroach upon each others 'territory' - They are not free to move as particles in an ideal gas, but are constrained to move only when other electrons move. It is as if the entire mass of electrons are geared together.

“Degeneracy”Certain kinds of particles

(like electrons and neutrons) don’t like looking EXACTLY like others…

“THE EXCLUSION PRINCIPLE”example: electrons in

atomic orbitals

if forced together, they must have:

® different spin orientation

OR® different motion

TWIN CLOSET

This Star has a ProblemLet's summarize the situation,The star is converting H to He furiously in a

shell about a contracting He core. The core is resisting the pressure because of the electron degeneracy pressure, but that doesn't stop the heat from increasing causing the H to go to He more and more rapidly.

Eventually the excess heat, radiation and pressure overwhelm the force of gravity on the hydrogen.

Balance is lost and the star begins to expand. (Of course, the He core is still trying to compress)

Red GiantAs the star expands, the gas cools.This has the following effect,

1. The color of the star changes gradually to red showing the cooling gas temperature

2. The surface area becomes larger and larger, causing the brightness to increase despite the cooling temperature.

3. The He core is still getting hotter causing the 'burning' shell to produce more and more

4. This star begins to travel to the Red Giant region of the HR Diagram

Leaving the Main Sequence

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Helium Flash• At this point the core is still contracting against the

electron pressure – It's like a pressure cooker with the lid on tight.

• Finally the temperature exceeds 100 million degrees Kelvin

• At this temperature, the He nuclei have enough energy to begin to react

• The Triple-Alpha process begins to convert helium into carbon: He2

4He24+

4Be8 +

He24+4Be

86 C12 +

Triple-Alpha Process

Helium Flash• One property of the degenerate electron gas is that it

conducts temperature very well, so as soon as the energy is released in one part of the core, it is transmitted throughout the core in seconds, producing a rapid heating of all of the He there.

• The He burning accelerates like an explosion – the He Flash.

• The new energy expands the core rapidly which in turn cools things abruptly reversing the growth of the red giant. Without the overwhelming heat and pressure, the outer atmosphere begins to contract again; the triple-alpha process ceases once the temperature has dropped less than 100 million degrees.

• The giant reduces its size (at the cost of heating up and shifting color toward the blue once again)

• This moves the star down and to the left once again

After the He Flash

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And next…• The pressure has been released, the star

has reduced it size (and consequently gotten a hotter surface changing color again)

• Now it begins all over,Hydrogen is begin converted to Helium in a shell about the remaining Helium. The star once again collects He in its core, and everything happens all over again --- back to the Red Giant stage as the He compresses heating the Hydrogen shell

Only this time there is a difference…

Red Giant again• This time there isn't enough time to

form the electron gas – these changes have occurred over tens or hundreds of millions of years, not billions

• This time when the core temperature reaches 100 million degrees and the He C, there won't be a He Flash instead the star will be converting H to He and He to C simultaneously.

Late stage evolutionThin, cool atmosphere

Hydrogen burning shell

HeliumWhat's THIS??

It's a core forming – Carbon 'ash'

The final stagesOur star is now creating a carbon core, as

that becomes substantial, gravity begins compressing it, making it denser and hotter

(Sound familiar?)The heat from the compressing carbon gas

causes the helium shell to burn furiously; that in turn increases the rate of burn for the hydrogen shell. Making the star larger and hotter (moving it left on the HR)

The pressure finally overcomes gravity in the helium and hydrogen and the outer layers begin to expand and lift off into space

Solar Life-Cycle

Sun: Main Sequence

SunEarth1 A.U.

Sun: Red Giant

1 A.U.

The Red Giant Sun

The Fate of the EarthThree possibilities:• The earth enters the supergiant sun.

– The Earth will vaporize.– The Earth will melt into a cinder, but

remain• The earth remains just outside the

supergiant sun. – The Earth will melt into a cinder, but

remainBut what happens next to the Sun?

Planetary Nebulae

M57 – The Ring Nebula

As the outer layers lift off they form one of the most beautiful sights in space. Emitting mostly in blue and red the gas above the core moves into space. The ring is an illusion as the gas is spherical about the core.At one time we thought that this was a gentle, graceful process. The Hubble telescope has changed our mind

Planetary Nebulae

Exposedcore

Ejected atmosphe

re

Planetary NebulaeIn the Cat's Eye Nebula, we can see the complex jets and interactions of the expanding gas

Hubble's eyesight has shown us that the stars do not "gently go into the night"

NGC 2440

Planetary Nebulae

In the 'Twin Jet Nebula', the gas is a bipolar flow moving at 200 miles/second. The left-over core of this star has a surface temperature of 200,000 ºK

Planetary Nebulae

• low-mass star pushes most of its gas into

space, exposing hot core

nuclear reactions stopstar cools forever…• gas is illuminated by

hot white dwarf

White DwarfWhat is left is the carbon core of the original star.

It is very small, very hot

About the size of the Earth, and many times hotter than the Sun.

This is a White Dwarf. It is held apart by the degenerate electron pressure. It will slowly cool over billions of years to become a burned out carbon core – a black dwarf.

This will be the fate of our Sun.

White Dwarf• Remember what's left at this point is a '

carbon core' – It's outer atmosphere has lifted away leaving a very dense, very hot core.

• The core's intense gravitational field is balanced by the pressure of the degenerate electron gas

White Dwarf• Most of the mass of a

solar sized star is concentrated in a core about the size of the Earth– This means it is very

dense: A sugar cube’s worth of material at the Earth’s surface could weigh up to 200 tons

0.4 kg 0.8 kgChocolate cakes grow larger when their mass increases

There is an inverse relationship between the mass and the radius --- the more massive, the smaller the white

dwarf

0.4 M0.8 M

White dwarfs grow smaller as their mass increases. (More gravity, but same pressure)

Low-Mass Star Corpses: White Dwarfs

• pressure comes from “degenerate” electrons• star about same size as Earth• higher mass white dwarfs are smaller

Earth Sun1.0 M⊙

1.3 M⊙

White Dwarf• The hot core now slowly cools without

losing pressure support• The cooling process takes billions

(perhaps trillions) of years• When it becomes cool enough, it can

crystallize• At some point, when it is cool enough, we

declare it to be a black dwarf

Solar (low mass star) Evolution

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White DwarfIt becomes natural to ask, "What if the star has more mass than the

electron gas can balance?"

In order to become a white dwarf, a star cannot have more mass than Chandrasekhar's Limit,

Mstar < 1.4 Msun

Exceeding Chandrasekhar's Limit

If a star starts out with more than 1.4 M it cannot become a white dwarf so its evolutionary path must be different (which we will discuss in the next lecture)

Is there any other way to exceed Chandrasekhar's Limit?

Multiple Star Systems• Let's digress for a moment and consider

multiple star systems. • Until now, we have been considering only

'solitares' – stars isolated in space. But this is actually the rarity. Most stars are found with one or more companions.

• Their spacing is anywhere from about 2000 AU down to 'almost touching'

• For simplicity, let's consider only binary star systems

Algol – The Demon StarAlgol, Beta Persei, was seen to be the blinking eye in

Medusa's head. It fades and brightens in just under 3 days.

A very frightening sight Algol is an 'eclipsing binary' Two stars in close orbit oriented so that one passes in front of the other as seen from Earth

Binary Star SystemsThere are other types of variable stars (we will discuss

some later). For now let's take a closer look at that image of the Algol system

The dotted line about Algol A represents its Roche Limit; Notice that Algol B is deformed. Material from Algol B is being pulled into Algol A.

Binary Systems• Suppose one of the companion stars is a white

dwarf• As its partner reaches the red giant stage, its

atmosphere may impact on the white dwarf's Roche Limit.

• Material from the companion will be accreted onto the surface of the white dwarf

Dana Berry

Nova• If the layer of slowly-accreting hydrogen is

heated to the appropriate temperature, it may explode – vastly, but temporarily, increasing the lumenosity of the system.

• This is a Nova, or "New Star"• This, generally, does not do lasting damage

to the star and, in fact, may be re-occurring – after burning off the accumulated Hydrogen, the capturing process begins again

SupernovaWhat if the hydrogen layer is deposited more

quickly and so that it doesn’t have the time to heat up enough to 'flash' into a nova, but instead just adds mass to the white dwarf?

Once Chandrasekhar's Limit is exceeded, the star reacts by undergoing a cataclysmic explosion.

Supernova

This explosion totally destroys the star.

It is a Type Ia Supernova

It has an Absolute Magnitude of -19.3 or about 5 billion times brighter than the Sun

SN Ia LightCurves

From P. Hultzsch, et al

Notice that the light curves from the various supernova are nearly all the same. This implies that the mechanism is very similar (and we will be able to use them as distance indicators

Very distant supernovae

Since they are so bright, they are used to measure the expansion of the Universe