Conversations with the Earth

Tom Burbinetburbine@framingham.edu

Quiz on Thursday

• Sun• Hertzsprung-Russell Diagram• Death of stars

Main Sequence

• Is not an evolutionary track– Stars do not evolve on it

• Stars stop on the main sequence and spend most of their lives on it

Sun ends it time on the main sequence

• When the core hydrogen is depleted, nuclear fusion stops

• The core pressure can no longer resist the crush of gravity

• Core shrinks

Why does the star expand?

• The core is made of helium• The surrounding layers are made of hydrogen

And ..

• Gravity shrinks the inert helium core and surrounding shell of hydrogen

• The shell of hydrogen becomes hot for fusion• This is called hydrogen-shell burning

And …

• The shell becomes so hot that its fusion rate is higher than the original core

• This energy can not be transported fast enough to surface

• Thermal pressure builds up and the star expands

And ..

• More helium is being created• Mass of core increases• Increases its gravitational pull• Increasing the density and pressure of this region

When• When helium core reaches 100 million Kelvin,• Helium can fuse into a Carbon nucleus

Helium Flash

• The rising temperature in the core causes the helium fusion rate to rocket upward

• Creates a lot of new energy

However

• The core expands• Which pushes the hydrogen-burning shell

outwards• Lowering the hydrogen-burning shell’s

temperature

And

• Less energy is produced• Star starts to contract

Now

• In the core, Helium can fuse to become Carbon (and some Oxygen)

• Star contracts• Helium fusion occurs in a shell surrounding the

carbon core• Hydrogen shell can fuse above the Helium shell• Inner regions become hotter• Star expands

http://upload.wikimedia.org/wikipedia/commons/8/8d/Triple-Alpha_Process.png

• Some carbon fuses with He to form Oxygen• 12C + 4He → 16O + gamma ray• Harder to fuse Oxygen with Helium to produce

Neon

Planetary Nebulae

• There is a carbon core and outer layers are ejected into space

• The core is still hot and that ionizes the expanding gas

Planetary Nebulae

White Dwarf

• The remaining core becomes a white dwarf• White dwarfs are usually composed of carbon and

oxygen• Oxygen-neon-magnesium white dwarfs can also

form• Helium white dwarfs can also form

High-Mass Stars

• The importance of high-mass stars is that they make elements heavier than carbon

• You need really hot temperatures which only occur with the weight of a very high-mass star

Stages of High-Mass Star’s Life

• Similar to low-mass star’s• Except a high-mass star can continue to fuse

elements• When the fusion ceases, the star becomes a

supernova• Supernova is a huge explosion

Fusion

• The temperatures of high-mass stars in its late-stage of life can reach temperatures above 600 million Kelvin

• Can fuse Carbon and heavier elements• Helium Capture can also occur where Helium can

be fused into heavy elements

“Deaths” of Stars

• White Dwarfs• Neutron Stars• Black Holes

White Dwarfs

• White Dwarfs is the core left over when a star can no longer undergo fusion

• Most white dwarfs are composed of carbon and oxygen

• Very dense– Some have densities of 3 million grams per cubic

centimeter– A teaspoon of a white dwarf would weigh as much as

an elephant

White Dwarfs

• Some white dwarfs have the same mass as the Sun but slightly bigger than the Earth

• 200,000 times as dense as the earth

White Dwarfs

• Collapsing due to gravity• The collapse is stopped by electron degeneracy

pressure

Electron Degeneracy Pressure• No two electrons can occupy the same quantum

state

The Sun

• Will end up as a White Dwarf

Neutron Star

• Neutron stars are usually 10 kilometers acroos• But more massive than the Sun• Made almost entirely of neutrons• Electrons and protons have fused together

How do you make a neutron star?

• Remnant of a Supernova

Supernova

• A supernova is a stellar explosion. • Supernovae are extremely luminous and cause a

burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months.

Type Ia Supernova

Type II Supernova

This stops with Iron

• Fusion of Iron with another element does not release energy

• Fission of Iron does not release energy• So you keep on making Iron

Initially

• Gravity keeps on pulling the core together• The core keeps on shrinking• Electron degeneracy keeps the core together for

awhile

Then

• The iron core becomes too massive and collapses• The iron core becomes neutrons when protons

and electrons fuse together

Density of neutron star

• You could take everybody on Earth and cram them into a volume the size of sugar cube

Explosion

• The collapse of the core releases a huge amount of energy since the rest of the star collapses and then bounces off the neutron core

• 1044-46 Joules• Annual energy generation of Sun is 1034 Joules

How do we know there are neutron stars?

• The identification of Pulsars• Pulsars give out pulses of radio waves at precise

intervals

Pulsars

• Pulsars were found at the center of supernovae remnants

Pulsars

• Pulsars were interpreted as rotating neutron stars• Only neutron stars could rotate that fast• Strong magnetic fields can beam radiation out

Black Holes

• If a collapsing stellar core has a mass greater than 3 solar masses,

• It becomes a black hole

Black Hole

• After a supernova if all the outer mass of the star is not blown off

• The mass falls back on the neutron star• The gravity causes the neutron star to keep

contracting

Black Hole

• A black hole is a region where nothing can escape, even light.

Event Horizon

• Event Horizon is the boundary between the inside and outside of the Black Hole

• Within the Event Horizon, the escape velocity is greater than the speed of light

• Nothing can escape once it enters the Event Horizon

Black Hole Sizes

• A Black Hole with the mass of the Earth would have a radius of 0.009 meters

• A Black Hole with the mass of the Sun would have a radius of 3 kilometers

http://www.astronomynotes.com/evolutn/remnants.gif

Can you see a Black Hole?

No

• Black Holes do not emit any light• So you must see them indirectly• You need to see the effects of their gravity

Evidence

• The white area is the core of a Galaxy

• Inside the core there is a brown spiral-shaped disk.

• It weighs a hundred thousand times as much as our Sun.

http://helios.augustana.edu/~dr/img/ngc4261.jpg

Evidence• Because it is rotating we can measure its radii

and speed, and hence determine its mass. • This object is about as large as our solar

system, but weighs 1,200,000,000 times as much as our sun.

• Gravity is about one million times as strong as on the sun.

• Almost certainly this object is a black hole.

Any Questions?