Conversations with the earth tom burbine tburbine@framingham edu
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Conversations with the Earth Tom Burbine [email protected] 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.

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Conversations with the Earth Tom Burbine [email protected]

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Conversations with the EarthTom [email protected]


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


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.

Evidence

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?


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