Gravity simplest

fusion

The life of a star has a complex relationship with gravity:

1. Gravity is what brings the original dust together to make a star

2. Gravity wants to crush the star

•Gravity pulls together old “star-dust”.

•Cloud 100x as large as our solar system.

•Gravity pulls in cloud to a spinning disk.

•As it compresses, temp. increases

What is the key to “igniting” a star?

Temperature . . .

Needs to induce thermo-nuclear fusionby reaching 18-million K.

Main sequence star, like the sun, fuses atoms into larger atoms in order to be in equilibrium with gravity.

Once “ignited” from high temperature, star is self-luminous until fuel runs out.

Fuel runs out, but gravity does not . . .

If the fusion stops then the star will give in to gravity, then a few things could happen:

-small ones just fade and die out

-larger ones get to explode!

We can apply Planck’s curve to find temp.

- Red = cooler- Blue/Violet = hotter

Our yellow sun burns somewhere in the middle

Small cool red stars, like Proxima Centauri

½ to 1/10 the mass of the sun

Surface temp at less than 7000 Farenheit

Dim/faint so you don’t see

20 times the mass of the sun

Surface temp at 45,000 Farenheit

10,000 times as bright as sun

Size means everything!

Larger the star, the shorter the life

Opposite what you’d think, i.e. big-time gambler!

Higher mass—higher temp—higher pressure—higher fusion rate.

The sun, and other main sequence stars, will run out of fuel eventually . . .

Gravity will not stop and will contract the star further.

However, this compression causes the core to superheat . . .

Takes “loan” and uses the superheat to start burning Helium.

It takes 10 billion years to go through all Htakes 100 million to go through all He

Fuses He to Carbon and other elements

Heat of Helium fusion causes outer shell to expand.

Outer atmosphere begins to evaporate.

Cause planetary nebula.

Gravity keeps coming.

Look for a “ledge”

Electrons hate being compacted, the pressure created prevents the collapse

Cools bizarrely and forms “White Dwarf”

Very dense300,000x mass of earthSize of earth

“Retired Stars” releasing light from all accumulated energy – spending savings

Shines for 1billion more years

Will be the fate of our sun

In binary system, WD can siphon Hydrogen material.

White Dwarf mass increases to become unstable and create a catastrophic explosion – Type 1A supernova

Type 1Awhite dwarf.0001% of all energy is given off as light

Type IIlarger stars 10x size of sun

Massive stars have layers of different fusion reactions like an onion with heaviest elements in core:HHe, HeC and O,ONe and Mn, Si, S, And then . . . Iron and heavier

--requires energy, not building energyIRON CORE COLLAPES AND BLOWS AWAY STAR!

Spread out elements that lead to life.

How do Neutron Stars Form?

• After a supernova occurs, there are 3 possible paths for stellar evolution to take– Stellar cores with up to 1.4 times the mass of the Sun

become white dwarfs.

– Cores with between 1.4 and 3 times the mass of the Sun become neutron stars. Type II

– Cores with more than 3 times the mass of the Sun become black holes.

• All of these transformations take place the same way; gravity compresses what is left over from a supernova explosion. There are 3 possible transformations because the amount of mass in a star affects the force of gravity.

After Type II supernova, the core is left intact. To crush core . . . Gravity wins!?

Electron-pressure eliminated when electrons combined with protons and turned to neutrons!

Gravity compresses more, neutrons don’t like being packed-in either

What are Neutron Stars?

• Neutron stars are the remnants of very large collapsed stars

• They are approximately 10-15 km in diameter and weigh about 1.4 times the mass of the Sun

– Due to the combination of small size and tremendous mass, their force of gravity is about 300,000 times that of Earth.

• They do not glow– no nuclear reaction occurs

• Some neutron stars do emit radiation, in the form of x-rays and radio waves

Why are Neutron Stars so Dense?

• When a neutron star forms, the pull of gravity is so great that it overrides the electron degeneracy pressure of the atoms of the star.

• The electrons are forced into their respective nuclei, where they combine with protons to form neutrons. This greatly decreases the size of each atom, and allows them to be packed together much more tightly.

Even more dense than the White Dwarf; squeezed to size of Manhattan.

“Pulsars” spin rapidly and shine “lighthouse” strobe

Pulsars• Some neutron stars are called “Pulsars”

because of the way they seem to “pulse” radiation when viewed from Earth

• They do not really pulse; they emit radiation along an axis. As this axis sweeps across the Earth, it looks like the flow of radiation is pulsing on and off.

Neutron Stars vs. Pulsars• All pulsars are neutron stars, but not all neutron stars

are pulsars.

– Rotation-powered pulsars have a very active magnetic field . . . this is what enables the “pulse” effect

• Many neutron stars were never able to be pulsars.

– As time goes on, the pulsar loses its rotational momentum, and eventually stops pulsing.

• By astronomical standards this happens quickly, usually within a few million years.

PulsarCrab Pulsar- “On” Crab Pulsar- “Off”

This particular pulsar cycles every 33 milliseconds.

These pictures are in the X-ray spectrum.

Diagram of a Rotationally-Powered Pulsar

Black Holes

Formation of a Black Hole • Thought to form from stars which have collapsed in on

themselves.

• They form from the core remnant after a supernova.

• The core 3x the mass of the sun, but much smaller

• What does that mean about density?

Stars that are 25-40x size of the sun are too massive to be able to become Neutron Stars.

Gravity crushes them even further—inifinte density!

Gravity’s victory over mass!

Huge star collapses to a point where the gravitational field is inescapable, even a flashlight beam cannot escape (it will bend)

Not a “cosmic vacuum” – doesn’t pull everything to it, but does have an effect on neighbors’ motions

The Center of a Black Hole

• Singularity - The point at the center of a black hole

• Event Horizon - Within a certain distance of the singularity, the gravitational pull is so strong that nothing, not even light can escape.

Event Horizon

Supernova 2006 GY not even leaves a blackhole?

100x energy emitted of normal SN death of star 200x as massive as sun?

May be a clue to early first stars?

Likely gave us early heavy elements (like iron)

Collisions are not likely, unless in binary systems or in globular clusters

•2 neutron stars can collide to release more energy than sun does during its whole life.

•White Dwarf vs. Sun would distort sun from immense gravity

Globular clusters has collision ever 10,000 years. Two small old stars can come together to make brand new, large “Blue stragglers”

Three ways to end the life of a star:

1. Black hole

2. White Dwarf

3. Neutron Star

And . . .

“Failed star”

Less than 80% of sun’s mass, so not enough mass to sustain fusion behaves like a planet.

Looks and behaves like even more massive Jupiter—very dim and similar cloud structures of Iron vapor.

Disks left around them that may turn into stars?