Download Gravity simplest fusion

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 fusion
by 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 selfluminous 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 H
takes 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 dense
300,000x mass of earth
Size 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 1A
white dwarf
.0001% of all energy is given off as light
Type II
larger stars  10x size of sun
Massive stars have layers of different fusion
reactions like an onion with heaviest elements in
core:
HHe, HeC and O,
ONe and Mn, Si, S,
And then . . . Iron and heavier
--requires energy, not building energy
IRON 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.
Pulsar
Crab 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?