Stars Part Two:Stars Part Two:

Stellar EvolutionStellar Evolution

Overview of the life of a star:1. Formation of protostar from

a cloud of mostly Hydrogen gas.

2. Main sequence star3. Red giant

• White dwarf or…• Supernova -

• Neutron star or • Black hole

Formation of protostar:1. Gaseous clouds contract

under their own gravity.2. Regional areas of initial high

density accrete more and more gas.

3. Gravitational potential turns to heat.

4. Heat and pressure start fusion.

Birth of a star

IP Demo: Star_Birth.ip

Birth of a star1. As the cloud of gas and dust collapses,

a small rotation becomes big (Ice skater pulls in their arms...)

2. The rapidly spinning protostar often needs to get rid of angular momentum before it can start fusion.

3. The magnetic field channels rapidly spinning material out of polar jets

Birth of a star1. Eventually, the spin slows enough to

allow fusion2. The newly born star often blows away

the nebula it came from with its radiation.

3. The remaining material (still spinning) stays around the newly formed star in an accretion disk.

Birth of a solar system:

The New Star

Rocky Stuff

Icy and Gassy StuffAccretion Disk

The Solar System

National Geographic Magazine

The Inner Planets::

Mercury Venus Earth Mars Asteroids

•Close together (Relatively)•Terrestrial (made of rock like Earth)

The Outer Planets::

Jupiter Saturn Uranus Neptune Pluto

•Spread out (Relatively)•Gas giants

Life on the Main Sequence:1. Energy comes primarily from

the Proton-Proton cycle:(Hydrogen fusion)1H + 1H = 2H + e+ + ν1H + 2H = 3He + γ3He + 3He = 4He + 1H + 1H(requires heat and pressure)Hydrogen becomes Helium

Gravity - CrushingPressure

Heat - ThermalAgitation& Radiation Pressure

Thermal agitation and radiation pressure balances the tendency of gravity to crush a star:

4He accumulates in the core of the star:

Displacing the hydrogen

1. The rate of burn depends roughly on the cube of the mass

2. Even though larger stars have more fuel, they burn the fuel they have at a much faster rate.

3.Big stars are Brief, Bright, and

Blue

4.Diminutive stars are Durable,

Dim and reD

From Robert Garfinkle’s “Star Hopping”

.01 Billion Years

.1 Billion Years

1 Billion Years

10 Billion Years

100 Billion Years

500 Billion Years

From Jay Pasachoff’s “Contemporary Astronomy”

From Jay Pasachoff’s “Contemporary Astronomy”

A Star trying to be too big

The death of a star:1. When most of the Hydrogen in the core

has been used up, leaving a Helium core, the star cools down. (The Helium displaces the fusing Hydrogen)

2. Heat energy no longer balances gravity.3. Gravity collapses the He core.4. The heat generated by the implosion of

the core spurs more fusion of the remaining Hydrogen.

5. The outer envelope of the star expands, and cools. It is now a Red Giant

Collapse of the He Core:

Cools Down

Expands

Turning into a Red Giant :1. A star the size of the sun would expand

to the orbit of Venus, or maybe the earth.

2. As a red giant, the star blows off a great deal of its mass into space.

3. A star 8 time as massive as the sun will have a residual mass of 1 or 2 times the mass of the sun after its red giant stage.

4. Stunning image from the Hubble:

Helium Fusion:1. When the core gets hot and dense

enough, He begins to fuse:4He + 4He = 8Be + γ4He + 8Be = 12C + γ

2. The star contracts slightly and heats up, moving along the horizontal branch

3. Before the He is used up these reactions also occur:

4He + 12C = 16O + γ (mainly)4He + 16O = 20Ne + γ4He + 20Ne = 24Mg + γ

Carbon accumulates in the core of the star:

Displacing the Helium

Helium Fusion:Heats up and contracts

Carbon Fusion:1. When most of the Helium in the core

has been used up, leaving a Carbon core, the star cools down.

2. Heat energy no longer balances gravity.3. Gravity collapses the Carbon core.4. The heat generated by the implosion of

the core spurs more fusion of the remaining Helium.

5. The outer layer of the star expands, and cools briefly.

Cools Down

Expands

Collapse of the Carbon Core:

Carbon Fusion:1. If the remaining part of the star is more than

.7 times the mass of the sun, the core gets hot and dense enough to start Carbon fusion:

12C + 12C = 24Mg + γ16O + 16O = 28Si + 4He

2. Nuclei as heavy as 56Fe and 56Ni can be created if the star core is hot enough.

3. Nucleosynthesis and fusion stop with 56Fe and 56Ni as larger nuclei would require the input of energy, because of binding energy

From Douglas Giancoli’s “Physics”

Most tightly bound nuclei(If you go from less to more bound you release energy)

56Fe and 56Ni

So far:

Hydrogen Fusion stops

Helium FusionCollapse of C core

Carbon Fusion(if > .7 Msun)

Collapse of He Core

How do we know all this?By observing Globular clusters…

How do we know all this?

1. Globular clusters are thousands of stars that all formed at more or less the same time.

2. Globular clusters are much smaller than galaxies.

3. Galaxies create stars in an on-going process.4. The stars in a globular cluster accrete

suddenly and nearly simultaneously.

By observing Globular clusters…

Planetary Nebulas:

1. Some stars with mass 1-7 times the sun’s mass.2. While the star is fusing carbon, it shrinks and

gets hotter.3. The material blown off by the red giant phase

is overtaken by the material blown off by the carbon core collapse.

4. The rapidly spinning core creates a strong magnetic field that channels the expulsion of the outer envelope.

5. Some planetary cores might have a companion.

If the residual mass of the star is less than 1.4 times the current mass of the

sun, our story ends here.A star with the mass of the sun

becomes a White dwarf about the size of the earth.

The Pauli exclusion principle prevents the star from collapsing any further.It gradually runs out of Carbon fuel, getting dimmer and dimmer, until it

becomes a black dwarf.

If the residual mass of the star is less than 1.4 times the current mass of the

sun, our story ends here.A star with the mass of the sun

becomes a White dwarf about the size of the earth.

The Pauli exclusion principle prevents the star from collapsing any further.It gradually runs out of Carbon fuel, getting dimmer and dimmer, until it

becomes a black dwarf.

If the residual mass of the star is less than 1.4 times the current mass of the

sun, our story ends here.A star with the mass of the sun

becomes a White dwarf about the size of the earth.

The Pauli exclusion principle prevents the star from collapsing any further.It gradually runs out of Carbon fuel, getting dimmer and dimmer, until it

becomes a black dwarf.

Now for something completely different….

Wanna hear a scary story?

Do not adjust your television set

We are on a special schedule…

Life After the Main SequenceStarring:

Marcela SupernovaJoe Neutron Star

Bob QuasarMary Pulsar

Freda Black HoleMusic by “Warped Space Time”

If the mass of the star is greater than 1.4 times the mass of the sun. (This is called the Chandrasekhar limit) it don’t care about no Pauli exclusion principle.

When the Carbon Fusion fires burn down, gravity crushes the star.

The collapse of the star releases an incredible amount of energy. The star becomes a supernova, increasing in brightness by billions of times for a few days, and then dies out.

The terrific energy released by the collapse of the star creates elements heavier than Iron, and forces electrons and protons to combine creating neutrons.

Dogs become cats.

Republicans support campaign finance reform, and Democrats cut taxes

In February of 1987, a supernova occurred in the Large Magellenic Cloud, 170,000 ly from Earth. It was briefly visible to the naked eye.(Assuming your eye was naked in Australia)

1. The remnant of the supernova is composed almost entirely of neutrons.

2. White Dwarfs are the size of planets.3. Neutron stars are the size of towns.4. Some Neutron stars spin a thousand times a

second.5. The pressure is so high in the core atomic

nuclei cannot exist.6. The outer envelope is about a mile thick - a

crust of nuclei and electrons.7. The core is a super-fluid.

Neutron Stars:

From Jay Pasachoff’s “Contemporary Astronomy”

Picture of a Neutron Star:

Ticks are 5 seconds

1. In 1967, Antony Hewish of Cambridge University in England was studying the scintillation of radio sources due to the solar wind.

2. A graduate student named Jocelyn Bell Burnell discovered a strong night time source of “twinkling”.

3. Its location was fixed with respect to the stars.

Pulsars:1. Pulsars emit pulses some as short as 1/40th of a

second.2. There are many things they could not be.3. The only thing small enough, and rotating fast

enough was a neutron star

From Jay Pasachoff’s “Contemporary Astronomy”

Pulsars Movies

Real photos from hubbleAnimation

Black Holes:1. If the mass of the neutron star is bigger than

about 2 or 3 solar masses, it don’t care about no neutron exclusion principle.

2. Gravity collapses the neutron star even further.3. The star becomes a black hole - an object from

which even light cannot escape.4. Light is really fast.5. The curvature of space-time becomes infinite.6. General relativity doesn’t work.7. Um… we don’t yet have a quantum theory of

gravity.

Black Holes:1. Black holes actually do radiate energy from the

event horizon due to the Heisenberg uncertainty principle.

2. When stars orbit a black hole, we can see their orbit, but not the black hole. We can infer the mass from the mass of the star and its orbit.

3. The Andromeda galaxy has stars orbiting a dark object that is 30 to 70 million times the mass of the sun.

Picture of a Black Hole:

Quasars: (Quasi-stellar radio source)1. Massively bright.2. Intense radio source.3. Red shifted radiation.4. Black holes eating matter.5. Usually located in the centers of galaxies

Quasars:1. In falling material forms an accretion disk.2. Quasars are ravenous beasts.3. The black hole’s magnetic field pumps energy

into the accretion disk.4. The accretion disk gets hot.5. The accretion disk has tornadoes that create jets6. Predictions

1. Old bright Quasars are rare, young ones common

2. Recently disturbed galaxies should have bright quasars.