The Life and Death of Stars

We are “star stuff” because the elements necessary for life were made in stars

• Stars are born in molecular clouds consisting mostly of hydrogen molecules

• Stars form in places where gravity can overcome thermal pressure in a cloud

Infrared light from Orion

Orion Nebula is one of the closest star-forming clouds

Solar-system formation is a good example of star birth

As gravity forces a cloud to become smaller, it begins to spin faster and faster

Conservation of angular momentum

Protostar to Main Sequence

• Protostar contracts and heats until core temperature is sufficient for hydrogen fusion.

• Contraction ends when energy released by hydrogen fusion balances energy radiated from surface.

• Takes 50 million years for star like Sun (less time for more massive stars)

Temperature

Lu

min

osi

tyStars more massive than 100 MSun blow apart

Stars less massive than 0.08 MSun can’t sustain fusion

How massive are newborn stars?

Degeneracy Pressure:

Laws of quantum mechanics prohibit two electrons from occupying same state in same place

Pressure Gravity

If M > 0.08 MSun, then gravitational contraction heats core until fusion begins

If M < 0.08 MSun, degeneracy pressure stops gravitational contraction before fusion can begin

Life as a Low-Mass Star

• What are the life stages of a low-mass star?

• How does a low-mass star die?

High-Mass Stars> 8 MSun

Low-Mass Stars< 2 MSun

Intermediate-Mass Stars

Brown Dwarfs

Most of life is relatively boring

• Things get interesting after about 9 billion years….

• A star remains on the main sequence as long as it can fuse hydrogen into helium in its core

Thought Question

What happens when a star can no longer fuse hydrogen to helium in its core?

A. Core cools offB. Core shrinks and heats upC. Core expands and heats upD. Helium fusion immediately begins

Thought Question

What happens when a star can no longer fuse hydrogen to helium in its core?

A. Core cools offB. Core shrinks and heats upC. Core expands and heats upD. Helium fusion immediately begins

So the core is contracting and heating up…

• The core is inert He, but outside the core, in the radiation-zone, there is still plenty of H.

• As the core contracts it, of course, brings some of the rest of the star with it. A layer of H in the radiation-zone gets sufficiently hot to start fusing!

• We call this hydrogen-shell burning when you have a shell of H (around the He core) fusing into He.

So the thermostat is broken. Shell burning doesn’t do anything for the core. But it does fight back against the gravity of the rest of the star.

But don’t forget -- the core is still shrinking, even though there is some fusion in the shell going on.

That means that the star poofs out and expands into a Red SubGiant. Radius and Luminosity are bigger.

Stage 1: H-shell burning SubGiant

Helium fusion requires higher temperatures than hydrogen fusion because larger charge in bigger atoms leads to greater repulsion.Fusion of two helium nuclei doesn’t work (the beryllium barrier), so helium fusion must combine three He nuclei to make carbon.

But what about the core? Eventually the contraction of the core heats it up high enough for Helium Fusion to start

BUT! Note that Electron Degeneracy Pressure is supporting the core when

He-burning begins.

• That means that He-burning won’t immediately cause the core to expand back outward.

• Instead there is a Helium Flash where a huge amount of fusion occurs quickly and a lot of energy is released. The Degeneracy Pressure keeps the core’s temperature hot so there’s no lessening of the fusion rate.

Fortunately, this lasts only a short time.

• Thermal Pressure eventually does become larger than Degeneracy Pressure, and so lets the core expand.

• Core expansion means the H-burning shell expands too and cools off a bit.

• The thermostat is fixed, and the star goes back into equilibrium for a while, burning He in the core and some H in the shell.

• The star shrinks back out of Red Giant phase, but not all the way back to the main sequence.

Helium burning stars neither shrink nor grow because thermostat is temporarily fixed.

…except a solar-mass star will never get hot enough to fuse Carbon into something else. No more fusion can happen!

You’ll have two shells burning around the core -- a H shell and a He shell, but no more fusion in the core.

Again, the shells do nothing for the core, but they poof out the star even larger than before.It is at this point that probably our Sun will engulf Earth!

Stage 3: He and H shell burning Red Giant

A star like our sun dies by puffing off its outer layers, creating a planetary nebula.

Only a white dwarf is left behind

Stage 4: Outer layers lost to planetary nebula

Stage 5: White Dwarf

A white dwarf is about the same size as Earth

White dwarfs shrink when you add mass to them because their gravity gets stronger

Shrinkage of White Dwarfs

• Quantum mechanics says that electrons in the same place cannot be in the same state

• Adding mass to a white dwarf increases its gravity, forcing electrons into a smaller space

• In order to avoid being in the same state some of the electrons need to move faster

• Is there a limit to how much you can shrink a white dwarf?

The White Dwarf Limit

• Einstein’s theory of relativity says that nothing can move faster than light

• When electron speeds in white dwarf approach speed of light, electron degeneracy pressure can no longer support it

• Chandrasekhar found (at age 20!) that this happens when a white dwarf’s mass reaches 1.4 MSun

• He actually puzzled this out on the boat from India to England before he started his grad studies in physics. (Once at Cambridge his advisor told him he was crazy and to drop this work…..it won him the Nobel Prize)

S. Chandrasekhar

Hydrogen that accretes onto a while dwarf builds up in a shell on the surface

When base of shell gets hot enough, hydrogen fusion suddenly begins leading to a nova

Nova explosion generates a burst of light lasting a few weeks and expels much of the accreted gas into space

Thought Question

What happens to a white dwarf when it accretes enough matter to reach the 1.4 MSun limit?

A. It explodesB. It collapses into a neutron starC. It gradually begins fusing carbon in its core

Thought Question

What happens to a white dwarf when it accretes enough matter to reach the 1.4 MSun limit?

A. It explodes B. It collapses into a neutron star

C. It gradually begins fusing carbon in its core

Two Types of Supernova

Massive star supernova: Iron core of massive star reaches white dwarf limit and collapses into a neutron star, causing explosion

White dwarf supernova: Carbon fusion suddenly begins as whitedwarf in close binary system reaches

white dwarf limit, causing total explosion

These two types have different patterns of luminosity, so we can tell them apart….

Nova or Supernova?

• Supernovae are MUCH MUCH more luminous!!! (about 10 million times)

• Nova: H to He fusion of a layer, white dwarf left intact

• Supernova: complete explosion of white dwarf, nothing left behind

Low-Mass Star Summary

1. Main Sequence: H fuses to He in core

2. Red Giant: H fuses to He in shell around He core

3. Helium Core Burning: He fuses to C in core while H fuses to He in shell

4. Double Shell Burning: H and He both fuse in shells

5. Planetary Nebula leaves white dwarf behind

Not to scale!

Reasons for Life Stages

Core shrinks and heats until it’s hot enough for fusion

Nuclei with larger charge require higher temperature for fusion

Core thermostat is broken while core is not hot enough for fusion (shell burning)

Core fusion can’t happen if degeneracy pressure keeps core from shrinking

Not to scale!

High-Mass Stars> 8 MSun

Low-Mass Stars< 2 MSun

Intermediate-Mass Stars

Brown Dwarfs

Life as a High-Mass Star

High-Mass Star’s Life

Early stages are similar to those of low-mass star:

• Main Sequence: H fuses to He in core

• Red Supergiant: H fuses to He in shell around inert He core. But the extra mass soon produces the temperatures and pressures necessary to start He fusion.

• Helium Core Burning: He fuses to C in core (no flash)

High-mass stars become supergiants after core H runs out

Luminosity doesn’t change much but radius gets far larger

The fusion in the high-mass star is a sequence of similar events that repeat themselves:

• X is fusing in the core, making Y, but the core eventually runs out of X.

• Core contracts, allowing layers around the core to heat up, initiating an X-burning shell around the core.

• The shell burning does nothing for the core, but does change the star’s overall radius.

• Core continues to contract, eventually getting hot enough to let Y start fusing into Z.

How do high mass stars make the elements necessary

for life?

I.e.: how do these stars actually make those heavier elements?

Big Bang made 75% H, 25% He

stars make everything else

Helium fusion can make carbon in low-mass stars

CNO (explained in the book) cycle can change C into N and O

Helium-capture reactions add two protons at a time and make more common elements

Helium capture builds C into O, Ne, Mg, …

Advanced nuclear fusion reactions require extremely high temperatures

Only high-mass stars can attain high enough core temperatures before degeneracy pressure stops contraction

Advanced reactions make heavier elements

Advanced nuclear burning occurs in multiple shells --Like an onion!

Iron is dead end for fusion because fusion reactions involving iron do not release energy

(Iron has the lowest mass per nuclear particle)

Evidence for helium capture:

Higher abundances of elements with even numbers of protons

How does a high mass star die?

Core degeneracy pressure goes away. Gravity is so strong that it forces the electrons to combine with protons, making neutrons and neutrinos

Neutrons collapse to the center, forming a neutron star.

The collapse of the core also produces a HUGE explosion, a

Supernova

Energy and neutrons released in supernova explosion enables elements heavier than iron to form AND releases them into space.

Elements made during supernova explosion

Crab Nebula: Remnant of supernova observed in 1054 A.D.

Supernova 1987A is the nearest supernova observed in the last 400 years

before after

The next nearby supernova?

Life Stages of High-Mass Star

1. Main Sequence: H fuses to He in core

2. Red Supergiant: H fuses to He in shell around He core

3. Helium Core Burning: He fuses to C in core while H fuses to He in shell

4. Multiple Shell Burning: Many elements fuse in shells

5. Supernova leaves neutron star behind

Not to scale!

A neutron star is the ball of neutrons left behind by a massive-star supernova

Degeneracy pressure of neutrons supports a neutron star against gravity

What is a neutron star?

A neutron star is about the same size as a small city

• The first neutron stars were discovered using radio telescopes that found very regular pulses of radio emission coming from a single part of the sky.

• These are Pulsars

Pulsar at center of Crab Nebula pulses 30 times per second

A pulsar’s rotation is not aligned with magnetic poles

Pulsars are rotating neutron stars that act like lighthouses

Beams of radiation coming from poles look like pulses as they sweep by Earth

X-rays Visible light

We can even see the radiation coming out of pulsar’s poles

Thought Question

Could there be neutron stars that appear as pulsars to other civilizations but not to us?

A. YesB. No

Thought Question

Could there be neutron stars that appear as pulsars to other civilizations but not to us?

A. YesB. No

Matter falling toward a neutron star forms an accretion disk, just as in a white-dwarf binary

What happens to a neutron star in

a close binary system?

Accreting matter adds angular momentum to a neutron star, increasing its spin

Episodes of fusion on the surface lead to X-ray bursts

Thought Question

According to conservation of angular momentum, what would happen if a star orbiting in a direction opposite the neutron’s star rotation fell onto a neutron star?

A. The neutron star’s rotation would speed up.B. The neutron star’s rotation would slow down.C. Nothing, the directions would cancel each other out.

Thought Question

According to conservation of angular momentum, what would happen if a star orbiting in a direction opposite the neutron’s star rotation fell onto a neutron star?

A. The neutron star’s rotation would speed up.B. The neutron star’s rotation would slow down.C. Nothing, the directions would cancel each other out.

A black hole is an object whose gravity is so powerful that not even light can escape it.

What is a black hole?

Thought Question

What happens to the escape velocity from an object if you shrink it?

A. It increasesB. It decreasesC. It stays the same

Thought Question

What happens to the escape velocity from an object if you shrink it?

A. It increasesB. It decreasesC. It stays the same

Hint:

The “surface” of a black hole is the radius at which the escape velocity equals the speed of light.

This spherical surface is known as the event horizon.

The radius of the event horizon is known as the Schwarzschild radius.

A black hole’s mass strongly warps space and time in vicinity of event horizon

No Escape

Nothing can escape from within the event horizon because nothing can go faster than light.

No escape means there is no more contact with something that falls in. It increases the hole mass, changes the spin or charge, but otherwise loses its identity.

BLACK HOLES DON’T SUCK

• As one gets farther away from the object, the strength of gravity decreases (remember…inverse square law).

• Away from the black hole, it doesn’t matter if it is a black hole or a star of the same mass.

• You will orbit a black hole exactly the same as you would a star of the same mass.

Neutron Star Limit

• Quantum mechanics says that neutrons in the same place cannot be in the same state

• Neutron degeneracy pressure can no longer support a neutron star against gravity if its mass exceeds about 3 Msun

• Some massive star supernovae can make black hole if enough mass falls onto core

Beyond the neutron star limit, no known force can resist the crush of gravity.

As far as we know, gravity crushes all the matter into a single point known as a singularity.

3 MSun Black Hole

The event horizon of a 3 MSun black hole is also about as big as a small city

Neutron star

Thought Question

How does the radius of the event horizon change when you add mass to a black hole?

A. IncreasesB. DecreasesC. Stays the same

Thought Question

How does the radius of the event horizon change when you add mass to a black hole?

A. IncreasesB. DecreasesC. Stays the same

If the Sun shrank into a black hole, its gravity would be different only near the event horizon

Black holes don’t suck!

What would it be like to visit a black hole?

Light waves take extra time to climb out of a deep hole in spacetime leading to a gravitational redshift

Black Hole Verification

• Do Black Holes really exist? After all you can’t see them…..

• Need to measure mass Use orbital properties of companion Measure velocity and distance of orbiting gas

• It’s a black hole if it’s not a star and its mass exceeds the neutron star limit (~3 MSun)

The jet emitted by the galaxy M87 in this image is thought to be caused by a supermassive black hole at the galaxy's centre

At the center of the Milky Way stars appear to be orbiting something massive but invisible … a black hole?

Orbits of stars indicate a mass of about 4 million MSun