Neutron Stars, Neutron Stars, Black Holes, and Black Holes, and

RelativityRelativity

Low Mass (M < 8 M) Stellar Evolution

• Main Sequence (core hydrogen fusion)

• Red Giant Star (core contraction, shell hydrogen fusion)

• Helium Burner (helium fusion in core)

• 2nd Giant Branch (core contraction, shell hydrogen and helium fusion, mass loss)

• Due to mass loss, the star is now less than 1.4 M (the Chandrasekhar limit)

• Planetary Nebula (ionization of mass lost as a giant star)

• White Dwarf star (inert carbon/oxygen core)

Planetary Nebulae

The Endpoint – A White Dwarf

Note: Electron Degeneracy only works if the star is less than 1.4 M. This is the Chandrasekhar Limit. If the star is more massive than 1.4 M, something else must happen.

In stars with final masses over the Chandrasekhar limit, the gravity becomes so great that even carbon and oxygen can fuse. The result is a host of products, including neon, sodium, magnesium.

The Death of a High Mass Star

Since 24Mg weighs less than two 12C atoms, the result is energy!

The Death of a High Mass Star

The products of fusion are getting heavier!

• Carbon-burning (temporarily) supplies energy to core. The core expands, shell-burning stops, and the star contracts.

• It doesn’t take long to burn all the carbon/oxygen. When the C/O is gone, the core again contracts, and C/O fusing is forced into a shell around the core.

The Death of a High Mass Star

Eventually, magnesium, etc., will begin to fuse. When it does, the result is …

The Death of a High Mass Stars

Aluminum, Silicon, Phosphorus, Sulfur , and Energy!

• Magnesium-burning (temporarily) supplies energy to core. The core expands, shell-burning stops, and the star contracts.

• The magnesium, etc., fuses very quickly, and when it’s gone, the core again collapses, and shell burning begins.

The Death of a High Mass Star

Soon, the core fuses silicon. When it does, the main products are

The Death of a High Mass Star

Iron, Cobalt, Nickel, and Energy!

• This time silicon-burning (temporarily) supplies the energy. The core expands, shell-burning stops, and the star contracts.

• Silicon fuses extremely quickly, and when it’s gone, the core again collapses, and shell burning begins.

The Death of a High Mass Star

When the star’s core turns to iron, it again collapses. The increased pressure and temperature then causes iron to fuse. However…

The products of iron fusion weigh more than the initial iron nucleus. According to E = m c2, this means that iron fusion does not make energy, it absorbs energy.

The Death of a High Mass Star

Up to iron, the products are lighter than the ingredients: + m c2

After iron, the products are heavier than the ingredients: - m c2

For heavy elements, you make energy by fission.

Fission and Fusion

When the star’s core turns to iron, it again collapses. The increased pressure and temperature then causes iron to fuse. However…

The products of iron fusion weigh more than the initial iron nucleus. According to E = m c2, this means that iron fusion does not make energy, it absorbs energy.

The Death of a High Mass Star

The more iron that fuses, the more energy is taken out of the core. The temperature decreases, the gas pressure decreases, the core collapses faster, more iron fuses and …

The star explodes! In that explosion, every element heavier than iron is created. This is the only way these heavier elements (such as silver, gold, etc.) can be created – in a supernova explosion.

Supernova

The Products of Supernovae

In a supernova, all the elements previously made in a star are thrown out into space. In addition, every element heavier than iron is made and ejected as well.

The Supernovae

For about a month, a supernova will outshine an entire galaxy of 100,000,000,000 stars!

Many of the elements made in a supernova explosion are radioactive, i.e., they make energy by nuclear fission. This is keeps the material bright for some time.

Supernova Remnants

Galactic Supernovae

In a galaxy such as the Milky Way, a supernova should occur once every 50 to 100 years. The last few were

Crab Supernova (1054 A.D.)

Tycho’s Supernova (1572 A.D.)

Kepler’s Supernova

(1604 A.D.)

Casseopia A (1680 A.D.?)

SN 1006 (1006 A.D.)

Neutron StarsIn addition to ejecting a large amount of (nuclear processed) matter into space, a supernova explosions will leave behind a stellar remnant. In the remnant, the electrons of atoms are crushed into their nucleus. The star becomes one gigantic atomic nucleus made up only of neutrons – a neutron star.

Neutron Stars

Neutron stars have masses that are similar to that of the Sun, but they are extremely small – only a few miles across!

And because neutron stars are so small, they spin very rapidly, due to conservation of angular momentum. Neutron stars rotate about once a second!

Pulsars

Neutron stars are extremely small, so, by L = 4 R2 T4 , their blackbody emission is minimal. However, they can beam light out from their magnetic poles via synchrotron emission.

If the “searchlight” points towards earth, we see a pulsar.

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Pulsars

Pulsar light comes out at all wavelengths, but is especially bright in the radio and the x-ray. The Crab pulsar is detectable in the optical.

(When first detected, these objects were dubbed “LGMs” for Little Green Men)

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What Supports a Star Against Gravity?

Type of Star What Holds it up? Limitation

Normal Stars Gas Pressure Must continually generate energy

White Dwarfs Electron DegeneracyMass must be less

than 1.4 M

Neutron Stars Neutron DegeneracyMass must be less

than ~ 3 M

What if a neutron star is greater than ~ 3 M? The neutrons will get crushed! There is nothing left to hold up the star. You get a Black Hole!Black Hole!

The Speed of Light

Imagine yourself in a river. The time it takes for you to swim upstream is longer than it takes for you to swim downstream.

The equivalent should be true for light. The time it takes for light to move upstream (against the motion of the Earth) should be longer than the time it takes to go downstream.

But it isn’t! The speed of light is always the same!

Special Relativity

Premise: constant velocity motion is relative (i.e., are you moving, or is the entire world moving past you?)

Since the speed of light is always the same, this has some weird implications.

Implication: A RealReal Speed Limit

Imagine holding a flashlight. You turn the flashlight on, and the light illuminates your path ahead.

Now perform the same experiment while running, i.e., while racing a beam of light. Can you win?

ANSWER: NO! For you are not running – you are standing still, and the whole world is running past you. And the speed of light as you measure it is always the same!

Wacky Addition of Velocities

Imagine running at ¾ the speed of light in one direction, while another person runs at ¾ the speed of light in the other direction.

0.75 c0.75 c

0.94 c

You do not observe the other person going away at 1.5 times the speed of light. The addition of velocities always add to < 1.0 c .

Implication: Time DilationImagine yourself in a large stationary spaceship. It takes light 1 second to get from the back of the spaceship to the front.

1 second

Implication: Time Dilation

1 second

1.5 seconds

“Pinky … you are a little slow.”

Light is traveling 1.5 rocket-ship lengths

Now the spaceship is moving. To you, the ship is standing still, and light still takes 1 second to go the length of the ship.

But to someone outside, the light has traveled more than one rocket ship length. Therefore, more than 1 second has elapsed.

Imagine yourself in a large stationary spaceship. It takes light 1 second to get from the back of the spaceship to the front.