Lesson 8: White Dwarfs, Neutron Stars, Blackholes

Question 1

What is a white dwarf? How is it supporting itself against gravity?

Answer 1

• White dwarfs are the remaining cores of dead stars

Electron degeneracy pressure supports them against gravity
You can’t crush electrons, they can only get really close

Question 2

Why does a white dwarf that has MORE mass also have a SMALLER size?

Answer 2

• White dwarfs with the same mass as the Sun are about the same size as Earth
• Higher-mass white dwarfs are smaller

Question 3

What is the upper limit to a white dwarf mass? What’s it called? Why does it exist?

Answer 3

• As a white dwarf’s mass approaches 1.4 MSun, its electrons must move at nearly the speed of light

Because nothing can move faster than light, a white dwarf cannot be more massive than 1.4 MSun, the Chandrasekhar Limit (also known as the white dwarf limit)

Question 4

Describe what can happen to a white dwarf in a binary system.

Answer 4

• In a close binary, one star can steal mass from the other
• Mass falling towards a white dwarf from its companion has some angular momentum
• the matter therefore orbits the white dwarf in an accretion disk

So:

• Stars can feed off each other, mass transfer
• Main Star can go from M to G star

As a result:

• The star that gained mass evolves and gives mass back to the white dwarf, the white dwarf has degeneracy pressure… explodes as a supernova!

Question 5

What is an accretion disk?

Answer 5

When diffuse material is attracted to a massive central body, the flattened shape of the accretion disk is due to angular momentum.

Question 6

What is a nova? How does it compare to a supernova?

Answer 6

when the white dwarf, “steals” gas from its nearby companion star.
- The temperature of accreted matter eventually becomes hot enough for hydrogen fusion

Fusion begins suddenly and explosively, causing a nova

*Accretion blows up; the star doesn’t actually blow up
*

Comparing?
* Supernova are MUCH MUCH more luminous than novae (about 10 million times!!!)

Nova: H to He fusion of a layer of accreted matter; white dwarf left intact

Supernova: complete explosion of white dwarf; nothing left behind

Question 7

Compare the two different types of supernovas. What causes them? How do they appear different on our sky?

Answer 7

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

White dwarf supernova:
* Carbon fusion suddenly begins as white dwarf in close binary system reaches white dwarf limit, causing a total explosion

How they appear different?
* White dwarf supernovas reach a higher brightness than massive star supernova
* light curves differ
* Spectra differ (exploding white dwarfs don’t have hydrogen absorption lines)

Question 8

What is a neutron star? How does it support itself against gravity?

Answer 8

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

Question 9

What is neutron degeneracy pressure?

Answer 9

• The degeneracy pressure of neutrons supports a neutron star against gravity

Question 10

How does a neutron star compare in size and mass to a white dwarf or the Earth?

Answer 10

a dot compared to the White Dwarf and Earth

Question 11

If a supernova goes off, what may it leave behind as a remnant?

Answer 11

One sign of a supernova is where there’s a burst of neutrinos in all directions and hits earth before we see the bright light form the explosion

Question 12

What is a pulsar? How are they related to neutron stars?

Answer 12

Pulsars are rapidly spinning neutron stars that blast out pulses of radiation at regular intervals from seconds
- beams radiation along a magnetic axis as a way to get rid of angular momentum

Question 13

How fast can a pulsar pulse?

Answer 13

Spin Rate of fast pulsars ~1000 cycles per second

Question 14

Is it possible to see a neutron star on the sky, but that same neutron star could be a pulsar from the perspective of some alien civilization?

Answer 14

Yes

Question 15

Why do pulsars spin so fast?

Answer 15

When a rotating object shrinks in size, it spins faster

Conservation of Angular Momentum (demands the star to spin faster)

• When a star’s core collapses into a Neutron Star, it must speed-up to conserve angular momentum

Question 16

Is there a limit to neutron star mass?

Answer 16

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

Question 17

What is space-time? How does mass affect space-time? Compare a small mass object (like the Sun) with a large mass like a neutron star.

Answer 17

Space-time: the fabric of space, a single continuum where space and time are intertwined

The mass of an object determines how much space-time is bent

• The mass of an object determines the strength of its gravitational force
• Gravity bends spacetime

The Sun would bend space-time less than the Neutron star since it has a larger mass=larger gravitational force = larger bend

Question 18

What is a black hole?

Answer 18

• A black hole is an object whose gravity is so powerful that not even light can escape it
• Some massive star supernovae can make a black hole if enough mass falls onto the core

Question 19

What is the event horizon? Schwarzschild radius?

Answer 19

This spherical surface is known as the event horizon

The radius of the event horizon is known as the Schwarzchild radius

Question 20

How big would the Schwartzchild radius of 1.5MSun black hole be?

Answer 20

increases when you add mass to it
1.5 x 2 = 3..
3 times the radius

Question 21

What is a singularity?

Answer 21

Theory of relativity predicts gravity crushes all the matter into a single point known as a singularity

Question 22

What is gravitational redshift? Time dilation?

Answer 22

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

Gravitational Time Dilation:

• Time passes more slowly near the event horizon
  ○ Therefore time curves - spacetime

Question 23

What is tidal stretching? Why doesn’t the Earth stretch us?

Answer 23

Tidal force - something that is closer to the mass will feel a stronger gravity (feet are being pulled more than head)
We on Earth aren’t close to a large mass object

• Tidal forces near the event horizon of a ~3MSun black hole would be lethal to humans
• Tidal forces would be gentler near a super massive black hole because its radius is much bigger

Question 24

What are the two different types of black holes

Answer 24

Stellar Black Hole: 10 Solar Masses
Super Massive Black Hole: 1,000,000 or billion Solar Masses… usually one at the centre of every galaxy

Question 25

How do we know black holes exist?

Answer 25

Founded by UofT professor at the David Dunlop
Confirmed the existence of blackholes

• 1960s: The strongest Galactic X-ray source was discovered (Cygnus X-1) and proposed to be from an accretion disk around a stellar black hole

Question 26

What is the significance of the silicon fusing into iron? Why is this the ‘end of the line?’

Answer 26

Light nuclei (carbon, hydrogen) give up some of their binding energy in the process of fusing into more tightly bound, heavier nuclei (iron)

○ It’s the released energy that maintains the outward pressure in the core so that the star does not collapse
- Iron is the most tightly bound and stable

Question 27

What pressure is supporting the iron core before it collapses? Which other stellar object uses this type of pressure to support itself?

Answer 27

• The electrons at first resist being crowded closer together, and so the core shrinks only a small amount.
• Ultimately, however, the iron core reaches a mass so large that even degenerate electrons can no longer support it.
• When the density reaches 400 billion times the density of water, some electrons are actually squeezed into the atomic nuclei, where they combine with protons to form neutrons and neutrinos.
• Means the collapsing core can reach a stable state as a crushed ball made mainly of neutrons, which astronomers call a neutron star

Question 28

What is the maximum mass of a neutron star (roughly)? And what happens if a
neutron star is larger than that?

Answer 28

• When the collapse of a high-mass star’s core is stopped by degenerate neutrons, the core is saved from further destruction, but the rest of the star is literally blown apart
  Such an explosion requires a star of at least 8 MSun, and the neutron star can have a mass of at most 3 MSun.

Question 29

While the star is completely destroyed, why are supernovas so important in creating
life in the universe (namely, us!)

talk about Supernovae and Earth

Answer 29

• Supernovae — source of many of the high-energy cosmic ray particles
  ○ the particles from exploded stars continue to circulate around the vast spiral of the Milky Way.
• Scientists speculate that high-speed cosmic rays hitting the genetic material of Earth organisms over billions of years may have contributed to the steady mutations—subtle changes in the genetic code—that drive the evolution of life on our planet.

Question 30

How many neutrinos were detected from supernova SN1987A?

Answer 30

○ supernova actually released 1058 neutrinos - Earth captured 19 neutrinos

Question 31

How were pulsars discovered? By who?

Answer 31

Jocelyn Bell
* Found in the constellation of Vulpecula, was a source of rapid, sharp, intense, and extremely regular pulses of radio radiation
Pulsars — “pulsating radio sources.”

Question 32

describe the core mass limits/maximums for white dwarfs, neutron stars, and black holes

Answer 32

• Stars whose core masses are less than 1.4 MSun at the time they run out of fuel end their lives as white dwarfs
• Dying stars with core masses between 1.4 and about 3 MSun become neutron stars
• Stars with core masses are greater than 3 MSun result of the death of such massive stellar cores (called a black hole)

Question 33

How does ‘escape velocity’ apply to black
holes?

Answer 33

Escape velocity is the speed you need to have to escape the curve in space
○ The deeper the ‘hill’, the higher required velocity an objects needs to get out

Question 34

What does the ‘black holes have no hair’ phrase mean?

Answer 34

meaning that nothing sticks out of a black hole to give us clues about what kind of star produced it or what material has fallen inside.

Question 35

How would we go about searching for a black hole? How do Kepler’s laws factor in? How do accretion disks factor in? How do X-rays factor in?

Answer 35

Looking for a star whose motion (determined from the Doppler shift of its spectral lines) shows it to be a member of a binary star system
- find if the mass is greater than 3Msun using Kepler’s and Newtons Laws

Accretion and X-Rays:
If matter falls toward a compact object of high gravity, the material is accelerated to high speed. Near the event horizon of a black hole, matter is moving at velocities that approach the speed of light. As the atoms whirl chaotically toward the event horizon, they rub against each other; internal friction can heat them to temperatures of 100 million K or more. Such hot matter emits radiation in the form of flickering X-rays.