1. Neutron Stars and Black Holes
Star Chapter 22

2. Standards Understand the scale and contents of the universe.
Explain how objects in the universe emit different electromagnetic radiation and how this information is used.
Examine investigations of current interest in science.

3. Neutron Stars What remains after a supernova explosion?
Type I: there is probably nothing left.

4. Neutron Stars
Type II (core-collapse supernova): part of star may survive.
Tiny ultra-compressed remnant of matter in a strange state.
Called a neutron star.

5. Neutron Stars Neutron stars are extremely small and very massive.
They are composed purely of neutrons, packed together in a ball about 20 km across (about size of a city).

6. Size of typical neutron
star next to
Manhattan Island

7. Neutron Stars They have a mass greater than the sun’s.
They are incredibly dense (~1 billion times denser than a white dwarf).
A single thimbleful of neutron star material would weigh 100 million tons, about as much as a good-sized mountain.

8. Neutron Stars They are solid objects with extremely powerful gravity.
On a neutron star, a 150 pound human would weigh the Earth-equivalent of 1 million tons!
The severe pull of gravity would flatten you thinner than the piece of paper you’re writing on!

9. Neutron Stars
They rotate rapidly (many times/s) and have strong magnetic fields.

10. Pulsars
The 1st observation of a neutron star was in 1967 in the form of a pulsar.
Pulsars emit radio radiation in the form of rapid pulses.
Some also emit visible, x-ray and gamma radiation.
Regular change in
intensity of radio
radiation

11. Pulsars Their periods of pulses range from 3 – 30 times/s.
Best current model: they are spinning neutron stars that periodically flash radiation toward Earth, like a revolving lighthouse beacon.
All pulsars are neutron stars but not all neutron stars are pulsars.

12. a: Crab Pulsar, blinks on & off 30 times/s
b: visible image showing pulsing
c: x-ray image showing pulsing

13. Chandra x-ray image
of Crab Pulsar
superimposed on
Hubble optical image.
Shows central pulsar,
disk & jets of
outflowing material

14. X-ray Bursters
Some neutron stars, that are members of binary systems, emit violent bursts of x-rays in events that are thousands of times more luminous than our sun, but lasting only a few seconds.

15. X-ray Bursters
These x-ray bursters occur when matter torn from the surface of the neutron star’s companion builds up on the neutron star’s surface.
Surface temperature rises until it’s hot enough to fuse hydrogen and a large burst of energy is released.

16. Gamma Ray Bursts
Bright, irregular flashes of gamma rays, lasting a few seconds.
One of deepest mysteries in astronomy today.

17. Gamma Ray Bursts
They never repeat in the same location and they originate at cosmological distances (i.e., far beyond the Milky Way).
Each burst generates more energy than a supernova explosion, but lasts only seconds.
They come from a source only a few hundred km across.

18. Gamma Ray Bursts There are 2 leading models for these bursts:
Merger of 2 ultradense stars in a binary system .
Failed supernova called a hypernova.
Instead of collapsing to a neutron star, the core of a massive star collapses to a black hole

19. Models showing the 2 theories for gamma ray bursts
Both predict fireballs that release energy in the form of jets

20. Gamma Ray Bursts
The Fermi Gamma-ray Space Telescope was launched in August 2008 to study gamma ray bursts.
With the data we get from the telescope, we may be able to determine which of the models is correct.

21. Fermi Space Telescope:
3 month composite map of gamma-ray sources

22. Black Holes The evolution of a star depends critically on its mass.
Low mass stars (<1.4 solar masses) leave behind a white dwarf when they die
High mass stars (1.4 – 3 solar masses) leave behind a neutron star.
For stars beyond 3 solar masses, not even tightly packed neutrons can withstand a star’s gravitational pull.

23. Black Holes
If enough material is left behind after the supernova, gravity finally wins and the central core collapses forever.
As the core shrinks, the gravitational pull becomes so great that not even light can escape.
We now have a black hole!

24. Einstein’s Theories of Relativity
Einstein’s theories of relativity are needed to describe black holes.
Special relativity states that:
The speed of light, c, is the maximum possible speed in the universe, and all observers measure the same value for c, regardless of their motion.

25. a: bullet fired from moving car would appear to outside
observer to have a speed equal to the sum of the speeds of
the car and the bullet
b: beam of light shining forward from spacecraft would still
be observed to have a speed of c, regardless of the speed of the aircraft

26. Einstein’s Theories of Relativity
2. There is no absolute frame of reference in the universe (i.e., there is no “preferred” observer relative to whom all other velocities can be measured. Instead, only relative velocities between observers matter (hence the term “relativity”)

27. Einstein’s Theories of Relativity
3. Neither space nor time can be considered independently of one another. Rather, they are each components of a single entity: spacetime. There is no absolute, universal time – observers’ clocks tick at different rates, depending on the observers’ motions relative to one another.

28. Einstein’s Theories of Relativity
General theory of relativity:
Einstein’s description of the universe is equivalent to Newton’s for situations encountered in everyday life, but they diverge radically in circumstances where speeds approach the speed of light & in regions of intense gravitational fields (more on this later).

29. Einstein’s Theories of Relativity
Two important points from these theories are that:
Nothing can travel faster than the speed of light.
All things, including light, are attracted by gravity.

30. Escape Speed
Escape speed is the speed needed for an object to escape the pull of another object.

31. Escape Speed
For an object that retains the same mass, but that is squeezed to a smaller radius, the escape speed increases.
Earth’s radius is 6400 km, and its escape speed is 11 km/s.
If Earth’s radius shrank to ¼ its present size, the escape speed would be 22 km/s.
With a radius of 1 km, Earth’s escape speed would be 630 km/s.

32. Escape Speed
And with a radius of 1 cm, the escape speed would be 300,000 km/s (the speed of light).
In other words, if Earth were squeezed to the size of a grape, the escape speed would exceed the speed of light and nothing could escape.
Earth would be invisible and uncommunicative (no signal could be sent to the universe beyond). Only Earth’s gravitational field would be left to signal the presence of its shrunken mass.

33. The Event Horizon
The event horizon is the critical radius at which the escape speed from an object would equal the speed of light, and within which the object could no longer be seen.

34. The Event Horizon
This critical radius is called the Schwarzschild radius.
It is proportional to an object’s mass:
for Earth = 1 cm
for Jupiter = 3 m
Rule of thumb: Schwarzschild radius is 3 km x an object’s mass, measured in solar masses

35. The Event Horizon
Every object has a Schwarzschild radius, which is the radius to which the object would have to be compressed for it to become a black hole.

36. The Event Horizon
The surface of an imaginary sphere with a radius equal to the Schwarzschild radius, and centered on a collapsing star is the event horizon.
It defines the region within which no event can ever be seen, heard or known by anyone outside.

37. The Event Horizon
Even though there is no matter associated with it, we can think of the event horizon as the “surface” of a black hole.
It is not a physical boundary, just a communications boundary.

38. The Event Horizon
If at least 3 solar masses of material remain after supernova, the remnant core will collapse catastrophically, diving below the event horizon in less than a second.
The core simply “winks out”, disappearing & becoming a small, dark region from which nothing can escape – a literal black hole in space

39. The Event Horizon
This fate awaits any star whose main sequence mass exceeds 20 – 30 times the mass of our sun.

40. Black Hole Properties
Only relativity can properly account for the bizarre physical properties of black holes.
General relativity states:
All matter tends to warp or curve space in its vicinity.
The greater the mass, the greater the warping.

41. Black Hole Properties
All objects, such as planets & stars, react to this warping by changing their paths (this replaces Newton’s gravity).
Close to a black hole, the gravitational field becomes overwhelming & the curvature of space extreme.
At the event horizon itself, curvature is so great that space “folds over” on itself, causing objects within to become trapped and disappear.

42. Space warping: any mass causes space to be curved.
As people assemble at the x on the sheet, the curvature
grows progressively larger (frames a, b & c).
The people are eventually sealed inside the bubble (d),
forever trapped & cut off from the outside world.

43. Black Hole Properties Analogy of a black hole to a pool table:
Top of pool table made of thin rubber sheet.
Heavy rock placed on it causes distortion & sheet sags.
The heavier the rock, the larger the distortion

44. Black Hole Properties
Trying to play pool, the balls passing near the rock would be deflected by the curvature.
In a similar way, matter & radiation are deflected by the curvature of space near a star.
The more massive an object, the more space around it is curved.

45. Pool table analogy

46. Space Travel Near Black Holes
Black holes are not cosmic vacuum cleaners.
They don’t cruise around space sucking up everything in their path.
They don’t go out of their way to drag in matter.
However, if matter gets too close to the event horizon, it will be unable to get out.

47. Space Travel Near Black Holes
If you fell into a black hole:
Falling in feet first, you would experience tidal forces that would stretch you enormously in height and squeeze you unmercifully laterally – a process that some astronomers call “spaghettification”.

48. Space Travel Near Black Holes
You would be torn apart before ever reaching the event horizon because the pull of gravity would be much stronger at your feet than at your head.
Your torn up pieces would be accelerated to high speeds and would collide with each other.

49. Space Travel Near Black Holes
Frictional heating from these collisions would cause your torn up pieces to emit their own radiation in the form of x-rays (a.k.a. Hawking radiation).
Your pieces would continue to emit x-rays until reaching the event horizon.

50. Space Travel Near Black Holes
Humans can’t enter orbit near a black hole because the stress is too great.
The human body can’t withstand stress greater than about 10 times the pull of Earth’s gravity, which corresponds to about 3000 km from a 10 solar mass black hole (with a 30 km event horizon).
We would therefore need to send a robot to the black hole.

51. Space Travel Near Black Holes
Imagine that you are in a spaceship at a safe distance from the black hole and, observing through a telescope, a robot that is approaching the black hole.
The robot has a light and a clock on it.
You will observe the following:

52. Space Travel Near Black Holes
Light:
Becomes more and more redshifted.
Redshift even if robot stays motionless.
This is gravitational redshift

53. Space Travel Near Black Holes
Photons (particles of light) are attracted by gravity & must work to escape gravity, so they expend energy.
They don’t slow down - always move at speed of light – they just lose energy

54. Space Travel Near Black Holes
A photon’s energy is proportional to the frequency of its radiation, so light that loses energy must have its frequency reduced (or, equivalently, its wavelength lengthened).
Therefore, radiation coming from the vicinity of a massive object will be redshifted; the greater the gravitational field the greater the redshift.

55. Space Travel Near Black Holes
Clock
Close to the black hole, the clock would tick more slowly than clock on observing spacecraft.
On reaching event horizon, the clock would seem to stop.

56. Space Travel Near Black Holes
All action would be frozen in time – an observer would never see the robot fall below event horizon.
This is called time dilation and it is closely related to gravitational redshift - imagine each “tick” of the clock as the passage of a wave crest.
The clock thus ticks at the frequency of the radiation & appears to slow down as the frequency decreases.

57. Space Travel Near Black Holes
From the robot’s perspective, the light is not redshifted and the clock keeps perfect time.

58. Deep Down Inside
No one knows what lies within the event horizon of a black hole.
This raises fundamental issues that lie at the forefront of modern physics.

59. Deep Down Inside
General relativity predicts that the core will collapse all the way to a point at which both its density and its gravitational field become infinite.
This point is called a singularity.
Singularities always signal the breakdown of the theory producing them.

60. Observational Evidence for Black Holes
Methods of detecting black holes include:
Stellar transits – black hole transits (crosses) in front of a star. Large mass of black hole deflects star light. Hard to detect in this way.

61. Observational Evidence for Black Holes
Binary system black holes – observe their effects on other objects.
There are many binary systems in our galaxy in which only one member can be seen and the invisible member is massive & emits large amounts of x-rays (Hawking radiation).

63. Observational Evidence for Black Holes
Ex: Cygnus X-1 – an x-ray source in the constellation of Cygnus. The source is less than a few hundred km in diameter.

64. Brightest star in photo is part of
binary system whose unseen
companion is Cygnus X-1

65. Cygnus X-1: artists conception

66. Observational Evidence for Black Holes
3. The strongest evidence comes from observations of the centers of other galaxies.
High resolution observations in many wavelengths show that the stars and gas near many galactic centers are moving extremely rapidly & orbiting a very massive, unseen object.

67. Observational Evidence for Black Holes
Masses inferred from Newton’s laws range from millions to billions of times the mass of the sun.
The intense energy emissions & short-timescale fluctuations in the emissions suggest the presence of massive, compact objects.

68. Observational Evidence for Black Holes
The leading (and presently only) explanation is that these objects are black holes that are supermassive or intermediate in mass.
Ex: M82 – unusual looking galaxy in which x-ray images taken by Chandra show several bright x-ray sources near the center.

69. Galaxy M82
X-ray observations (inset) show collection
of bright sources thought to be the result
of matter accreting onto black holes