1. ASTR 1040 – October 12 Next Observatory Opportunity: Tonight at 8:00
Then next Tuesday at 8:00
Homework 3 posted due next Thursday
Homework 4 posted soon
Second Mid-term November 2
LA Session UMC235 today .
Website

2. Chandrasekhar Limit The Chandrasekhar Limit for a White Dwarf is 1.4M
A peculiarity of Degeneracy Pressure is that it has a maximum mass.
Each electron added must find its own quantum state by having its
own velocity.
But what happens when the next electron has to go faster than light?
The Chandrasekhar Limit for a White Dwarf is 1.4M
No White Dwarf Can have more than 1.4M
Otherwise it will groan and collapse under its own weight.
We’ll come back to this later.

3. WDs are Common
Every star with less than 5M will end up as a White Dwarf
Most stars with mass above 1.3M have reached end of MS life.
White Dwarfs are VERY common ~ 10% of all stars
Closest is only 2.7pc away. (Sirius B)
Will become increasing common as universe ages.

4. Immortal Stars
Regular stars need thermal pressure to balance gravity, and they
need nuclear reactions to maintain the pressure, so the die when they
run out of fuel.
Not so White Dwarfs. They are as stable as a rock. Literally.
A quadrillion years in the future all the stars will be gone, but the
White Dwarfs will still be here.
Their glow is fossil energy left from their youth as a regular star.
Might die in 1031 years if protons prove to be unstable themselves.
That’s 10,000,000,000,000,000,000,000,000,000,000 years!
Really don’t know if universe will still be here.

5. Binary Stars
Optical Double appear close together but aren’t really binary
Visual Binary orbiting, but we can see them both
Astrometric Binary proper motion wiggles to show orbit
Spectrum Binary spectra of two stars of different type
Spectroscopic Binary Doppler shift shows orbital motion
Eclipsing Binary light varies
Half of all stars are in binaries….
Binary stars are formed at birth.
Both components will have same age and composition.
Can vary in mass
Can be very distant (0.1pc) or touching

6. Spectroscopic Binary

7. Variable Stars Some stars just expand and contract. Eclipsing Binary
Algol – “The Devil Star”

8. Russian Variable Star Catalogue
Compilation of all the stars that vary.
Letter starting with R, followed by Constellation Name
After Z starts RR through ZZ, then AA
SS Cygni
VY Hydrae
W Ursa Majoris
Gets funny on occasion
RU Lupi
EZ Sextans

9. Close Binaries
Gravity
Mid-Point
Equal Energy Curves

10. Contact Binaries Very Close Touching Common Envelope Two Nuclear Cores
“W Ursa Majoris” star

11. Periods of Contact Binaries
By Kepler’s Law: P ~ R(3/2)
R = (1/200) AU
So P = (1/200)(3/2) years = 3.5x10-4 years = 104 s = 3 hours
These contact binaries swing around each other
every few hours!

12. Mass Transfer Huge Flow of Material from One to Other
Can stop evolution of one and speed up other
Gets complicated
“Dog Eat Dog” Scenario

13. Mass Transfer Binary
White Dwarf & Star
The Roche Lobe

14. Mass Transfer Binary Accretion Disk Material Swirls In
Friction allows the material to fall
and heats while it falls.
All the way to the surface

15. Energy Released
Huge amounts of energy are released as the material swirls in.
Material get hot. Really hot. Like a million degrees Kelvin.
Emits ultraviolet and x-rays.
We can see these accretion disks with x-ray telescopes!

16. Material Reaches Surface
Carbon White Dwarf
Layer of H build
up on surface
Pressure builds on the hydrogen.
Material pouring in heats it.

17. Nova One day the hydrogen ignites in huge nuclear rush.
Burns like a brush fire from one end of the star to the other.
This is called a “Nova”. A new star appears in the sky. Often visible to the unaided eye.
Lasts a few weeks to months.

18. The Sun can never go nova!
Novae
Hydrogen explodes into space
to create a shell of expanding
gas.
Gas expands outward at 500km/s
The Sun can never go nova!
It’s not a white dwarf in a close binary.

19. 3 Kinds of Novae Classical Novae Only seen once
Recurrent Novae Seen several times over last few hundred years
Dwarf Novae Pop off every few weeks to months
It’s just a matter of how fast material is transferring and how much needs to accumulate before the spark.

20. Supernovae Nature’s Biggest Explosion
10,000BC
185AD m pc
Crab II
Tycho’s I
Kepler’s I
1667 > Cas-A II
,000 SN1987A II
We now see a dozen or so every year in distant galaxies.

21. Supernovae Occur about once every hundred years per galaxy.
Briefly outshines the other 100Billion stars in the galaxy.

22. Type I Supernovae White dwarf is gaining mass.
Over time, the mass will approach the Chandrasekhar Limit
Remember, at 1.4M, electron degeneracy fails.
What happens?

23. White Dwarf Collapse
As WD starts to collapse, the material falls through the
gravitational field of the star.
It heats very rapidly.
In just a few seconds it reaches >100,000,000K.
Carbon and Oxygen ignite and burn by fusion to even heavier elements.
The whole star explodes in a frenzy of nuclear burning.
Blows completely apart.
All that remains is an expanding shell of gas that used to be a white dwarf
and the companion star slingshot into space.

24. Explosion
Explosion Starts at Center
where pressure is highest

25. Energy Released
Nuclear Energy Generates 2MeV per atom in forming molecule (burning)
2MeV = 3x10-13 Joules
Number of Atoms in Star:
Available Energy
About 1044J release in just a few seconds.
That’s as much energy as the Sun emits during its entire lifetime.
In a few seconds!!!!
This is so titanic we can see it across the universe
A billion trillion trillion atomic bombs
Gas returning to interstellar space has more CNO etc.

26. SN1987A – Before and After

27. The Crab Nebula Supernova Dominated Sky in 1054 AD
Observed by Chinese (not in Europe)
Recovered in 18th Century by Messier
Called a “Supernova Remnant”
1pc in diameter
Expanding Rapidly

28. Tycho’s Supernova Seen in X-ray Gas at 10,000,000K
Expanding at 5000km/s

29. Type II Supernovae High Mass Star --- M > 5M
In low mass star, envelope is blown off into space, creating
planetary nebula, before Carbon in core can flash.
High mass star has enough gravity to hold onto the gas.
Get a Carbon flash just like the Helium Flash
Carbon burns to Neon
Then Neon flash
Gets very complicated

30. Onion Skin Model

31. Nuclear Reactions neon shell 12C + 12C  20Ne +4He
20Ne + g  16O + 4He
oxygen shell
16O + 16O  28Si + 4He
silicon shell
iron core
28Si + 28Si  56Fe
Iron cannot nuclear burn at any temperature
(On border between fusion and fission)
Develops degenerate iron core than cannot flash
Just gets hotter and heavier down in the middle of the star

32. Collapse
When the degenerate iron core exceeds the Chandrasekhar limit,
electron degeneracy can no longer support it.
It will start to collapse.
Electrons do not have individual quantum states left.
They hide by merging with protons to form neutrons:
P + e-  n + n
Every time this happens, a neutrino is also created.
Neutrinos are free to escape to infinity and carry energy with them.

33. Reversal of the Nuclear Reactions
Every iron nucleus in the core was formed in nuclear burning.
There is one electron for each proton.
After electrons are absorbed, the nucleus consists of 56 neutrons.
That’s unstable and the nucleus dissolves into free electrons.
Millions of years of fierce nuclear burning is reversed in a few seconds!
The star keeps shrinking.
By the time it has shrunk from 6000 to 600km, this process is complete.
So it’s a ball of neutrons.
Still nothing to stop its collapse.
Keeps shrinking.
Finally, when radius is about 7km, it stops.
Has at least 1.4M, but is a speck the size of Boulder

34. Neutron Degeneracy
Neutrons, like electrons, must have individual quantum states.
What stops the descent is “neutron degeneracy”
Conceptually identical to electron degeneracy.
Because a neutron is 1838 times more massive than an electron,
the radius of the degenerate star is 1838 times smaller.
This is called a Neutron Star.
It is roughly 14km in diameter and has 1.4 times the mass of the Sun.
They are formed in the middle of Type II Supernovae.

35. Energy of Collapse
As neutron ball collapses it releases gravitational energy.
Sun will only emit 1044 J in its entire life.
This is about a thousand times greater than the energy released
in at Type I supernova.

36. Where did the energy go?
Neutron Stars were found in supernova remnants in the 1960’s.
Type I and Type II Supernovae have comparable brightness.
Type II’s are NOT 1000x brighter.
Where did 99.9% of the energy go??

37. Neutrinos
Ball of neutrons radiates thermal neutrinos the same way that
a ball of electrons will radiate photons.
These elusive particles carry away 99.9% of the energy.
So poorly coupled to regular matter that they travel unimpeded through
the Universe at close to the speed of light.
In fact, at this moment, each and every one of us has about 10 neutrinos
per second passing through our bodies. They were generated in distant
supernova in galaxies far, far away – long, long ago.

38. Neutrinos So what if we view this as a personal violation?
Let’s go to the Department of Homeland Security and ask them to
put up a shield that will protect us from these nasty neutrinos.
We’ll make it out of one of the best materials for stopping them – lead.
How thick will the shield have to be?
Answer:
About a parsec!!
Looks like neutrinos have a mass about a million times lower than
electrons.

39. Core Bounce When star reaches neutrons star
size it is collapsing so fast that it
overcompresses. It reverses direction
and grows some.
This outward shock wave couples into
the rest of the star and drives the
stellar envelope into space.
That’s the 1044J we see.

40. Explosive Nucleosynthesis
Expanding shell starts out very hot.
Nuclear reactions are taking place rapidly.
As it expands it cools rapidly.
Some very heavy elements can’t survive long at high
temperature. A few get frozen in during cooling.
That’s where all the elements heavier than iron come from.
That’s why heavy elements are expensive.
They’re made in supernovae! And they’re trace resultants at that.

41. SN1987A First naked eye supernova since 1604.
Discovered by Ian Shelton. Feb 23, 1987 UT
Showed hydrogen escaping at 30,000km/s
In Large Magellanic Cloud
Tiny galaxy orbiting the Milky Way.
Huge international response of the astronomy community.

42. Progenitor Star that exploded was tracked down.
Not terribly prominent. SK
B3 Supergiant m= M=-7.8
T=16,000K, R= 40R
Distance 55,000pc – actually outside Milky Way.

43. Stellar History Burned H 10,000,000 years He 1,000,000 C 300
Ne months O
Si days
POW!!!!

44. Rings HST image of SN1987A a few years after the event. Center is dim
3 Bright rings
Illuminated by the flash
Hourglass shape again
Means there was mass loss
prior to the explosion

45. Impending Collision
Blast wave is about to hit the ring.

46. Big Rings 30 Light Years With this geometry there is only a few light
months delay.

47. Neutrino Astronomy Ground Level Fill mineshaft with water
Put photo detectors around the inside

48. Neutrino Going Through Water
(scattering)
light produced
by secondary interactions n

49. Super Kamiokande

50. Detection of SN1987A Kamiokande II Japan 12 events 15s
IMB Ohio 8 events 5.6s
Mont Blanc neutrino detector saw a marginal signal 4.7 hours earlier. Real???
In 1987A we detected the formation of a new neutron star!

51. Pulsars Neutrons Stars considered unobservable
Forgot the effect of magnetic fields
When a magnetic spins it creates electric fields
Electric fields create accelerated electrons
Accelerated electrons create strong radio signals
Pulsar is a spinning, magnetized neutron star

52. Intense Magnetic Field
Field not necessarily aligned with spin axis
Particles get thrown out along the polar axes (cannot cross field lines)
Beam radio signal along magnetic axis too.

53. From Above
Every time beam sweeps by we see a pulse

54. Lighthouse Analogy Some of the Cosmic Rays that
reach Earth could have been
created in Pulsars

55. Pulse Trains Listen to the Pulsars: PSR B0329+54 1.4Hz
Vela Pulsar 11Hz

56. Center of the Crab Nebula
Chandra (X-rays) and HST (Visible)

57. Pulsar Power Energy Source is the Rotation
As Pulsar emits, rotation slows
As pulsar slows, it becomes less luminous
Rapidly fade out
Period
Glitches or Starquakes
Time

58. The Binary Pulsar Pulses get closer on approaching
side of orbit (Doppler)
Can map out orbit
Two neutron stars spiraling
toward each other. Will merge
into black hole in about 200,000
years
Friction that causes the inward
spiral caused by Gravity Waves
First confirmation of Einstein’s
prediction
Period
Time

59. An X-ray Pulsar Hercules X-1 Discovered in 1971
Pulses every 1.24 seconds in the x-ray
Eclipses every 1.7 days due to binary orbit
Turns off every 35 days due to accretion disk wobble
The Rosetta Stone of X-ray Astronomy
Proved the bright x-ray sources were mass transfer binaries
with a neutron star.
There are about 100 of these “Classical X-ray Binaries” in the Milky Way

60. X-ray Bursters A Neutron Star Nova Intensity Time
Every few hours the x-ray source will “burst” for 30 seconds
Explanation: Helium accumulates on surface of the neutron star in binary
Periodically it explosively burns
Nuclear release insufficient to blow it out into space
Collapses back and cools
A Neutron Star Nova