1. Black Holes and Neutron Stars
Dead Stars
Copyright – A. Hobart

2. Goals What are black holes? How do we see black holes?
What happens when black holes are in binaries?
Supermassive Black Holes

3. Concept Test
Which of the following lists, in the correct order, a possible evolutionary path for a star?
Red Giant, Neutron Star, White Dwarf, Nothing
Red Giant, White Dwarf, Black Hole
Red Giant, Supernova, Planetary Nebula, Neutron Star
Red Giant, Planetary Nebula, White Dwarf
Red Giant, Planetary Nebula, Black Hole

4. Density Density = mass per volume
From Red Giant cores to White Dwarfs to Neutron Stars, density has been increasing.
As density increases, the force of gravity on the surface increases.
The greater the force, the higher the escape velocity:
How fast you need to go in order to escape the surface.
How dense can something get?
How strong can the force of gravity be?
What if the escape velocity is faster than light?

5. Singularity
When a high-mass star’s core is greater than ~3 x Msun, then, when it collapses, neutron degeneracy pressure can’t balance gravity.
The star collapses to form a singularity.
No size at all.
Density infinite.
Escape velocity > c

6. Black Hole Diagram
Singularity
Event Horizon .
Schwarzschild Radius

7. Schwarzschild Radius Distance from object where vesc > c Object
Mass
Radius
Earth
6 x 1024 kg
1 cm
Jupiter
300 x Earth
3 m
Sun
300,000 x Earth
3 km

8. Concept Test A black hole is best defined as:
a star which sucks all matter into itself.
a window to another Universe.
any object which is smaller than its event horizon.
the final result of all stellar evolution.
none of the above

9. Seeing Holes
Can’t see black hole itself, but can see matter falling into a hole.
Gravitational forces stretch and rip matter: heats up.
Very hot objects emit in X-rays (interior of Sun)
Cygnus X-1.

10. Binaries
Gravitational tides pull matter off big low density objects towards small high density objects.
Cygnus X-1

11. Holes Don’t Suck
Newton’s Laws of gravity only depend on mass and separation.
Kepler’s Laws of orbits only depend on mass and separation.
At 1 AU, force of gravity from a 1 Msol B.H. is same as from a 1 Msol star.
At surface of each, force of gavity is very different!

12. Tides M m m m M m
While each m is attracted to each other m, the difference in force from M is greater.
The closer you are to the object M, the more extreme this is!

13. Accretion disk
Tides
Frictional Heating

14. Concept Test We can see X-rays from black holes because?
X-rays are more energetic than visible light and so can escape from the event horizon.
X-rays can pass through ordinary matter showing us things we can’t normally see.
Light given off by objects as they enter the event horizon are gravitationally redshifted to X-rays.
Material flowing into a black hole is heated so much that the thermal radiation peaks in X-rays.
None of the above

15. Cygnus X-1
1970s
Intense source X-rays.
“Near” star HDE

16. HDE226868 Doppler shifts of HDE226868
Like before, we get mass of star and unseen companion.

17. The Companion
Result:
Period = 5.6 days
Total Mass ~ 28 x Msun
From spectral type of HDE we estimate its mass ~18 Msun.
Companion M = 10 Msun!
Massive!
But where is its light?
Dark!
Can’t be a normal star, or even neutron star.

18. X-ray Source? Star brightness fluctuates every 5.6 days.
X-rays drop off every 5.6 days!
Companion must be source of X-rays!
REH = 30 km!

19. Supermassive Black Holes
Photograph the center of a galaxy.
Distance
Velocity
Make spectrum of light from center.

20. Heart of Darkness From Doppler shift get a velocity.
From picture get a separation.
From Kepler’s Laws get a Total Mass.

21. The Dark Truth Observe: V = 400 km/s within 26 LY of center. So:
Period = 121,600 yrs
Separation = 26 LY = 1,600,000 AU
Total Mass in central pixel:
300,000,000 x Mass of Sun!
But where’s all the light?
Small, massive, dark  black hole?
REH = 6.5 AU!

22. For next week For 3/14: Review for Exam #2
For 3/14 after class: re-air Cosmos Chapter 9
For 3/16: Exam #2