stellar mass black holes
TRANSCRIPT
Stellar mass black holes• Now we know that massive stars make black holes
• Black hole mass: A few solar masses
• But how do we find such black holes?
Gravity on other star?
Light bending?
Radiation?
Most stars are “binaries”
• They have a stellar companion
• They orbit each other following Kepler’s laws
• Most massive stars should have a companion
• Massive stars evolve into black holes
Some hold on to their companions when they make black holes
⇒ Some black holes have stellar companions
Accretion• When a black hole comes close
enough:
It can syphon off matter from the companion star!
This matter must ultimately fall into the black hole
This process is called accretion
• This is how black holes grow
• This is how we find and study most black holes
Binary accretion• How can matter flow from one
star to another?
• Both stars orbit each other
Now we have two figure skaters...
2x gravity + centrifugal barrier
The “Roche potential”• Recall the effective potential
• The same for two masses:
Rotation + gravity
Curves: Lines of equal energy
“Free” to move along these lines
• Four classes of lines
Around both masses
Around one
Around none
The “Roche lobes” (thick black lines)
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RS unstable stable
Roche lobe overflow• Surfaces enclosing one mass:
“Roche Lobes”
Inside: objects are “bound”
At the solid black line: Can move freely from one object to other
In the middle: unstable point
• Two options:
Companion star smaller
Nothing happens
Companion star bigger
Matter will flow...
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Accretion disks• Recall the skater...:
Matter cannot fall straight in
It must lose angular momentum
• Forces:
Gravity (pulls in and down)
Centrifugal (pushes out)
Accretion disks• Recall the skater...:
Matter cannot fall straight in
It must lose angular momentum
• Forces:
Gravity (pulls in and down)
Centrifugal (pushes out)
⇒ Matter forms rotating disk
Matter slowly spirals in (like water going down a drain)
It speeds up (just like going down a slide)
Friction heats it up and it begins to shine brightly
Accretion disks
Accretion power: gravity
en
ergy
• Einstein: E=mc2
Gasoline powered (chemical) car: 45 mpg
Efficiency: 1 gram of energy for every 2000 tons of fuel
Efficiency (chemistry)
• Einstein: E=mc2
Nuclear (fission) powered car: 200 million mpg
Efficiency: 1 gram of energy for every 2 kilograms of fuel
Efficiency (nuclear power)
• Einstein: E=mc2
Nuclear (fusion) powered car: 4 billion mpg
Efficiency: 1 gram of energy for every 100 grams of fuel
Efficiency (nuclear power)
• Einstein: E=mc2
Black hole (accretion) powered car: 45 billion mpg
Efficiency: 1 gram of energy for every 10 grams of fuel
Efficiency (gravity)
Artist’s rendition
Gra
vita
tiona
l ene
rgy
Gravity power• Most of the energy release:
Close to the object:
• half the distance = twice the energy
The smaller the object, the more energy
• If you drop an object onto
sun : white dwarf : black hole
• you get energy return of
1 : 1000 : 500,000
Distance from object
Accretion disks• What transports the angular
momentum?
Friction (viscosity)
• Friction in astronomy always small
Matter must move in slowly
⇒ Almost perfect circular orbits
• Particles on Kepler orbits!
Inner orbits move faster
Inner orbits: more energy
⇒ Inner disk is hotter
Accretion disks• What transports the angular
momentum?
Friction (viscosity)
• Friction in astronomy always small
Matter must move in slowly
⇒ Almost perfect circular orbits
• Particles on Kepler orbits!
Inner orbits move faster
Inner orbits: more energy
⇒ Inner disk is hotter
side view
20 million degrees
X-rays!
X-ray binaries
X-ray binaries• Accretion onto neutron stars
and black holes:
Bright X-ray sources
First detected X-ray source: Scorpius X-1 (neutron star)
• How bright?
100,000 x sun
Easy to see in other galaxies
A binary in our Galaxy can destroy X-ray instruments
Messier 83, 15 million lightyears away
• Accretion is violent:
A planet would be shredded in seconds
Magnetic eruptions constantly dissipate energy
⇒ Flares, bursts, flashes...
X-Ray Binaries
How do we know magnetic things flare?
The sun in X-rays: Magnetic dissipation
X-Ray Binaries• Accretion is violent:
A planet would be shredded in seconds
Magnetic eruptions constantly dissipate energy
⇒ Flares, bursts, flashes...
Time (20 seconds)
Inte
nsity
The Eddington limit
The Eddington limit• So: 10% efficiency
• That means:
The more you accrete, the more you shine
• Is there a limit?
The Eddington limit• Where does all the radiation go?
Outward
It must go through incoming gas
⇒ Radiation pressure
⇒
The Eddington limit• Where does all the radiation go?
Outward
It must go through incoming gas
⇒ Radiation pressure
• When radiation pressure outweighs gravity:
No more accretion
No more radiation
⇒ Maximum power:
⇒ 500,000 x Psun x M/Msun