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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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