Do black holes really exist?

Dr Marek Kukula, Royal Observatory Greenwich

Now have strong evidence for two classes of black hole

Stellar-mass black holes:few times the mass of the sun. Found throughout our own Galaxy.

Supermassive black holes:up to 10 billion times the mass of the sun. Found only in the centres of large galaxies.

What is a black hole?

• A region of space with such intense gravity that not even light can escape.

• First suggested in the 18th Century by Laplace.

• Idea confirmed by Einstein’s General Theory of Relativity.

Escape velocity

If enough mass is concentrated into a small enough volume its gravity will be so strong that even light will not be able to escape.

Strength of gravity depends on:•Mass of object

•Distance from centre of mass

A Black Hole

Event horizon

Background light distortedby intense gravitational fieldclose to the black hole.

Event horizon: escape velocity = speed of lightNothing can escape the gravitational pull insidethis radius.

Singularity: allmatter inside theevent horizon iscrushed to a pointof ZERO SIZE andINFINITE DENSITY.

How might black holes form?

Where should we look for them?

very large mass very small volume

To be sure that we’ve found a black hole astronomers need to demonstrate the object has:

Physicists and mathematicians might also like to see evidence for:

an event horizon a singularity( )

Stellar mass black holes

Nuclear reactions in the stellar core support a star against the inward force of its own gravity.

When the star’s nuclearfuel runs out it should begin to collapse…

Is this a way to form a black hole?

Death of a star like the sun

• When the the Sun’s helium fuel is exhausted it will have no further source of energy

• The outer layers of the star are gently expelled into space, forming a glowing “planetary nebula”

• The hot, dense stellar core is left behind to cool slowly over billions of years – a White Dwarf star

Everything depends on the mass of the star…

White Dwarf starThe mass of the sun in a volume the size of a planet.

Composed of “degenerate matter”.

… but it’s not a black hole

Planetary nebulae

Stars more massive than the Sun end their lives in Supernova

explosions:

Much of the star’s mass is lost in the explosion

A dense, compact coreis left behind.

If the remaining stellar core has a mass less than 3 times the mass of the sun it will form a Neutron Star:

Neutron star: a ball of subatomic particles Neutron star: a ball of subatomic particles supported by nuclear forcessupported by nuclear forces

Mass: 1.4 Mass: 1.4 3 times the Sun 3 times the SunRadius: 10 kmRadius: 10 km

Density: Ben Nevis per teaspoonful!Density: Ben Nevis per teaspoonful!

Do neutron stars really exist?

Radio signals from the centre of supernova remnants:

Lovell radio telescope, Jodrell Bank

“pulsars”

The discovery of pulsars

Jocelyn Bell-Burnell & Anthony Hewish 1967Jocelyn Bell-Burnell & Anthony Hewish 1967

Such rapid radio pulsations could only come from a verysmall, dense object with anintense magnetic field.

Exactly the properties expected for a rapidly spinningneutron star.

But this still isn’t a black hole!

For really massive stars (> 10 solar masses) the remaining stellar core will have a mass more than 3 times that of the sun. even neutrons cannot support this amount of mass.

The core is crushed down to a point of INFINITE DENSITY witha gravitational field so intense that even light cannot escape…

A Black Hole

How can we detect them?

Can’t see the black hole directly

But can try to observe the effects of its gravity on its surroundings…

Binary star systems

Many stars occur in binary pairs, orbitingeach other.

If one of the stars goes supernova, the collapsed core of the star will remain in orbit around its companion.

X-ray Binary SystemsThe collapsed stellar core is too small to be directly detected but we can infer its presence from its effect on the visible companion star.

Gas is stripped from the companion starand heated as it spirals in towards the neutron star or black hole.

This gas emits huge amounts of X-rays.

Anatomy of an X-ray binary system

Gravity of compact objectpulls matter off companion star

Accretion disc: shines in X-rays

Jets of material ejected at high speed, giving off radiowaves

Measuring mass in X-ray Binaries

Binary orbit around common centre of mass causes a wobble in the position of the visible star:

If the mass of the compact companion is greater than 3 times the mass of the sunit CANNOT be a neutron star.The object must be a black hole.

Speed of wobble gives mass ofinvisible compact companion.

Cygnus-X1: the best candidate for a stellar-mass black hole

From the ‘wobble’ of the visible star we can weigh the mass of the companion to be ~10 solar masses. Astronomers are 95% certain that Cyg-X1 is a black hole.

X-ray source associated with a binary star. 1 billion timesmore luminous in X-raysthan the Sun.

8 such black hole candidates are now known, with masses estimated at >3 solar masses

The case for stellar-mass black holes looks good

The evidence for stellar mass black holes

• Intense X-ray emission from gas falling onto an extremely compact object (< 3km across)

• Wobble of companion star indicates a mass of over 3 times the mass of the Sun

Physics suggests such an object can only be a black hole

Supermassive Black Holes

• 1963: radio source 3C273 associated with a blue star-like object.

• Implied distance is 2 billion light years.

Optical luminosity 250 times brighter than the milky way.

Quasi-stellar radio sources (Quasars)

Many similar objects soon discovered, all with highly unusual properties.

3C273

Imaging quasars with Hubble

Quasars lie at the centres of distant galaxies

Quasar properties

Luminous at all wavelengths

Jets compact, stable energy source

Rapid variability object is small

Powering quasars

• Extremely luminous• Extremely small

Only plausible energy source is an accretion disc around a black hole with millions of times the mass of the Sun.

The black hole’s accretion disc is only the size of the solar system, yet it emits more light than the 100

billion stars in the Milky Way.

X-rays from iron atoms

• Gas moving with velocities up Gas moving with velocities up to 100,000 km/s - exactly the to 100,000 km/s - exactly the speed we’d expect at the Event speed we’d expect at the Event HorizonHorizon

• Broad “emission tail” Broad “emission tail” evidence for gravitational evidence for gravitational redshift predicted by General redshift predicted by General Relativity close to a BHRelativity close to a BH

• High temperatures cause iron atoms to give off X-High temperatures cause iron atoms to give off X-raysrays• High speeds close to the black hole change the High speeds close to the black hole change the frequency of these X-rays frequency of these X-rays “Doppler Shift” “Doppler Shift”

X-Ray frequency

More evidence from the Hubble Space Telescope

Hubble finds signs of dormant black holes in most large galaxies, not just quasars

Stellar velocities:very massive,very compactobject ingalaxy centre.

Is there a Supermassive Black Hole in the Milky Way?

Radio image of the Galactic Centre

Sag A*

Infrared images Reveal the central star cluster:

The La Silla Observatory ChileThe La Silla Observatory Chile

The SHARP-1 CameraThe SHARP-1 Camera(Speckle-Interferometry)(Speckle-Interferometry)

Special technique counteracts atmospheric blurringto give accurate positions for the stars in the Galactic centre.

High resolution infrared imagingHigh resolution infrared imaging of the galactic centreof the galactic centre

1994 1997 2000

can track the motions of individual stars

Stellar motions in the Galactic centreStellar motions in the Galactic centre

mass of central object = 3 million suns

Chandra launch, July 23 1999Chandra launch, July 23 1999

Measure X-ray emission from the Galactic centre

Our black hole takes a snackOur black hole takes a snack

Before:

After:

What does the black hole look like?

The Evidence for Supermassive Black Holes

Energy source for quasars Quasar variability Stability of radio jets X-rays from iron atoms at the Event Horizon

Motion of gas in nearby galaxies Stellar motions in centre of Milky Way

only plausible explanation is a black hole

So do black holes really exist?

• Extremely compact stellar-mass objects in X-ray binary systems

• Extremely massive compact objects in the centres of most galaxies

We have found:

Their properties are exactly what we’d expectif they are powered by black holes

(BUT we still haven’t seen a black hole directly!)

Answer: yes (probably)

The End