Introduction to Astrophysics

Lecture 16: Black holes in astrophysics

A black hole this size will have a mass 100 times that of the Earth!!

Black holes

So far we have met black holes in two contexts:

The endpoint of evolution of very massive stars.

The supermassive black hole thought to inhabit the centre of our Galaxy.

In fact black holes are implicated in a very wide range of astronomical phenomena.

Black hole properties

A black hole arises when the force of gravity around an object is so intense that not even light can escape.

They were first envisaged in 1783 by Cambridge mathematician John Michell, who used Newtonian gravity to work out the escape velocity and conjectured it could exceed light speed. Laplace also studied them in the eighteenth century.

Nowadays we use Einstein’s theory of general relativity to describe black holes. The name black hole was invented in 1969 by John Wheeler.

Black hole properties

The surface from within which even light cannot escape is called the event horizon. Its radius is known as the Schwarzschild radius, after the German mathematician who first solved Einstein’s equations (though it would be fifty years before it was properly appreciated what a black hole is).

The Schwarzschild radius is given by the formula

r = 2GM /c 2

where M is the mass of the black hole, G is Newton’s constant and c is the speed of light.

Mass Characteristic object

Radius

6 x 1024 kg Earth 1 cm

2 x 1030 kg Sun 3 km

2 x 1036 kg A million Suns 3 million km (4 Sun radii)

Some examples:

Black hole properties

Anything which crosses the event horizon will never be able to escape again, and will inevitably be crushed out of existence at the centre of the black hole, which is known as the singularity.

Time does weird things in the vicinity of the event horizon. An astronaut falling in will perceive that they have crossed the event horizon and then are ripped to shreds and crushed to death at the centre after a finite time (well, they might not perceive that last bit too clearly).

However, to an observer outside the black hole, time seems to pass more and more slowly at the event horizon, and the astronaut appears `frozen’ at the surface for an infinite time.

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Weird properties of light: what do you see if you orbit a black hole?

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Weird properties of light: what do you see if you plunge into a black hole?

NB: black holes do not `suck stuff in’. Their gravity acts like a star’s; you are no more likely to plunge into a black hole than you are with a star.

Astrophysical black holes

Surprisingly, black holes tend not to be hard to see!

The extreme gravitational forces sucking material in tend to create very energetic environments which lead to very bright radiation.

In fact, some of the very brightest astrophysical phenomena may be due to black holes!

X-ray binaries

Like normal stars, black holes are quite likely to reside in binary systems, with a star and the black hole in mutual orbit.

Especially if the star is a red giant, it may be easy for the black hole to pull material away.

Because of angular momentum conservation, the material will form a disk, and friction leads to radiation.

X-ray binaries

Such systems often emit strongly in the X-ray part of the spectrum. In binary systems the masses can be determined by Kepler’s Laws.

If the mass exceeds 3 solar masses, then a black hole is likely to be present as that is too massive to be a neutron star.

An example is the famous system Cygnus X-1.

Galactic nuclei

It is thought that supermassive black holes reside at the centre of some, or maybe even all, galaxies.

Mainly they give away their presence because the stars near the centre have extraordinarily rapid orbits, indicating an intense concentration of mass.

Active Galactic nuclei

In some galaxies the central black hole gives away its presence more dramatically, by intense radiation from a disk of material being consumed by the central black hole. These are known as Active Galactic Nuclei (AGN).

The Circinus galaxy

AGN are often best seen in X-rays.

This is the centre of our galaxy.

The supermassive black hole candidate Sagittarius A* is indicated by the arrow.

Chandra satellite image

Not all galaxies have bright AGN emission, and it is not easy to tell by looking in the optical which ones do.

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Recent news: stellar orbits around the central black hole

This movie shows the motions of stars in the vicinity of the galactic centre.

Recent news: stellar orbits around the central black hole

This movie shows the motions of stars in the vicinity of the galactic centre.

The closest approach was about seventeen light hours – about three times the size of the solar system.

From the orbit, the central black hole is found to have a mass of 2.6 million times the Sun.

Quasars

Quasars (short for quasi-stellar object) are the brightest known objects in the Universe, and are seen at the greatest distances. They were discovered by Maarten Schmidt in 1963.

They are thought to be extremely powerful AGN, housed mostly in the first generation of large galaxies.

They are probably bright because there is lots of material to `eat’ at that early stage.

Quasars

Quasars exhibit variation on timescales of days, which implies a maximum size of light days.

Given their prodigious energy emission, they probably need to contain black holes with masses up to ten billion solar masses.

Because they are highly visible, they may prove an extremely useful probe of the young Universe. X-ray image

The course quiz!

Takes place during the class on Friday 2nd December. It will start promptly at 9am. If you arrive late you lose that time.

There will be 40 multiple choice questions and you will have 40 minutes to complete them.

The quiz contributes 60% of the total assessment for this course.

The material covered will be up to and including the lecture on Monday 28th. The material in example sheets is also part of the syllabus.

NB: Tomorrow’s lecture is cancelled and will take place on Monday instead in the usual slot.