Transcript Introduction to Astrophysics, Lecture 16

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.
Some examples:
Mass
24
6 x 10
Characteristic
object
kg
Earth
30
kg
36
kg A million Suns
2 x 10
2 x 10
Sun
Radius
1 cm
3 km
3 million km
(4 Sun radii)
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.
Weird properties
of light:
what do you see if
you orbit a black
hole?
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
Weird properties
of light:
what do you see if
you plunge into a
black hole?
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
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.
Recent news: stellar orbits
around the central black hole
This movie shows the motions
of stars in the vicinity of the
galactic centre.
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
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.