Click the above image to enlarge. This image shows a supermassive black hole, one of the most energetic
phenomena in the known Universe, and one of Nature's engines of destruction. These objects may outshine a
galaxy of 100 billion stars by one hundred times! Alongside gamma-ray bursts they are the most energetic
phenomena observed in the known Universe.
When a massive star ends its life in a supernova explosion, the core may remain as a neutron star or pulsar.
However, if the core is above a critical mass then it will collapse further. As it collapses its density will increase,
and so its gravitational field will become stronger and stronger. When a rocket launches from the surface of the
Earth it needs a certain minimum speed to break free from Earth's gravity, otherwise it will simply fall back to
Earth. This minimum velocity is called the escape speed. On Earth this escape speed is 11.2 kilometres per
second. This is a great speed, and so rockets need tremendous thrust.
On the surface of a collapsing massive stellar core remnant, the gravitational field becomes so increasingly
intense as the core collapses that eventually the escape velocity exceeds the speed of light, about 300 000
kilometres per second! Since no massive object can travel this fast, nothing can escape the gravitational field
of such an object. Not even light is fast enough to escape, and so the object becomes completely black and is
known as a black hole. As the core collapses further we have noway to observe what happens to it, it is
shrouded in a black sphere from which neither light nor any object nor any signal can escape. The surface of
this mysterious sphere is called the event horizon. Any spaceship that crosses the event horizon is lost
forever and will be torn apart within seconds, if not instantly. The core continues to collapse inside the event
horizon, and Einstein's Theory of General Relativity predicts that it will collapse into a singularity (an object of
zero diameter!). However, nature seems to abhor singularities and usually some twist of physics prevents this
from occurring. In the case of a black hole, once it reaches the minute Planck length of 0.000 000 000 000
000 000 000 000 000 000 000 016 metres (1.6^-35 metres) then size loses its meaning and nothing can be
said to be smaller than this, so the black hole may end up as such an unimaginably minute object containing
several times the mass of the Sun! However, the event horizon remains at a distance of about 10 000
kilometres (depending on the black hole mass) at a distance called the Schwarzschild radius (pronounced
Schvarzschilt). From our point of view, the event horizon is the size of the black hole.
What is an AGN?
The black holes described so far are called stellar black holes, since they form from dead stars and have
masses a few times that of the Sun. However, there are objects that appear to be supermassive black holes,
with masses of a million to a billion or more times that of the Sun! These black holes have Schwarzschild radii of
about one billion kilometres. Being so large the density does not have to be very high for the escape velocity to
exceed the speed of light (the strength of a gravitational field depends upon both the mass of the object and its
density). Indeed, for an object this massive if the density is only that of water, then a black hole will form! It is
hard to avoid the notion that black holes exist (although this has not been irrefutably proven) since if they do
not exist then some very strange physics must be invoked to explain why the escape velocity cannot exceed
that of light. Such a supermassive black holes is found in the centre of many galaxies. Your Milky Way Galaxy
has a black hole in its centre (or nucleus), called Sagittarius A* (pronounced 'Sagittarius A star') or Sgr A*
(pronounced 'Sag A star') which is about one million times the mass of the Sun. When these black holes are
being fed with plenty of material, such as from in-falling stars that spiral into the black holes, unable to escape
the immense gravitational pull, they emit vast amounts of radiation, in the form of gamma-rays, X-rays, radio
waves and ultraviolet light and they are then called active galactic nuclei (AGNs).
If black holes are black how can they be observed - the case of the AGN?
A black hole's immense gravitational field will start to disturb any nearby object that is outside the event horizon.
In many so-called 'active galaxies', including quasars, Seyfert galaxies and blazars, the galactic nucleus is
an active supermassive black hole. Stars at the centre of a galaxy, in the so-called galactic nucleus, are usually
very closely packed together and they will slowly spiral in to the black hole in the galaxy's centre. As they do so,
the stars are ripped apart by the huge gravitational (tidal) forces and their plasma streams into the black hole,
probably forming a hot blue accretion disc of material slowly spiralling into (accreting onto) the black hole at
its centre. Surrounding this disc there appears to be a torus (doughnut-shaped ring) of hot ionised gas that
perhaps feeds into the accretion disc. Hot clouds of gas (up to 16 000 degrees) may extend for up to one
hundred light years from the black hole and travel at speeds of up to 10 000 kilometres per second. These
clouds glow as they are struck by the intense ultraviolet light and X-rays given off by the hot accretion disc
and also by gamma-rays that may be given off by gas near to the black hole. Stars can be seen to spiral
rapidly around what is clearly a supermassive and compact object - a black hole as they slowly get sucked in.
Finally tremendous jets may be seen blasting from each side of the central object, extending up to 10 000
light years into space at 100 000 kilometres per hour and emitting intense radio waves. Such jets must have
been active for millions of years to travel so far.
The conclusion seems inescapable - no other theory seems able to account for all these tremendously
energetic phenomena, other than that there is a supermassive black hole at the centre of such galaxies. Some
galaxies even contain two such black holes, probably as they formed from two once separate galaxies colliding
What about the Milky Way?
The black hole at the centre of the Milky Way is not so energetic, since it appears to have swallowed up most of
the stars near to it and become inactive as it is starved of fuel. Occasionally, however, material falls into it and it
can be seen to suddenly flare up for a while as it 'digests' its meal.
The Sun is a star, so will the Sun be swallowed by the Sgr A*?
The Sun is very far away from Sgr A*, close to the edge of the Milky Way Galaxy, and the Sun will be long dead
before its material ever gets sucked into the galactic nucleus, or into a smaller stellar black hole (unless on the
off chance a rogue black hole wanders into the Solar System, which is probably very unlikely). Eventually,
however, most of the Universe's mass may well end up inside black holes. Current theory predicts that the
Universe will eventually die. The Universe is now some 13.7 billion years old (since the Big Bang) and is
currently expanding in size and in another one thousand billion years or so the stars will run out of material and
no new stars will be born and all life in the Universe will probably die. In about 1 000 000 000 000 000 000 000
000 000 (10^27) years time the black hole in the Milky Way nucleus will have devoured all the burnt out stellar
remnants (white dwarfs, black dwarfs, neutron stars and smaller black holes) and brown dwarfs and planets in
the Milky Way Galaxy. Eventually these galactic black holes may well be devoured by even larger black holes.
What becomes of black holes?
So most scientists consider a time when the Universe will consist of black holes and not much else. However,
black holes are not immortal, because in fact they are not expected to be entirely black! Although nothing can
escape the gravitational pull of a black hole once it crosses the event horizon, there is an odd way in which
black holes can decrease their mass as if by radiating photons, called Hawking radiation. This is a very slow
process, however, though it should accelerate as the black hole loses mass until finally the black hole explodes
in a ball of light. For the largest black holes this should take something like 10^109 (1 followed by 109 zeros!)
years! At the end of their unfathomably long existence these black holes are expected to finally die and the
Universe itself will be no more than cold, dilute radiation - the Universe will be dead, the so-called heat death
of the Universe. This is currently the opinion of most cosmologists, however, as some have pointed out, there
are alternative possibilities ... time will tell ... eventually!
Space and Time do odd things near a Black Hole!
Gravity is the apparent force that attracts massive objects to one another, such as the Earth to the Sun,
keeping the Earth in its yearly orbit, but how does gravity work? The Sun actually warps space and time
(spacetime) around itself, curving it, and the Earth, which is trying to go in a 'straight' line ends up circling the
Sun because of the curvature of spacetime. It is the presence of energy that causes spacetime to curve. (Note
that this is not curvature in the ordinary dimensions that we can visualise, but rather curvature in the 4th spatial
dimension). All forms of energy curve spacetime and so create a gravitational field. The more energy the object
has, the more it curves spacetime and the stronger the gravitational field. For most objects, including the Sun,
most of this energy is locked up as mass, and so we tend to speak of the 'gravitational attraction between two
masses' but all forms and sources of energy generate gravitational fields - including light, pressure, momentum
and gravity itself!
A black hole has the ultimate gravitational field that warps spacetime so tremendously, that nothing can escape
once it has crossed the event horizon - once past this point of no return, all paths in space and time lead to the
centre of the black hole and certain annihilation. No amount of energy can escape, because there is nowhere
else to go! Some scientists, however, think that a black hole may actually open into another Universe (or
another part of this one) as a white hole, but this is not known for certain. There remains a possibility of
travelling through a rotating black hole and emerging elsewhere from a white hole, who knows?
As a spaceship nears the event horizon of a rotating black hole, it will find that it is sent spiralling in complex
paths, not because the force of gravity pulls it about, as such, but because space and time are so distorted as
they are dragged around by the black hole that it will be impossible to travel in a straight line. However, escape
is still possible from this region, called the ergosphere, so long as one does not venture over the event
Black holes do even stranger things to space and time, but there is not enough space to coverall of these
Above and below: a section through and AGN, seen edge-on. Remember that this is the nucleus of a
large spiral galaxy, but the rest of the galaxy will be faint in comparison and from great distances may
be invisible. The nucleus consists of a vast extended torus of gas and dust with the mysterious
nuclear supermassive black hole in its centre, surrounded by an accretion disc (which is probably
less than 0.1 pc in diameter (pc = parsec = 3.26 light years = 3.08E+13 km)). Ionisation cones
extend outward as funnels of ionising radiation that escape from above and below the torus, radiating
clouds of gas within the host galaxy which glow. Also present are the two radio jets.
The clouds closest to the accretion disc (the blue clouds in the centre of the torus) have broad
emission lines and form the broad-line region (BLR). Broad spectral lines means that the narrow
peeks normally associated with spectra have been broadened and the most obvious mechanism in
this case is Doppler broadening - just as the wavelength of a police car siren increases as the car
moves away from you, making the sound deeper, so movements within the clouds can broaden the
wavelengths of light. Likewise, movement towards the observer will shorten the wavelengths.
High-speed motions in these clouds will broaden the range of wavelengths over which the emission
peek is visible. The broadening seen in the BLR corresponds to cloud velocities of about 10 000
Further away (50-100 pc) in the ionisation cone, are clouds of the narrow-line region (NLR) where
velocities are lower and so the emission lines broadened less, although the peeks are still broad
compared to those seen in non-AGN galaxies and correspond to velocities of several hundred km/s.
Further out still is the extended narrow-line region (ENLR).
Seyfert galaxies are galaxies with active nuclei. In type 2 Seyfert galaxies, only narrow lines are
visible (sometimes broad-lines can be faintly seen) whereas in type 1 Seyfert galaxies, both narrow
and broad-lines can be seen. (There are intermediate classifications, such as Seyfert 1.5, 1.8 and
1.9 galaxies, in which the broad-lines become increasingly few and harder to detect in the higher
subclasses). The reason for these differences is possibly due to our line-of-sight when we view
This AGN is seen face-on and the BLR is clearly visible, as well as the NLR - characteristic of a
Seyfert 1 galaxy.
Above: a Seyfert galaxy seen at an angle, such that the dusty torus hides the BLR, making it hard to
detect - characteristic of a Seyfert 2 galaxy.
In a Seyfert galaxy, the AGN is dim enough to allow the host galaxy to be seen and a Seyfert look
like a spiral galaxy with a bright star at its centre. Quasars, however, are the brightest class of AGN
and the nucleus far outshines the host galaxy which is often invisible (as its light is swamped by that
of the nucleus) or detectable as a quasar fuzz. Quasars also have stronger broad-lines compared
to their narrow-lines. Apart from line-of-sight effects, one possible explanation for the difference is
that the quasar central black hole is more active, undergoing an active phase of swallowing matter
by accretion. Normal stellar black holes are relatively placid - something has to get quite close to be
swallowed, although it might slowly spiral in. Supermassive black holes, however, seem quite
capable of stripping off clouds of plasma from nearby stars, which probably forms the clouds of the
BLR which are spiralling rapidly around the black hole before being swallowed.
It is hard to be certain that a black hole exists in AGNs. However, there is enough mass in the
central structures to easily form a structure whose escape velocity exceeds the speed of light (this
can be done even if the matter has an average density equal to that of liquid water, since there is
so much mass present!). It all depends how concentrated the central mass is, and this can only be
settled with higher resolution imaging. If indeed most of the mass is central, then it is hard to see
how a black hole could be prevented - if light cannot escape then we have a black hole by
definition, irrespective of the true state of matter within. It is very hard to see how exceeding the
escape velocity of light can be avoided if sufficient mass is present in a sufficient volume - either
case requires bizarre physics beyond current understanding, so those people who readily dismiss
the existence of black holes ought to think more carefully to find an alternative explanation! Indeed
all alternatives appear to have failed, except perhaps one - could the inner nucleus be a region of
very dense supermassive stars that coalesce until their combined masses become unstable,
resulting in rapid mass loss and supernova/hypernova explosions? This theory is not currently the
favourite, the black hole theory is, however, it should be born in mind that the upper mass of stars
has been considerably revised upwards with the discovery of very massive hypergiants. How large a
star can become by coalescence of several smaller stars is also unknown. Would such a
supermassive object be able to become a black hole or would it blow itself apart first?
The fact is that AGNs are very strange and mysterious objects that exhibit fascinating physics, much
of which no doubt awaiting discovery!
AGN Types and the Unified Model of AGNs