Download Part II: Ideas in Conflict.

An Introduction to Astronomy
Part XIII: Black Holes, Quasars and
Active Galaxies
Lambert E. Murray, Ph.D.
Professor of Physics
Einstein’s Theory of Special Relativity
The Special Theory Deals with Reference Frames
that move at constant velocity.
 Einstein was able to show that:

– In order to preserve the notion of “cause and effect”
nothing can travel faster than the speed of light.
– Because of this fact:
Events that are simultaneous in one reference frame are not
simultaneous in another.
 Moving objects contract in the direction of motion [Lawence
Contraction].
 Moving clocks run slower [The Twin Paradox].
 Objects that move at the speed of light can get anywhere in the
universe in no time at all.

Einstein’s Theory of General
Relativity
The General theory deals with accelerating
reference frames – and gravity accelerates objects.
 Einstein was able to show that mass “warps” space
to create the effect of gravity.
 This warping of space causes:

– Light to bend (or curve) in the presence of massive
objects.
– Time to slow near move massive objects.
– A gravitational redshift.
An Example of Curved Space
• The flat surface shown in (a) represents two dimensional space in
spacetime. You can think of it as a large flat sheet of rubber. In the
absence of any matter, straight lines are straight in our intuitive sense
(the sheet is flat).
• In the presence of matter, spacetime curves, as shown in (b) by the
curvature of the sheet when mass is laid on it (just like the rubber
would stretch). Straight lines, defined by the paths that light rays take,
are no longer straight in the “usual” sense, but follow the curvature of
the warped spacetime, just as a small marble would follow the
curvature of the rubber sheet.
• This curvature of space also creates gravity. If you place a large mass
on the rubber sheet and then release a much smaller marble near that
mass, the stretched rubber sheet will cause the marble to “roll down
hill” toward the larger mass.
Curved Spacetime and the Path of Light
• The bending of light by matter was confirmed in 1919
during a total solar eclipse. Photographs of stars in the
region of the sky near the sun during this eclipse showed
the exact displacement that Einstein’s theory predicted.
This is illustrated in the diagram above.
Time Slows Down Near Matter
• If two clocks are synchronized in space (a), and then
brought near the Earth and the Moon (b), the clock nearest
to the Earth will slow down more that the one nearer the
moon.
• This occurs because mass slows down the flow of time,
and Earth has more mass (and a higher density, which adds
to the effect) than the Moon.
Gravitational Redshift
• The color of light from the
same object located at
different distances from a
mass appears different as
seen from far away. The
photons that leave the
vicinity of the massive
object lose energy causing
a redshift. The closer the
light source is to the mass,
the redder the light
appears, and hence the
name gravitational
redshift. The same
argument applies to light
leaving the surfaces of
different stars.
Trapping of Light by a Black Hole
• (a) The paths and color of light rays
departing from a main-sequence,
giant, or supergiant star are affected
very little by the star’s gravitational
force.
• (b) Light leaving the vicinity of a
white dwarf curves and redshifts
slightly, whereas
• (c) near a neutron star, some of the
photons actually return to the star’s
surface.
• (d) Inside a black hole, all light
remains trapped. Most photons
curve back in. Those that move
straight upward become infinitely
redshifted, thereby disappearing.
Characteristics of Black Holes

Black Holes retain only three properties that it
possessed before forming:
– Its Mass
– Its Angular Momentum
– Its Electrical Charge
Black Holes lose their internal magnetic field upon
collapse.
 Most Black Holes are believed to maintain a
neutral charge.
 Thus, there are only two types of Black Holes to
consider:

– A Schwarzschild (non-rotating) Black Hole
– A Kerr (rotating) Black Hole
Structure of a Schwarzschild
(Nonrotating) Black Hole
• A nonrotating black hole has
only two notable features:
• its singularity, and
• its boundary.
• Its mass, called a singularity
because it is so dense, collects
at its center.
• The spherical boundary
between the black hole and the
outside universe is called the
event horizon.
• The distance from the center to
the event horizon is the
Schwarzschild radius.
Structure of a Kerr
(Rotating) Black Hole
• The singularity of a Kerr black
hole is located in an infinitely
thin ring around the center of
the hole. It appears as an arc in
this cutaway drawing.
• The event horizon is again a
spherical surface.
• There is a doughnut-shaped
region, called ergoregion, just
outside the event horizon, in
which nothing can remain at
rest. Space in the ergoregion is
being curved or pulled around
by the rotating black hole.
A Model of a
Rotating Black
Hole

Material in a region
surrounding a
rotating black hole
would form a
swirling “accretion
disk” just outside the
black hole.
What is the Evidence for Black
Holes?
 Since
solitary black holes are not directly
visible, we can infer their existence only
when they interact with other matter.
– This may occur in binary star systems.
– It may also occur when large number of stars
near the center of a galaxy fall into a
supermassive black hole.
– It may be evident by “gravitational lensing.”
X Rays Generated by Accretion of
Matter Near a Black Hole
• Stellar-remnant black holes,
such as Cygnus X-1, LMC X-3,
V404 Cygni, and probably
A0620-00, are detected in close
binary star systems.
• This drawing (of the Cygnus X1 system) shows how gas from
the 30 M companion star, HDE
226868, transfers to the black
hole, which has at least 11 Solar
Masses.
• This process creates an
accretion disk. As the gas
spirals inward, friction and
compression heat it so much
that the gas emits X rays, which
astronomers can detect.
Jets Created by a Black Hole in a Binary System
• Some of the matter spiraling inward in the accretion disk
around a black hole is superheated and redirected outward
to produce two powerful jets of particles that travel at close
to the speed of light.
Supermassive Black Holes
• The bright region in the center of galaxy M87 has stars and gas held in
tight orbits by a black hole.
• Doppler shift measurements allow us to calculate the central mass of
the galaxy.
• M87’s bright nucleus (inset) is only about the size of the solar system
and pulls on the nearby stars with so much force that astronomers
calculate its mass to be a 3-billion-solar-mass black hole.
Photograph of an Accretion Disk Around a
Supermassive Black Hole

Swirling around a 300million-M black hole in
the center of the galaxy
NGC 7052, this disk of
gas and dust is 3700 ly
across. The black hole
appears bright because of
light emitted by the hot,
accreting gas outside its
event horizon. NGC 7052
is 191 million ly from
Earth in the constellation
Vulpecula.
Image of OJ287 – a Supermassive Black Hole
Accretion Disk
Einstein rings - Light from distant galaxies is
bent into rings around intervening matter
(galaxies or black holes).
The Discovery of Quasars
In 1944, Grote Reber detected 3 strong radio
emission sources with a home-made radio
telescope in his back yard.
 Though two of these were identified as sources
within our galaxy (the galactic nucleus and a
supernova remnant), the third, Cygnus A, was not
clearly identified until 1951.
 In 1951 a visible source located at Cygnus A was
observed which looked like an odd-shaped
elliptical galaxy.
 When examined with a spectrometer, the light
from this galaxy exhibited unexpected emission
lines with a relatively large red-shift.

Cygnus A (3C 405) Today
 Today
we know that the red-shifted visible
spectra comes from the central galaxy and
the radio signals emanate from two radio
lobes located on either side of the galaxy.
Cygnus A (3C 405) Image
What Sized Red Shift?
A red-shift as large as that measured for Cygnus A
would imply (based upon Hubble’s Law) that this
galaxy was nearly 220 Mpc from us – farther from
us that any previously observed galaxy.
 For this radio source to produce a radio signal
large enough to be detected by a back-yard radio
telescope, and to be that far away it must be
emitting a HUGE amount of energy – hundreds of
times the output of the Milky Way.

Other Strange Stars
Around 1960 several “stars” were observed which
were also strong radio sources – unlike most stars
which are poor radio sources. These were 3C 48
and 3C 273.
 These stars exhibited strange spectral emission
lines that no one could identify.
 Initially, scientists believed that these “stars” were
in our galaxy. Finally, in 1963, Maarten Schmidt
realized that the emission lines were lines that had
been red-shifted by large amounts, placing these
“stars” at tremendous distances away from our
galaxy. Only then did we realize that these stars
were in fact the same type animal as Cynus A.

Note: 3C 405 refers to the object number 405 in the Third
Cambridge Catalogue of radio sources.
In 1963 Maarten Schmidt at CalTech identified the
strange emission lines of 3C 273 as significantly red
shifted lines of ordinary Hydrogen.
3C273
This Quasi-stellar object also
exhibited a luminous jet
Star-like Object 3C 48
This object
that looks
like a star
must be
enormously
luminous its redshift
indicates it
is 4 billion
light years
away!!
Quasars
Because these strange “stars” had looked so much
like a normal star, while also emitting radio waves,
they were originally dubbed “quasi-stellar radio
sources”, or “quasars”.
 Since these quasars are so distant and still have a
large apparent magnitude, we know that no single
star could emit that much energy – these objects
must be very bright galaxies.
 We have now identified a number of very distant
very “bright” objects exhibiting large red-shifts –
some of which are not radio sources. We associate
the name quasar, however, with both types of
galaxies.

An Indication of the Size of
Quasars
In the mid 1960’s it was discovered that some
quasars change in intensity over periods of
months, weeks, or even days.
 The variation in brightness of an object gives an
indication of the size of an object, since an object
cannot change in brightness faster than the speed
of light can travel across that object. Thus, an
object 1 ly in diameter cannot vary in brightness
with a period of less than a year.
 This means that some quasars must be relatively
small in size.

A Quasar Emits a Huge Amount of
Energy from a Small Volume
These changes in brightness could indicate that the
quasar cannot be larger than a few light years.
The Radio Galaxy M87
Visible
Elliptical Galaxy
Hubble Image
of Core of Galaxy
Radio Image
Galaxy is small bright dot in center.
The Nature of These Active
Galaxies
 Many
of these active galaxies have double
radio lobes associated with them.
 These double lobes appear to arise from jets
of electrons emitted from the galaxy at
relativistic speeds producing synchrotron
radiation in the radio part of the spectrum.
 The electrons are emitted along the
magnetic field lines coming from the poles
of the galaxy.
Computer Enhanced Radio Image of Cygnus A
Radio Emitting Lobes are Clearly Visible
(radio lobes are 320,000 lyrs wide)
Centaurus A - radio image superimposed on a
visible image...note no light from radio lobes
visible image
Radio lobes
Active galaxies lie at the center of double
radio sources
Giant Gas
Clouds
(surrounding the
galaxy)
Intergalactic
gas jet
Galaxy
(which is actually
quite large)
Supermassive Black Holes:
A Common Solution
 Astronomers
believe that supermassive
black holes at the center of these strange
and peculiar galaxies may explain what we
see:
– A relatively small, but massive energy source
– An energy source that radiates energy across
the spectrum
– An energy source that emits large quantities gas
in high-speed jets.
The location of the observer makes the difference in
what is seen ...
Model of the
center of an
Active Galaxy