Black Holes
Black holes are objects so dense that not
even light can escape their gravity,
and since nothing can travel faster than
light, nothing can escape from inside a
black hole . Loosely speaking, a
black hole is a region of space that has so
much mass concentrated in it that
there is no way for a nearby object to escape
its gravitational pull. Since
our best theory of gravity at the moment is
Einstein's general theory of
relativity, we have to delve into some results of
this theory to understand
black holes in detail, by thinking about gravity under
fairly simple
circumstances. Suppose that you are standing on the surface of a
planet. You
throw a rock straight up into the air. Assuming you don't throw it
too hard,
it will rise for a while, but eventually the acceleration due to the
planet's
gravity will make it start to fall down again. If you threw the rock
hard
enough, though, you could make it escape the planet's gravity entirely.
It
would keep on rising forever. The speed with which you need to throw the
rock in
order that it just barely escapes the planet's gravity is called
the
"escape velocity." As you would expect, the escape velocity depends
on
the mass of the planet: if the planet is extremely massive, then its
gravity is
very strong, and the escape velocity is high. A lighter planet
would have a
smaller escape velocity. The escape velocity also depends on how
far you are
from the planet's center: the closer you are, the higher the
escape velocity.
The Earth's escape velocity is 11.2 kilometers per
second (about 25,000 M.P.H.),
while the Moon's is only 2.4 kilometers per
second (about 5300 M.P.H.).We cannot
see it, but radiation is emitted by any
matter that gets swallowed by black hole
in the form of X-rays. Matter
usually orbits a black hole before being
swallowed. The matter spins very
fast and with other matter forms an accretion
disk of rapidly spinning
matter. This accretion disk heats up through friction
to such high
temperatures that it emits X-rays. And also there is some X-ray
sources which
have all the properties described above. Unfortunately it is
impossible to
distinguish between a black hole and a neutron star unless we can
prove that
the mass of the unseen component is too great for a neutron star.
Strong
evidence was found by Royal Greenwich Observatory astronomers that one
of
these sources called Cyg X-1 (which means the first X-ray source
discovered in
the constellation of Cygnus) does indeed contain a black hole.
It is possible
there for a star to be swallowed by the black hole. The pull
of gravity on such
a star will be so strong as to break it up into its
component atoms, and throw
them out at high speed in all directions.
Astronomers have found a half-dozen or
so binary star systems (two stars
orbiting each other) where one of the stars is
invisible, yet must be there
since it pulls with enough gravitational force on
the other visible star to
make that star orbit around their common center of
gravity and the mass of
the invisible star is considerably greater than 3 to 5
solar masses.
Therefore these invisible stars are thought to be good candidate
black holes.
There is also evidence that super-massive black holes (about 1
billion solar
masses) exist at the centers of many galaxies and quasars. In this
latter
case other explanations of the output of energy by quasars are not as
good as
the explanation using a super-massive black hole. A black hole is formed
when
a star of more than 5 solar masses runs out of energy fuel, and the
outer
layers of gas is thrown out in a supernova explosion. The core of the
star
collapses to a super dense neutron star or a Black Hole where even the
atomic
nuclei are squeezed together. The energy density goes to infinity. For
a Black
Hole, the radius becomes smaller than the Schwarzschild radius,
which defines
the horizon of the Black Hole: The death explosion of a massive
star, resulting
in a sharp increase in brightness followed by a gradual
fading. At peak light
output, supernova explosions can outshine a galaxy. The
outer layers of the
exploding star are blasted out in a radioactive cloud.
This expanding cloud,
visible long after the initial explosion fades from
view, forms a supernova
remnant. So, a black hole is an object, which is so
compact that the escape
velocity from its surface is greater than the speed
of light. The following
table lists escape velocities and Schwarzchild radii
for some objects: The black
hole masses ranging from 4 to 15 Suns (1 solar
mass = 1 Msun = 2 x 1033 grams.)
And are believed to be formed during
supernova explosions. The after-effects are
observed in some X-ray binaries
known as black hole candidates. The velocity
depends on the mass of the
planet. The scientists believe if our Sun dies, the
sun may turn into a black
hole. Black holes were theorized about as early as
1783, when John
Michell mistakenly combined Newtonian gravitation with the
corpuscular theory
of light . The concept of an escape velocity, Vesc, was well
known, and even
though the speed of light wasn't, Michell's idea worked the
same. He showed
that Vesc was proportional to mass/circumference and reasoned
that, for a
compact enough star, Vesc might well exceed the speed of light. His
mistakes
were twofold: he subscribed to the corpuscular theory of light, and
he
assumed that Newton's law of universal gravitation could apply to such
a
situation. These mistakes happened to cancel each other out, but when the
wave
theory of light gained favor, the astronomers abandoned these dark
stars. In the
beginning of the 20th century, Einstein proposed his theory of
general
relativity. The formula worked out by Michell and rederived, this
time without
mistakes in the derivation, by Karl Schwarzschild, gives the
Schwarzschild
radius for any massive body (that is, a body containing mass):
RS= 2GM/c2. Vesc
for any body smaller than this radius would exceed that of
light, and since
general relativity forbids this; any matter within RS would
be crushed into the
center. Thus RS can effectively be thought of as the
boundary of a black hole,
called an event horizon because all events within
RS are causally disconnected
from the rest of the universe. There aren’t many
physical features of a black
hole. In an aphorism coined by John Wheeler ,
"black holes have no
hair," hair meaning surface features from which details
of it's formation
might be obtained. There are no perturbations in its event
horizon, no magnetic
fields. The hole is perfectly spherical and in fact has
only three attributes:
it's mass, it's spin (angular momentum), and it's
electric charge. Of these
properties, it is only the mass that concerns
astronomers. As a cloud of gas
contracts, the interior heats up until the
core is so hot and dense that nuclear
reactions can occur. This
nucleosynthesis of hydrogen into heavier elements
generates a tremendous
pressure, according to the ideal gas law P=NkT, and this
pressure holds the
star up against further gravitational collapse. This state of
equilibrium,
during which a star is said to be on the main sequence, lasts until
the
hydrogen in the core is used up, about 10 billion years for a star like
the
sun, whereupon gravity will resume shrinking the star. Exactly what
occurs next
depends on the complicated interactions between different layers
of the star,
but generally, the star will explode in a supernova. If there is
any remnant of
this explosion, its further evolution depends almost
exclusively on it's mass. A
remnant below ~1.4 M (@) will collapse until it
can be supported by electron
degeneracy pressure and form a white dwarf. A
remnant between ~1.4 and ~3 M(@)
is halted by neutron degeneracy pressure and
forms a neutron star. Degeneracy
pressure is an effect that results from
quantum mechanical interactions when the
density of subatomic particles
increases. As it depends only on this density, it
is non-thermal and will
remain no matter how much the star cools down. Still for
remnants above ~3
M(@), not even degeneracy pressure can counter the force of
gravity, and a
black hole is born. This was the general base that general
relativity gave to
astronomers, but just because something is allowed to happen
doesn't mean
that it does. Most astronomers resisted such absurd
realities.
Astronomers are very conservative by nature, and some of the
most respected and
influential astronomers of the day rejected this idea so
soundly that it wasn't
until the 60's that any actual searches began. At
first, the only instruments
available were the old familiar optical
telescopes. Optical telescopes are just
what they sound like, telescopes
sensitive to the visible portion of the
electromagnetic spectrum . This
spectrum can reveal much information regarding
the source of the light. The
color indicates the temperature of a star. By
combining the type of star,
identified by observing lots of other stars with
similar characteristics, and
our models of stellar processes with a measurement
of the star's luminosity,
it is possible to calculate the distance to the star.
We can even
determine the chemical composition of the star by observing any
emission or
absorption lines in the spectra. Furthermore, these lines are
very
distinctive, and if they appear in the correct relation to each other
but have
been Doppler-shifted towards the red or blue ends of the spectrum, a
measurement
of the star's speed relative to the earth can be obtained. The
only
distinguishing feature of a black hole is its gravity, however, and
searching
for a black hole with an optical telescope is next to impossible. A
black hole
does not give off any light. It's too small to observe by blocking
out stars
behind it. It could act as a gravitational lens, but to do so it
would have to
be directly in line with the Earth and some bright object, and
even then there
would be no way to distinguish between a black hole or a very
dim star. Still,
there was on promising method proposed by Russian
astronomers Zel'dovich and
Guseinov in 1964. If the black hole was in a
binary system with another, normal
star, the light curve of the system would
give it away. Binary systems comprise
about half of all known stars, so it is
not unlikely that a black hole might be
found next to a normal star. In a
spectroscopic binary system, the stars rotate
about their center of mass and
the light will be Doppler shifted. The light
curve of a star is a graph of
the intensity or Doppler-shift of light from the
star versus time. Here the
light curve of the visible companion can yield much
information. The period
of rotation about the center of mass can be determined
by inspection of the
Doppler-shifted light curve itself, and the mass of the
visible star is given
by the type of star and how luminous it is. All that is
then needed is a
reasonable estimation of the inclination i of the system, and
several
important things can be calculated. The mass function f(M) = M2^3 sin i
/ (M1
+M2)^2 gives a relation between the masses of the two bodies, and
the
semi-major axis a1=AM2/(M1+M2)^2 sin i (where A is the separation of the
centers
of mass) gives the size of the orbit, which can also be related to
the
rotational velocities of the stars. A spectroscopic binary with no
visible
companion would be a candidate for a black hole, and if the dim
star's mass is
determined to be greater than that of the visible star, it
would be a promising
candidate. However, this method consists of many
uncertainties. Although there
were no hard cases for black holes any
scientist’s search, there arose another
way a black hole might show itself.
If the black hole were in a gaseous nebula,
the gas would fall into the black
hole. The inherent magnetic fields of the gas
create turbulence, generating
heat, which is in turn transformed into
electromagnetic radiation. The
luminosity of the gas could oscillate rapidly due
to the turbulence, and such
rapid oscillations would give the black hole away.
Another Soviet
scientist, Schwarzmann, developed the "Multichannel Analyzer
of Nanosecond
Pulses of Brightness Variation" in an effort to detect these
oscillations,
but that method also proved fruitless. X-ray novas are a special
class of
X-ray binaries where the system contains a late-type optical companion
(a
star near the end of its life) and a compact object, which can be either
a
neutron star or a black hole . Usually the spectrum of the companion in
this
type of system is very weak compared to that of the gas, but in X-ray
novae the
fraction of light from X-ray heating is negligible, and we have an
excellent
opportunity to study the system in detail. If the accretion disk is
due to a
black hole, then understanding the companion star in detail will
also allow
understanding of the processes of X-ray emission. Several X-ray
satellites
detected Muscae 1991 and calculations began to pinpoint an optical
companion. To
do this, the exact position of the X-ray source must be known.
If there is a
star in the visible range at that same position, it is most
likely related to
the X-ray star, and the light curve can then be studied in
detail. In this case,
a companion was found. The similarities of Muscae 1991
with one of the best
black hole candidates, V616 Mon, make it seem realistic
that it might be a black
hole. The evolution of the light curves, the decay
rate in magnitude of the
novae, and variations in brightness on the order of
a day are all similar in the
two systems. The spectrum of the nova, its
various emission lines and other
spectroscopic details, also does not
resemble a classical nova in the same
stages, but instead resembles that of
the black hole candidates Cen X-4 and V616
Mon. As it is not a classical
nova, the distance to Muscae 1991 must be
estimated from a known linear
relation of the width of the NaD line to distance.
This gives a result of
~1.4 kpc (kiloparsecs), which returns some typical values
for low mass X-ray
binaries and justifies confidence in its validity. Using this
distance and
the spectral features of the binary, the companion star seems to be
a late
main sequence star, which is in agreement with current theories of
low-mass
X-ray binaries. What this all boils down to is that the binary X-ray
nova
Muscae 1991 behaves very similarly to other black hole candidates in
the
galaxy, and gives a picture of the nova as a burst of gravitational
potential
energy released as matter from the disk accreted onto the compact
object. The
large amounts of energy released in the nova as X-rays indicates
the companion
is at least a neutron star and possibly a black hole, but no
obvious conclusions
can be made as to Muscae 1991's containing a black hole.
Cygnus X-1 is accepted
as a black hole by most astronomers, there is still
nothing about it that
demands unequivocally to be accepted as such. Cygnus
X-1 is the best X-ray
astronomy can give us. But X-rays and visible light are
not the only ways of
probing the sky. Radio astronomy was also discovered
accidentally. In the
1930's, a technician trying to clear up
intercontinental phone calls discovered
radio waves coming from the Milky
Way. Curiously enough, nobody really seemed to
care very much; an amateur
built the world's first radio telescope. A modest 9
meters in size, it had
extremely poor resolution, and the larger dishes that
were to slowly follow
did not fare much better. As in X-ray astronomy, the
astronomers couldn't do
anything really useful with cosmic radio waves until
they could identify an
optical counterpart. Since radio waves are on the order
of meters long,
diffraction effects would require unreasonably large dishes to
acquire any
decent resolution. To counter this, astronomers came up with
radio
interferometry. At first the bodies that shone most brightly in the sky
could
not be associated with an optical counterpart. As radio telescopes
improved, the
error boxes for these sources shrank until, in 1953, a team at
Cambridge had a
sufficiently accurate estimate that other astronomers at the
Palomar 5-meter
optical telescope could identify the radio source Cyngus A
with an optical
source. This source turned out to be a galaxy, and once it's
redshift, and hence
distance, were measured, it was found that this galaxy's
radio luminosity was
millions of times brighter than that of an ordinary
galaxy. The first radio
galaxy had been found. Now that the technology was in
place, more and more of
these galaxies were discovered and they began to be
studied in great detail. The
results troubled astronomers; radio galaxies had
two lobes of radio emissions
with the dim optical galaxy in the center. These
lobes stretched out millions of
light-years, indicating a stable source of
emission, and conservative estimates
of the energy involved in their
production was on the order of 10^61 ergs, as
much energy as would be
released in ten billion supernovas. Radio galaxies were
among the first in
what are today classified as AGN - active galactic nuclei.
Other types of
AGN include Seyfert galaxies, N galaxies, BL Lacertae objects,
and quasars.
They all demonstrate violent behavior that can't be associated with
the
ordinary behavior of stars and interstellar dust, whether it be matter
and
energy ejected from the nucleus to luminosities of truly
astronomical
proportions. While all these objects were regarded as puzzles,
it was really the
quasars that could not be explained by any astronomical
processes at all. Of
course they do exist, and astronomers rushed to find
explanations for them. It
was in this storm of hypotheses that the idea of a
super-massive black hole lost
it's exotic nature and became the most
reasonable explanation. In fact, many of
the other realistic explanations
also support this idea, for they could evolve
into a super-massive black hole
. If there are a lot of star-star collisions
occurring, the stars will lose
enough energy such that they become bound in a
binary which fairly rapidly
decays, if they do not coalesce directly with each
other. Such models of AGN
could have two natural results without invoking black
holes: supernova
explosions, or clusters of pulsars. The supernova explosions
are only as
efficient as regular nuclear burning in stars, and must occur at a
rate of
about 5 to 10 a year. Furthermore, these supernovas cannot be
ordinary
stellar supernovas but rather a sort of 'hypernova' , wherein
neutron stars must
pass through the cores of super-massive stars, due to
calculations of the
energies released. If the cluster evolves into a cluster
of pulsars, it is the
rotational energy of the pulsars that powers the
quasars. Through horrendously
complicated interactions of particles and
strong electromagnetic fields, this
energy could be released into the
universe, but both this and the supernova
model have another serious flaw;
there is no directionality of the radiation
that could result in the observed
jets of quasars and other AGN. To correct this
would require a flattened
cloud of gas that would either hasten the death of the
cluster and it would
collapse into a black hole, or the luminosity would be so
great that the
resulting wind of radiation would drive the gas into space,
thereby
destroying the model entirely. Other models involve the rotational
energies
of massive uncollapsed bodies. Known as super-massive stars, magnetoids,
or
spinars, they are all basically the same; a massive, spinning flattened
disk
(a super-massive rotating star will evolve into a disk). One way these
spinars
could liberate energy is by gravitational contraction, releasing up
to a few
percent of their rest mass as energy. However, to remain stable
against
collapse, a very large ultraviolet radiation pressure must be
present, and such
radiation is not found in radio galaxies, though they might
be in high-redshift
quasars. A pulsar is a rotating neutron star with skewed
magnetic poles.
Radiation is emitted in the direction of the magnetic
poles, and if this beam
passes earth, it has the same effect as a lighthouse.
The incredible angular
momentum of a pulsar makes its pulses extremely
regular, to a degree of accuracy
elsewhere found only in atomic clocks. As
such, the orbit of a binary pulsar can
be scrutinized in extreme detail, and
has been. The results are amazing; the
period of the stars is declining and
their orbit is slowly decaying to exactly
the degree predicted by general
relativity. A better proof of gravitational
radiation could hardly be
imagined. The first person to attempt to detect this
radiation was Joseph
Weber. He eventually came up with the first bar
gravity-wave detector. This
was a long aluminum cylinder, 2 m by 1/2 m, that
should be compressed with an
incoming gravity wave. To detect this compression
he wired piezoelectric
crystals, which respond to pressure by generating an
electric current, to the
outside surface of the bar. Although it didn't work,
other bar detectors were
built that used a device called a stroboscopic sensor
to filter out random
vibrations. This was an ingenious device, but it too proved
to be a
non-contributor in the advancement of learning more of the galaxy. Just
as
X-ray astronomy went from simple detectors in the noses of rockets to
full
fledged X-ray telescopes housed in orbiting satellites, and radio
astronomy went
from crude dishes to continent spanning arrays, gravity wave
detectors may show
a completely new spectrum. And, just as X-rays brought a
completely new universe
into focus, one can hardly imagine what a
gravitational view of the universe
will reveal. At the very least, we will
have definitive proof or denial of black
holes, but we may find that black
holes are some of the more subtle features of
the universe.