The question must be raising in your mind, “Why do
astrophysicists and physicists spend their money, energy and time when we know that these objects are out of our reach?” In short, I can say that the study of black hole reveals the functioning and evolution of universe. At the end of post, we will again return to the same question after knowing the importance of black holes. Let’s begin with the prediction of black hole.
Prediction of black hole
After the theory of gravitation, scientists start speculating whether it is possible to get out of the gravity of Earth. When we throw an object upward, its velocity decreases and it slowly comes to rest and then start falling back towards the surface. Application of larger force helps the object to reach more height but still it falls back. If we apply force large enough to help the object attaining a velocity known as escape velocity then the object will not come back to the surface of Earth. The escape velocity of Earth is around 11.2 km per second and it is independent of the mass of the object.
The escape velocity depends on the mass of the planet and it increases with the mass of the planet. Mercury which is smaller than Earth has escape velocity equal to 4.3 km per second whereas the giant planet Jupiter has escape velocity equal to 59.5 km per second. Hence, we can say that the massive bodies have more escape velocity.
Just after the theory of gravitation scientist started thinking about body so massive that its escape velocity exceeds the velocity of light. The light will never be able to escape from such massive body and body will appear black. In a paper published in 1784 , an English astronomer John Michell wrote, “ If there should really exist in nature any bodies whose diameters are more than 500 times the diameter of the sun. All the light emitted from such a body would be made to return to it by its own power of gravity. We could have no information from sight.” Michell called these objects “dark stars.”
In 1967, noted Princeton University Physicist John A. Wheeler coined a term "black hole". The name was perfectly descriptive and highly catchy, it immediately became popular. The concept of black hole attracted the attention of increasing number of physicists, astronomers, scientists and fiction fans.
Black hole Formation
To
understand the formation of black holes, we need to understand the stellar
evolution whose end product is black hole in case of heavy stars. Let’s briefly discuss stellar evolution, if you want to read in
detail, you may refer to following post:
The
stars are born from the clouds of gases known as nebulae which contain mainly
hydrogen. The gas of nebula collapses due to gravitational attraction among
molecules of gas, the decrease in volume raises the pressure and the
temperature. Once, the temperature reaches the threshold temperature required
to initiate the nuclear reaction of hydrogen to form helium, the collapse
stops. The energy produced in the nuclear reaction ejects from the star in the
form of light and heat. The inward pressure of gravity is balanced by the
outward pressure applied by the energy released in the nuclear reaction.
The
star will keep on glowing till it has hydrogen as fuel. Once the hydrogen gets
finished, the nuclear chain reaction proceed further for fusion of helium into
carbon. For star smaller than 1.4 times the solar mass, the fusion reaction of
carbon cannot be started due to less temperature. Thus, the core of star
collapses due to gravity because now the outward pressure is not present. The
collapse results in highly dense object called white dwarf. The outer layer of star is pushed outward into the
space.
The
stars which have mass between 1.4 to 8 times the solar mass, are able to
increases temperature high enough to initiate the nuclear chain reaction
further whose ultimate product is iron. After the formation of iron, the
nuclear reaction stops and the core of star collapses with violent explosion
called supernova. During the
collapse, the proton and electron are forced to merge and the result is a core
with only neutrons, this highly dense core is called neutron star. The escape velocity of neutron star is two third of
the speed of light which very close to the condition required for black hole.
For
the stars which has mass more than 8 times the solar mass, the collapse does
not stop at neutron star instead everything goes into a very small volume. The
resultant superdensed object is called black
hole. The escape velocity of black hole exceeds the speed of light so the
light is unable to escape and it appears black.
Detection of black holes
The
black holes are really mysterious objects because we don't know what the
structure inside them is. Even the light cannot come out of black holes which
make them invisible to human eyes and telescopes. Due to invisibility, the
direct detection of black holes was not possible. So, how do astronomers
confirmed that black holes really exist in nature?
The
evidence of first black hole was found by indirect measurements. The first
black hole was detected by the measurements of X-ray which is invisible but
more energetic and penetrable than visible light. The scientist were not aiming
to find black hole by detection of X-rays instead they wanted to know whether
the sun emits X-rays or not.
To
confirm this in 1948, the American scientists attached the detectors to the
rocket made by Germans in Second World War. The detectors confirmed that the
sun emits X-rays but in very small amount. The intensity of X-rays was found to
be million times smaller than the visible light emitted by the sun. Thus, the
sun was not found to be the strong source of X-rays.
In
June, 1962, American scientists attached the detectors to the rocket to measure
the X-rays emitted by the surface of moon. To their disappointment, they found
no X-rays emission from the surface of moon. But a very strong source of X-rays
was detected outside our solar system and later launches confirmed its location
in the constellation of scorpius. Astronomers named it Scorpius X-1 or Sco X-1
for short.
After
the detection of Sco X-1 many such strong source were detected. It soon became
clear to astronomers that these objects were neutron stars. Many astronomers
developed the model of binary star to explain how neutron star emits X-rays.
These neutron stars were required to be highly dense objects for producing such
shorts duration pulses of X-rays.
Now,
it was clear that these strong sources of X- ray were superdense objects.
Although most of the sources were neutron stars but there was one source
detected in 1960s which was a good candidate for black hole according to many
scientists. This source was present in the constellation of Signus the swan,
later it was known as Signus X-1 or Sig X-1 for short.
Cyg
X-1 was the part of a binary star system and producing bursts of X rays at an
astonishing rate of thousand or more per second. These millisecond pulses were
indicating that Cyg X-1 is an extremely tiny object because the object cannot
rotate in a time less than the time required by light to cross its diameter.
Thus, the size of this object should not be more than 300 km.
The
mass of the companion star was equal to 30 solar masses and measurements of its
orbits and time periods indicate that the mass of Cyg X-1 was about 9 solar
masses. Such a massive and tiny body cannot be a neutron star, it must be a
black hole.
Properties of a black hole
Just
like the stars, planets, moons and asteroids etc, the black hole also has
physical properties. However, the invisible and black appearance of black hole,
causes the difficulty in direct measurement of physical properties. Thus we
need to use indirect methods to measure the physical properties of black hole.
The
first and the most important property of any heavenly body is its mass. The
mass of black hole cannot be measured directly but in the presence of other
bodies like stars around the black hole, the measurement of mass becomes
possible. In such cases, the star and the black hole revolve around their
common center of mass. The scientists use sensitive instruments and mathematics
to measure the separation between the star and the black hole. After that their
orbital velocities are measured. Then, the scientists plug these values of
position and velocity into the equation of gravitation and calculate the mass
of black hole. The mass of black hole is usually represented as 8, 12, 20 or
more solar masses.
The
mass of black hole is very important property and it directly affects the
properties which cannot be measured directly. One of these properties is the
Schwarzschild radius. Whenever a black hole encounters objects like stars,
planets, asteroids or gas clouds, it tear them apart into atoms as they approach
the black hole. Then, these atoms move towards black hole and fall into it and
never come back.
In
process of falling into the black hole, there is a point which might be called
"the point of no return". This point is called event horizon of black
hole. The objects which are revolving outside the event horizon have
possibility of escape but once they cross event horizon, there is no
possibility of return. The distance between the center of black hole which is
also called singularity and the event horizon is called Schwarzschild radius, named after German astronomer Karl
Schwarzschild who discovered it in 1915.
The
Schwarzschild radius depends on the mass of black hole. The more massive black
hole has larger Schwarzschild radius and less massive has smaller. For example,
a black hole of one solar mass will have a Schwarzschild radius of 1.86 miles
and a black hole of 10 solar masses will have a radius of about 20 miles.
Karl
Schwarzschild worked out his equations for a hypothetical black hole which does
not rotate on its axis. But every object in space rotates and has angular
momentum which is the tendency of an object to keep on spinning. The rotational
energy of an object remains constant with the change in size. When the size of
an object increases, the rotational velocity decreases. And the decrease in
size results in the increase in rotational velocity. Similarly a star completes one rotation in 20 or 30
days but when it collapses and becomes neutron star, its rotation velocity
increases and it completes many rotations in one second. Thus, the black hole
must be spinning with very high rotational velocity.
The
non-spinning black hole is also called Schwarzschild black hole. The radius for
spinning black hole was successfully calculated by New Zealander astronomer Roy
P. Kerr who was then working at the University of Texas. After that the
spinning black hole is also called Kerr black hole in honor of Kerr.
Kerr's
solution of spinning black hole describes all parts beautifully like
singularity, event horizon and radius
etc but the situation becomes more complex due to spin of black hole. The
singularity is no longer a point instead it is a rapped region of space and
forms a shape like ring. The event horizon which separates the black hole from
the normal space spins in the direction of spin of singularity.
The
black hole spins like a tornado. As we know, the air present at the far
position from the center of tornado moves slowly similarly space at far
position from event horizon moves slowly. On the other hand, the air present
near the center of tornado moves faster similarly the space near event horizon
moves faster. At the event horizon, the space gets locked with event horizon
and moves with it.
When
any object comes close to black hole, it is forced to move along with the
spinning space. Thus the material around the black hole forms a thin disk like
ring of Saturn. This flat disk is called accretion disk.
 |
| Artist concept of matter swirling around a black hole. Credit: NASA/Dana Berry/SkyWorks Digital |
The
material spinning in accretion disk causes an important phenomenon in case of
some black holes, in which two narrow but strong jets of gases are ejected from
the black hole. Actually these
gases are not coming from inside the black hole instead they are coming from
outside the event horizon. Many theories are given to explain these jets.
According to one theory, the magnetic field lines are forced to move along the
accretion disk. The electrical force causes the hot plasma to move along
magnetic field lines outward. Then, this hot plasma is ejected outward in two
opposite directions in the form of jets.
The
black hole interacts with space time and distorts it in an interesting way.
According to General Theory of Relativity, the time near the black hole spends
slowly as compared to the time at some far point from black hole. To understand
this, consider two space ships moving around the black hole and one space ship
decides to move closer to black hole. As one space ship approaches the event
horizon, its time gets slower. What does it mean? The one hour shown by clock
of space ship moving near event horizon will be equal to many years of space
ship moving at a far distance from the event horizon.
You
must be thinking that this property of time dilation can be used for time
travel. You are right but this time travel is only in forward direction towards
future. Within few hours, you can travel many years or even centuries. But this
can't be used to go back into past. I must tell you that this time dilation is
not limited to black hole only, this also occurs around all gravitation bodies
but the effect is very small around normal bodies like planets.
Super Massive Black Hole
The
stellar black holes are mysterious objects but they are not dangerous to
universe, planets or life. It is true that during the formation of black hole,
the life on the nearby planets destroys completely. Even if life survives this
catastrophe, it will eventually freeze to death in the absence of light. Since
the distance between stars is very large of the order of 4 to 7 light years,
the effect of black hole's gravity and radiation is not dangerous at such a
vast distance.
But
the situation can be scary, if the stars lie close to each other, separated by
only light weeks, light days, or light hours. In such a situation, the black
hole will feed on other stars and over the span of millions and billions of
years it will eventually grow into a cosmic monster. Scientists have recently
discovered that this type of cosmic monster not only exist in nature instead
they are playing an important role in the evolution of galaxy and universe.
The
scientists have wondered how big the black hole can be. There are two types of
giant black hole, one is midsize black hole and others is supermassive black
hole. The stellar black holes usually have masses of the order of 8 to 20 solar
masses. The formation of midsize black hole required nearly tens of thousands
of stars densely packed in very small region of space. This type of densely
packed region of stars exists in nature and hundreds of them are present in our
galaxy. These densely packed region of stars are called globular cluster. Thus,
the star clusters are the good regions to look for midsized black holes.
Astronomers
examined several globular clusters for the search of midsized black holes and
they found high level of X ray just like emission from black hole of Cyg X-1.
In 2002, a team led by Roeland Van Der Marel at the Space Telescope Institute
found two midsized black holes. One, possessing four thousand solar masses, is
found in M15, globular cluster of Milkyway. Other found in G1, globular cluster
in neighboring Andromeda galaxy, possessing twenty thousand solar masses. Later
in 2003, two more midsized black holes, each having hundreds of solar masses,
were discovered in the spiral galaxy NGC 1313 lying at the distance of 10 million
light years from the Earth.
The
other giant, supermassive black holes are usually found in the center of
galaxies, due to this they are also called galactic black holes. But the
detection of these supermassive black holes took so long because the view of
center of galaxy is obstructed by the dust and gas cloud. Later, the more
sophisticated instruments and techniques revealed that the center of our galaxy
has large number of massive stars having masses equal to 120 solar masses of
more. The X-ray and infrared observations showed that these stars are revolving
around an invisible point which is also powerful source of X-ray. The records
of radio waves from outer space are also showing power sources in the center of
galaxies.
The
evidences were indicating that the center of Milky way galaxy is harboring a
supermassive black hole. After that the scientists started estimating mass of
this supermassive black hole, 1990 most of them were estimating it to be around
2.6 million solar masses. In 2002, a study by Rainet Schodel of Germany's Max Planck
Institute of Extraterrestrial Physics showed the mass of this black hole to be
equal to 3.7 million times the solar mass.
The
astronomers have intensified the study of galactic black hole and have found
many galactic black holes in various distant galaxies. The neighboring
Andromeda galaxy harbors a galactic black hole with 30 million solar masses. A
galaxy named NGC 4486B contains 500 million solar mass black hole in its core.
And finally a galaxy named 4261 has an unbelievable black hole with 1.2 billion
solar masses at its core.
 |
| Andromeda galaxy harbors a galactic black hole with 30 million solar masses. Credit: NASA. |
Conclusion
After
knowing about the supermassive black holes, it is clear that black holes are
integral part of the universe. Every galaxy possesses a galactic black hole at
its center. Scientists have tried to understand the relation between the black
hole and the galaxy. It is suspected that the structure, evolution and ultimate
fate of galaxy is related to galactic black hole. Now we can go back to our
question about the importance of studying the black hole.
Many
theories have been postulated to describe the relation between black hole and
evolution of galaxy. One says that the galaxies came first then the black holes
formed at the center from massive star. After feeding on the material of
galaxies, the black holes evolved into the supermassive black holes. Other says
that the black hole came first from big bang then gathered matter around them
in the form of giant accretion disk and acted as seed for the formation of
galaxies.
Whatever
came first, the thing is, they both seems to grow and develop together. In
2000, astronomer Michael Merrifield and his colleagues at University of
Nottingham, in England, found a telling correlation between the age of galaxies
and the masses of supermassive black holes at their core. In other words, the
older galaxies have more massive holes at their cores. The time scale over
which the black holes grow found to be comparable to the age of the host
galaxies which indicate that they have evolved together.
The
one question which may arise is, if the central black hole feeds on the
material of galaxy then why don't we see any galaxy at the last stage of their
life just about to completely finish by its black hole? The answer is
straightforward that the universe is not old enough. If you can wait enough or
travel to future, you may be able to view the galaxies completely eaten by
their central holes.
Now,
I can assume that now you have answer to the question, “What is a black hole?” And lot of new questions may have developed in your mind
which you can ask in comment box. If you want to understand the black holes in
detail then you can read the book given in the reference. I have learned a lot
from this book, hope you will also find this interesting. If you really liked
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