A black hole is a region of spacetime where gravity is
so strong that nothing—no particles or
even electromagnetic radiation such as light—can
escape from it. A black hole's "surface" called
its event horizon, defines the boundary where
the velocity needed to escape exceeds the
speed of light, which is the speed limit of
the cosmos. Matter and radiation fall in but
they can't escape. Two main classes of black
holes have been extensively observed. Stellar-mass
black holes with three to dozens of times
the Sun's mass are spread throughout our Milky
Way, while supermassive black holes weighing
100,000 to billions of solar masses are found
in the centers of most big galaxies, including
our own.
There's an in-between class called intermediate
mass black holes, weighing 100 to more than
10,000 solar masses, but only one has been
conclusively observed to date.
Once born, black holes can grow by the accretion
of matter, including gas stripped from the
neighbouring stars and merging with other
black holes. In 2019, astronomers using the
Event Horizon Telescope - an international
collaboration that networked eight ground-based
radio telescope into a single Earth - size
- dish captured an image of a black hole for
the first time.
It appears as a dark circle silhouetted by
an orbiting disk of hot, glowing matter.
The supermassive black hole is located at
the heart of a galaxy called M87, located
about 55 million light years away, and weighs
about 6 billion solar masses. Its event horizon
extends so far it could encompass much of
our solar system out to well beyond the planets.
Now, let's have a look at 10 amazing facts
about black holes.
Black holes can generate energy more efficiently
than our Sun. The way this works has to do
with the disk of material that orbit around
a black hole. The material that is nearest
to the fringe of the event horizon on the
inner edge of the disk will orbit much more
quickly than material at the very outer edge
of the disk. This is because the gravitational
pull is stronger near the event horizon. Because
the material is orbiting and moving so rapidly,
it heats up to billions of Kelvins, which
has the ability to transform mass from the
material into energy in a form called black
body radiation. To compare, nuclear fusion
coverts about 0.7 percent of mass into energy.
The condition around a black hole converts
10 percent of mass into energy. That's a big
difference!
Some people think that black holes are like
cosmic vacuum cleaners that suck in the space
around them, when, in fact, black holes are
like any other object in space, albeit with
a very strong gravitational field.
If you replaced the Sun with a black hole
of equal mass, Earth would not get sucked
in - it would continue orbiting the black
hole as it orbits the Sun, today.
It is important to realize that a black hole's
gravitational field is the same as that of
any other object in the space of same mass.
In other words, it wont "suck" objects in
any more than any normal star, with things
being more likely to just fall into them if
they got too close to the event horizon.
A stellar mass black hole forms when a star
with more than 20 solar masses exhausts the
nuclear fusion in its core and collapses under
its own weight. The collapse triggers a supernova
explosion that blows off the star's outer
layers. But if the crushed core contains more
than about three times the Sun's mass, no
known force can stop its collapse to a black
hole.
The origin of supermassive black holes is
poorly understood, but we know they exist
from the very earliest days of a galaxy's
lifetime. One hypothesis is that the seeds
are black holes of tens or perhaps hundreds
of solar masses that are left behind by the
explosions of massive stars and grow by accretion
of matter.
There are at least two different ways to describe
how big something is. We can say how much
mass its has or how much space it takes up.
Let's talk first about the masses of black
holes. There is no limit in principle to how
much or how little mass a black can have.
We suspect that most of the black holes that
are actually out there were produced in the
deaths of massive stars, so we expect those
black holes to weigh as much as 10 solar masses.
The more massive a black hole is, the more
space it takes up. In fact the Schwarzschild
radius and the mass are directly proportional
to one another: if one black hole weighs ten
times as much as another, its radius is ten
times as large. A black with a mass of three
Suns would have a radius of about 10 kilometers.
So a typical 30 solar mass black hole would
have a radius of about 100 kilometers.
A million-solar-mass black hole at the center
of a galaxy would have a radius of 3 million
kilometers. Three million kilometers may sound
like a lot, but its actually not so big by
astronomical standards. The Sun, for example,
has a radius of about 700,000 kilometers,
and so that the supermassive black hole has
a radius only about four times bigger than
the Sun.
Theoretically, anything can turn into a black
hole. If you shrunk our Sun to a radius of
only 3 kilometers, for example, then you would
have compressed all of the mass in our Sun
down to an incredibly small space, making
it extremely dense and also making a black
hole. You could apply the same theory to the
Earth, the Moon, or even your own body. But
in reality, we only know of one way that can
produce a black hole: the gravitational collapse
of an extremely massive star that's 20 to
30 times more massive than our Sun.
Quasars are supermassive black holes gobbling
up matter. A quasar is an extremely luminous
active galactic nucleus or AGN, in which a
supermassive black hole with mass ranging
from millions to billions of times the mass
of the Sun is surrounded by a gaseous accretion
disk. As gas in the disk falls towards the
black hole, energy is released in the form
of electromagnetic radiation, which can be
observed across the electromagnetic spectrum.
The power radiated by quasars is enormous:
the most powerful quasars have luminosities
thousands of times greater than a galaxy such
as the Milky Way.
3C 273 was the first quasar ever to be identified.
It is also one of the most luminous quasars
known, with an absolute magnitude of -26.7,
meaning that if it were only as distant as
10 parsecs or about 34 light years, it would
appear nearly as bright as the Sun in the
sky. Since the Sun's absolute magnitude is
4.83, it means that the quasar is over 4 trillion
times more luminous than the Sun at visible
wavelengths.
Quasars are found over a very broad range
of distances, and quasar discovery surveys
have demonstrated that quasar activity was
more common in the distant past. The peak
epoch of quasar activity was approximately
10 billion years ago.
Einstien didn't discover the existence of
black holes - though his theory of relativity
does predict their formation. Instead, Karl
Schwarzschild was the first to use Einstien's
revolutionary equations and show that black
holes could indeed form. He accomplished this
the same year that Einstien released his theory
of general relativity in 1915. From Schwarzschild's
work came a term called the Schwarzschild
radius, a measurement of how small you'd have
to compress any object to create a black hole.
Long before this, British polymath John Michell
predicted the existence of "dark stars" so
massive or so compressed that they could possess
gravitational pulls so strong that not even
light could escape; black holes didn't get
their universal name until 1967.
Black holes have incredibly powerful tidal
forces.The singularities within them contain
the most powerful gravitational fields in
the known universe. Because of this, objects
that venture past their event horizon, or
point of no return, cannot escape their pull,
whether it be an astronaut, an entire star,
or even light itself. Once an object passes
over this threshold, the strong tidal forces
stretch it out both vertically and horizontally,
similar to a spaghetti noodle, hence the name
spaghettification. The simplest way to describe
the process is by using a hypothetical astronaut
as an example, first discussed by Stephen
Hawking in his book, A Brief History of Time.
If an astronaut were to free-fall into a black
hole, he or she would be affected by the gravitational
gradient, which is the difference in the strength
of the gravitational pull depending on the
astronaut's orientation.
If the astronaut were falling feet-first into
the black hole, the gravity would be stronger
at his or her feet than at their head. This
difference in gravitational pull would cause
their body to be stretched out. Additionally,
as the astronaut's body was vertically stretched,
their body would also be compressed horizontally.
The right side of the body would be pulled
towards the left, and the left side pulled
towards the right, further stretching them
out in a noodle-like fashion. For supermassive
black holes, since their event horizon is
much larger, the point of death would most
likely be sometime after crossing the event
horizon.
However, a person wouldn't feel any immediate
effects while crossing over the event horizon.
Alternatively, in the case of a smaller black
hole with an event horizon much closer to
its center, a human would be killed before
crossing the event horizon. In both cases,
spaghettification would be very quick, occurring
in less than a second.
Some astronomers have begun labeling black
holes of at least 10 billion solar masses
as ultramassive black hole. Most of these
are associated with exceptionally energetic
quasars.
The most massive known black hole is in the
quasar TON 618, with a mass of about 66 billion
solar masses. The quasar has an absolute magnitude
of -30.7, making it about 140 trillion times
as luminous as the sun.
However, the ultramassive black hole at the
center of the supergiant elliptical galaxy
IC 1101 has a mass range of 40 - 100 billion
solar masses. If the latter estimate of it
mass is correct,then it would be the most
massive known black hole.
Back in the 1970's, Stephen Hawking came up
with theoretical arguments showing that black
holes are not entirely black: due to quantum-mechanical
effects, they emit radiation. The energy that
produces the radiation comes from the mass
of the black hole. Consequently, the black
hole gradually shrinks. It turns out that
the rate of radiation increases as the mass
decreases, so the black hole continues to
radiate more and more intensely and to shrink
more and more rapidly until it presumably
vanishes entirely.
The Universe is able to produce mass and energy
out of nowhere, but only if that mass and
energy disappear again very quickly. One particular
way in which this strange phenomenon manifests
itself goes by the name of vacuum fluctuations.
Pairs consisting of a particle and antiparticle
can appear out of nowhere, exist for a very
short time, and then annihilate each other.
Energy conservation is violated when the particles
are created, but all of that energy is restored
when they annihilate again. As weird as all
of this sounds, we have actually confirmed
experimentally that these vacuum fluctuations
are real. Now, suppose one of these vacuum
fluctuations happens near the horizon of a
black hole.
It may happen that one of the two particles
falls across the horizon, while the other
one escapes.
The one that escapes carries energy away from
the black hole and may be detected by some
observer far away. To that observer, it will
look like the black hole has just emitted
a particle.
This process happens repeatedly, and the observer
sees a continuous stream of radiation from
the black hole. Actually, nobody is really
sure what happens at the last stages of black
hole evaporation: some researchers think that
a tiny, stable remnant is left behind.
