- 4323 -
BLACKHOLES - the ways they exist? Black holes may sound like science
fiction, but there is significant evidence to prove they are real. Of all the far-out concepts in astronomy,
black holes may be the weirdest. A region of space where matter is so tightly
packed that nothing, not even light itself, can escape, these dark behemoths
present a pretty terrifying prospect, too.
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4323 - BLACKHOLES - the
ways they exist?
- Black holes were
found to be an inevitable consequence of Albert Einstein's theory of general
relativity. Black holes were predicted
in 1916 by Karl Schwarzschild, who found them to be an inevitable consequence
of Einstein's “theory of general relativity.” If Einstein's theory is correct
and all the evidence suggests it is, then black holes must exist.
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- Roger Penrose and
Stephen Hawking showed that any object collapsing down to a black hole will
form a “singularity” where the traditional laws of physics break down.
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- In the 1930s,
Indian astrophysicist Subramanian Chandrasekhar looked at what happens to a
star when it has used up all its nuclear fuel. The end result depends on the
star's mass. If that star is really big, say 20 solar masses, then its dense
core, which may itself be three or more times the mass of the sun, collapses
all the way down to a black hole.
-
- The final core
collapse happens incredibly quickly, in a matter of seconds, and it releases a
tremendous amount of energy in the form of a gamma-ray burst. This burst can
radiate as much energy into space as an ordinary star emits in its entire
lifetime. Telescopes on Earth have detected many of these bursts, some of which
come from galaxies billions of light-years away. We can actually see black holes being born.
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- Black holes don't
always exist in isolation, sometimes they occur in pairs, orbiting around each
other. When they do, the gravitational interaction between them creates ripples
in space-time, which propagate outward as gravitational waves, another
prediction of Einstein's theory of relativity.
-
- With
observatories like the “Laser Interferometer Gravitational-Wave Observatory”
and “Virgo”, we now have the ability to detect these waves. The first
discovery, involving the merger of two black holes, was announced in 2016, and
many more have been made since then.
-
- As detector
sensitivity improves, other wave-generating events besides black hole mergers
are being discovered, such as a crash between a black hole and a neutron star,
which took place way beyond our own galaxy at a distance of 650 million to 1.5
billion light-years from Earth.
-
- The short-lived,
high-energy events that produce gamma-ray bursts and gravitational waves may be
visible halfway across the observable universe, but for most of their lives
black holes, by their very nature, will be almost undetectable. The fact that
they don't emit any light or other radiation means they could be lurking in our
cosmic neighborhood without astronomers being aware of it.
-
- When observing the
ordinary-looking binary system, or pair of orbiting stars, known as “HR 6819”
in 2020, astronomers noticed oddities in the motion of the two visible stars
that could be explained only if there was a third, totally invisible, object
there.
-
- When they worked
out its mass, at least four times that of the sun. They knew there was only one possibility
left. It had to be a black hole, the closest yet discovered to Earth, a mere
thousand light-years away inside our own galaxy.
-
- The first
observational evidence for a black hole emerged in 1971, and this too came from
a binary star system within our own galaxy. Called Cygnus X-1, the system
produces some of the universe's brightest X-rays. These don't emanate from the
black hole itself, or from its visible companion star, which is enormous, at 33
times the mass of our own sun.
-
- Rather, matter is
constantly being stripped from the giant star and dragged into an accretion
disk around the black hole, and it's from this accretion disk that the X-rays
are emitted. As they did with HR 6819, astronomers can use observed star motion
to estimate the mass of the unseen object in Cygnus X-1. The latest
calculations put the dark object at 21 solar masses concentrated into such a
small space that it couldn't be anything other than a black hole.
-
- At the center of
our galaxy is a supermassive black hole in the region known as “Sagittarius A”.
It has a mass of about 4 million times that of our sun. In addition to black holes created through
stellar collapse, evidence suggests that supermassive black holes, each
millions or even billions of solar masses, have been lurking in the centers of
galaxies since early in the history of the universe.
-
- In the case
of “active galaxies”, the evidence for
these heavyweights is spectacular. The central black holes in these galaxies
are surrounded by accretion disks that produce intense radiation at all
wavelengths of light.
-
- We also have
evidence that our own galaxy has a black hole at its center. That's because we
see the stars in that region whizzing around so fast, up to 8% of the speed of
light, that they must be orbiting something extremely small and massive.
Current estimates put the Milky Way's central black hole somewhere around 4
million solar masses.
-
- Another piece of
evidence for the existence of black holes is … “spaghettification”. This is
what happens when you fall into a black hole, and it's pretty self-explanatory.
You get stretched out into thin strands by the black hole's extreme
gravitational pull.
-
- In October 2020,
astronomers witnessed this shredding as flash of light from a hapless star as
it was ripped apart. Fortunately, the spaghettifying didn't happen anywhere
near Earth, but instead in a galaxy 215 million light-years away.
-
- So far we've had
plenty of compelling indirect evidence for black holes: bursts of radiation or
gravitational waves, or dynamical effects on other bodies, that couldn't have
been produced by any other object known to science. But the final clincher came
in April 2019, in the form of a direct image of the supermassive black hole at
the center of active galaxy Messier 87.
-
- This stunning
photo was taken by the Event Horizon Telescope, a slightly misleading name, because
it consists of a large network of telescopes scattered all over the world
rather than a single instrument.
-
- The more
telescopes that can participate, and the more widely spaced they are, the
better the final image quality. The result clearly shows the dark shadow of the
6.5 billion-solar-mass black hole against the orange glow of its surrounding
accretion disk.
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January 21, 2023 BLACKHOLES - the
ways they exist? 4323
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