THE EXISTENCE OF BLACK HOLESESO/EHT^ COLLABORATION,^GETTY^ X2 ,^ NASA^ X3 ,^ ESOA handy list of the
terminology
connected to the
study of black holesACCRETION DISC
Rotating matter is pulled into a
disc shape by a star (part of the
formation process of a solar
s ys tem). In the ca se of black
holes, nearby matter is
accelerated intensely by
gravity, giving off a bright glow.JET
Streams of matter accelerated
to nearly the speed of light are
emitted at right angles to the
accretion disc. The cause of
these jet s is uncer t ain, though
they may be the result of a
complex magnetic field.PAULI EXCLUSION
PRINCIPLE
This principle of quantum
mechanics establishes that two
f ermions (a t ype of subatomic
par ticle) cannot be in an
identical quantum state. This
results in ‘exchange
interaction’, which is like a
short-range force keeping
fermions apart – except in
extreme conditions such as
black hole formation.SINGULARITY
In the ca se of a s trophysic s , a
singularity is a mathematically
predicted condition where
space-time becomes so locally
distorted by gravitation that
the force of gravity tends to
infinity and current theories
of physics break down.su re of t he mass of its ‘compact object’
companion. Many other candidates
have been detected since, but evidence
remains indirect and is based on
theoretical assumptions about the
maximum size of a neutron star that
may not be borne out in practice.
Supermassive black holes are
thought to exist at the heart of most
galaxies. Such black holes may play
a significant role in galaxy formation,
giving the galaxy a hub around which
to coalesce. Candidate Supermassive
black holes have been detected at
many galactic centres, thanks to
the odd motion of nearby stars and
the high electromagnetic emissions
from these regions.
A sta r called S2 orbits t he cent re of
the Milky Way at about four times the
radius of the orbit of Neptune. From
S2’s path, it seems likely it’s orbiting
something with a mass of about 4.3
million times that of the Sun. The
object matches the position of an
intense radio source called Sagittarius
A* and there is currently no other
explanation for this except a
supermassive black hole. Elsewhere,
stellar destruction gives a clue.
Unusually bright light signatures
in distant galaxies are thought to be
stars being ripped apart by
supermassive black holes.
All is not certain, though. A 2014
study suggested that black holes won’t
form at all. The authors believed that
as a star collapses, Hawking radiation
would reduce the mass of the star
sufficiently that the black hole never
reaches completion. There would be
an ultra-dense body acting like a black
hole, but without the singularity or
the event horizon. The paper isn’t
universally accepted, but illustrates
how our understanding of black holes
is primarily driven by theory. A theory
that was given a significant boost in
April 2019 when the first image of the
shadow of a black hole was captured
by an international team of scientists
using the Event Horizon Telescope.2012
The best evidence to date of a star being ripped
apart by a supermassive black hole is detected by
the Pan-STARRS telescope on Hawaii and analysed
by a team at Johns Hopkins University.
could reduce the energy threshold to
something accessible.
The best evidence we have for
conventional black holes, for med f rom
the collapse of a dying star, is X-ray
binaries. In these objects, material is
accelerated from one normal star into
an invisible star, giving off X-rays.
This can happen with a neutron star,
but if the ‘eating’ star has more than
about three times the mass of the Sun,
it should, in theory, be a black hole.
The first X-ray binary widely
recognised as containing a black hole
was Cygnus X-1. A powerful X-ray
sou rce was detected in 1964 a nd was
identified as a black hole candidate
in 1971. A blue supergia nt sta r in t he
binary was being stripped of material
by the X-ray source, which appeared
to have a mass in t he region of 9 to 15
times that of the Sun. In 1975, Kip
Thorne and Stephen Hawking made
a bet as to whether this was, indeed,
a black hole. Hawking, who was on the
‘no’ side, paid up in 1990 when better
observational data was obtained.
Since 1990, the identification of
Cygnus X-1 has become less cer tain.
This is because the companion star
is ver y la rge, ma k ing it diff icult to beby BRIAN CLEGG (@brianclegg)
Brian is a science writer and author. His latest
book is Professor Maxwell’s Duplicitous Demon.NEED TO KNOW