For centuries, scientists imagined objects so heavy and dense that their gravity might
be strong enough to pull anything in, including light. They would be, quite literally, a black
hole in space. But it's only in the past few decades that astronomers have conclusively
proved their existence. Today, Hubble lets scientists measure the effects of black holes,
make images of their surroundings and glean fascinating insights into the evolution of
our cosmos.
In science fiction, black holes are often portrayed as some kind of menacing threat
to the safety of the whole Universe, like giant vacuum cleaners that somehow suck up
all of existence. Now, in this episode, we're going to separate the fiction from the facts
and we're going to look at the real science behind black holes and how Hubble has contributed
to it.
Black holes come in different sizes. We've had solid evidence for the smaller ones since
the 1970s. These form when a huge star explodes at the end of its life. As the outer layers
are blown away, the star's core collapses in on itself forming an incredibly dense ball.
For instance, a black hole with the same mass as the Sun would have a radius of only a few
kilometers.
Before Hubble was launched, astronomers had noticed that the centers of many galaxies
were somehow much denser and brighter than they were expected to be. And so they speculated
that there must be some kind of huge, massive objects lurking in the centers of these galaxies
in order to provide the additional gravitational attraction.
Now, could these objects be supermassive black holes, that is, black holes which are millions
or even billions of times more massive than the stellar ones? Or was there perhaps a simpler,
less exotic explanation, like giant star clusters?
Fortunately, Hubble was on its way, along with a range of other high-tech telescopes.
When the space telescope was being planned, the search for supermassive black holes was
in fact one of its main objectives.
Some of Hubble's early observations in the 1990s were dedicated to these dense, bright
galactic centers. Where ground-based telescopes were just seeing a sea of stars, Hubble was
able to resolve the details.
In fact, around the very centers of these galaxies, Hubble discovered rotating discs
of gas and dust.
When Hubble observed the disc at the center of a nearby galaxy, Messier 87, the astronomers
saw that its color was not quite the same on both sides. One side was shifted towards
blue and the other towards red, and this told the scientists that it must have been rotating
very quickly.
This is because the wavelength of light is changed by the motion of an object emitting
it. Think about how the pitch of an ambulance siren drops as it drives past you, because
the sound waves are more spaced out as the vehicle moves away.
Similarly, if an object is moving towards you, the light's wavelength is squashed, making
it bluer; if it's moving away, it's stretched, making it redder. This is also known as the
Doppler effect.
So, by measuring how much the colours had shifted on either side of the disk, astronomers
were able to determine its speed of rotation. And it turned out that this disk was spinning
at a rate of hundreds of kilometers per second. This in turn allowed astronomers to deduce
that, hidden at the very center, there must be some kind of object which was two to three
billion times the mass of the Sun - and this was very likely a supermassive black hole.
Now, along with a lot of other observations, this was a key piece of evidence that led
to the notion that there is a supermassive black hole lurking at the center of most,
if not all, giant galaxies, including our own Milky Way.
Well, the science of black holes has moved along a lot since then. The mystery now isn't
whether they exist, but why they behave in the strange ways they do.
For example, Hubble observations have helped to show that the mass of a supermassive black
hole is closely related to the mass of its surrounding host galaxy. The bigger the black
hole, the bigger the galaxy.
A supermassive black hole is pretty big, and it packs a lot of punch, but you've got to
remember that compared to its host galaxy it's actually tiny. The region of space that
is most obviously and most immediately influenced by a supermassive black hole is in fact about
a million times smaller than its surrounding galaxy. That's about the same size difference
as between this coin and a whole city. So it's pretty hard to think of any processes
that would link the two in a long-lasting way.
So a big area in science just now is trying to find out what's going on here, and why
the two are linked. Do black holes regulate the size of galaxies, or do galaxies regulate
the size of black holes? Or is something altogether different happening?
Now when matter falls into supermassive black holes, it forms this big swirling disc that
heats up and gives off a lot of powerful radiation. The more matter falls into the black hole,
the more powerful the radiation.
Now these active, accreting black holes are called quasars, and they are among the most
luminous and most powerful objects in the Universe. The thing is, a quasar can get so
greedy that its radiation is powerful enough to actually blow away all the gas and dust
that's feeding it. And so it seems there's a natural upper limit to the rate at which
a black hole can grow.
Now, this implies that one wouldn't expect to see any really big and really powerful
quasars in the very early Universe, because there simply wouldn't have been enough time
to build up the supermassive black hole that is needed to power a quasar. But recent discoveries
have in fact shown that quasars do exist in the early universe just a billion years after
the Big Bang, which is much earlier than we had expected.
And so there you have it: another mystery for astronomers to pore over.