How many black holes are in the Milky Way?

Hi, I’m Dhara. I’m an Astronomer here at the
Royal Observatory Greenwich and in this week’s Space News, Astronomers have detected 12 black
holes around the supermassive black hole at the centre of our galaxy. And although this
seems like a small number of black holes detected, it actually provides great support to the
theory that there are many black holes around that supermassive black hole that lies at
the centre of the Milky Way. So what is a black hole? Well, it’s a very
dense and compact object. It’s lots of material crammed into a very very small space. In fact,
if we were to turn the Earth into a black hole, we’d have to take all the material on
the Earth and cram it down into something that was the size of your thumbnail. So we’re
talking about a very dense and compact object which has a huge gravitational pull. Around
the black hole is the event horizon – the point of no return. It’s only when you go
past the event horizon, close enough to the black hole, that even if you were traveling
at the speed of light, you wouldn’t be able to escape the black hole’s gravitational pull.
And that’s why lots of people say not even light can escape from it. There are two types
of black holes. You get the supermassive black holes – they’re the ones that are found at
the centre of large galaxies. The one at the centre of the milky way is called Sagittarius
A and it is 4 million times the mass of our Sun. So very very massive. The other types
are called stellar mass black holes. These are formed when very large stars end their
lives – stars that are much more massive than our Sun. They leave behind a remnant after
the star has gone supernova, a smaller black hole. Those ones are roughly tens of times
the mass of our Sun, so much smaller or much less massive. And the theory states that there
should be thousands of these smaller black holes around the supermassive black hole at
the centre of our galaxy and that’s because Sagittarius A has this huge halo of gas and
dust all around it and it’s from that material that stars are born. So once those stars are
born, they’ll live their lives and they’ll also end them too. When those massive stars
end their lives, they should leave behind these black holes and so we should find them
dotted around by this supermassive black hole. Now, the problem is we can’t actually see
these black holes. They’re dark in nature, they don’t emit any light or reflect it. So
they only way we can actually detect that they’re there is through the interactions
they have with the material around them. Now it’s a slightly windy day today, so although
the wind is here and it’s blowing my hair, I can’t see the wind itself – it’s invisible
to me but by looking at the effect it has on my hair, I know that the wind is here.
And it’s the same thing that Astronomers are trying to do to detect those black holes.
So what they’re looking at is the interaction of these black holes with the stars that they
have actually pulled close enough to become companions. So we call them black hole binaries.
Now, these black holes, when they interact with the material of these stars can produce
X-rays. Sometimes, you’ll get X-ray bursts and these are caused when the black hole itself
feeds on material from this star, so it’s very active. But, looking at these black holes
that are so far away at the centre of our galaxy, it means that these X-ray bursts are
only ever bright enough or strong enough to be seen maybe once every hundred or thousand
years – so not the kind of timescales we’re working to. Instead, Astronomers are looking
for the fainter but steadier X-ray signals that are given when these companion stars
shed their outer layers and that forms an accretion disk around the balck hole. That
accretion disk is full of very fast-moving and hot gas and that emits x-rays which we
can detect. So the chandra x-ray telescope was used to detect some of these X-ray signals,
they were using archived data, and they found these 12 black holes in these bianry systems
with their companion stars. But in fact, looking at where those black hole binaries are and
their properties, they actually assume that there could be about 500 of these blackhole
binaries around Sagittarius A at the centre of our galaxy. There are isolated black holes
too and their actually thought to be even more common that these black hole binaries.
The problem again is that without an interaction from these isolated black holes we can’t detect
they’re there. But from all the estimates, they predict there are around 10,000 black
holes around that supermassive black hole at the centre of our galaxy. And very timely,
about this time last year, the event horizon telescope started collecting data that would
allow us to make the first image of a black hole. Now, they say that if they combine the
data they’ve collected correctly, we might have the first image of a black hole later
this year. So something to keep our eyes on.

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