At the very heart of our galaxy, a giant monster is sleeping.
The supermassive black hole, known as Sagittarius A*, has a mass 4.3 million times larger than the sun and measures around 24 million kilometres (15 million miles) across.
For now, this colossal void is dormant, but scientists don’t think it will stay that way forever.
Now, researchers have predicted exactly when it is likely to wake up.
A supermassive black hole becomes active when it starts to swallow vast quantities of gas and dust from the galaxy around it.
But for this to happen, a black hole needs to be supplied with a mind-boggling quantity of matter to feed on.
The black hole at the centre of the Milky Way will wake up for its next feeding frenzy when we collide with a dwarf galaxy called the Large Magellanic Cloud.
And scientists have predicted that this is almost certain to happen exactly 2.4 billion years from now.
The enormous black hole at the centre of our galaxy, dubbed Sagittarius A*, is currently dormant. However, scientists say it will one day erupt into life
Why do black holes become active?
Black holes are formed when an extremely large star runs out of fuel at the end of its life and collapses under the enormous weight of its own mass.
However, these so-called stellar-mass black holes are pipsqueaks compared to the vast voids that sit at the core of nearly every galaxy.
While a stellar mass black hole might be one to 10 times the mass of our sun, a supermassive black hole can be millions or even billions of times bigger.
But despite how big they are, most of these giants are almost impossible for even our most powerful telescopes to directly detect.
That’s because their gravitational pull is so strong, not even light can escape.
However, that all changes when the black hole starts to pull in surrounding matter.
As the black hole ‘feeds’, it pulls matter into a swirling ring known as an accretion disk, where it orbits at incredible speeds.
Black holes become active when they start to consume massive amounts of matter from their host galaxy. This creates a blast of energy that we can see through our telescopes on Earth and in space
Dr Nathalie Degenaar, of the University of Amsterdam, told Daily Mail: ‘While swirling towards the black hole, the matter heats up to tens of thousands, even up to many millions, of degrees.
‘We can see the heat radiation that the matter gives off as visual light, but it will also radiate at ultraviolet, X-ray and infrared wavelengths.’
In some cases, however, black holes will put on an even more impressive display.
‘We see that black holes that feed on matter spit out gigantic, powerful streams of gas and energy,’ says Dr Degenaar.
‘We call these structures jets, and you could visualise them as sort of cosmic fire hoses.’
The fastest of those jets travels a sizeable fraction of light speed and releases huge bursts of energy that we can ‘see’ with radio telescopes on Earth.
When that happens, we say that a black hole has become ‘active’ rather than ‘dormant’.
Could Sagittarius A* become active?
Sagittarius A* is currently dormant because it is consuming so little matter; if it started to receive more matter, it could very suddenly come back to life. Pictured: A view of the magnetic fields around Sagittarius A*
At this very moment, Sagittarius A* is dormant, which means it isn’t giving off very much radiation.
Professor Christine Done, an expert on active black holes from the University of Durham, explains that this is because it isn’t swallowing enough material.
Professor Done told Daily Mail: ‘It’s a pathetic amount of material that’s falling in. Simply pathetic.’
Black holes have a maximum amount of material that they can swallow, known as the Eddington Limit.
The bigger a black hole is, the more it can absorb at a time, but the black hole in our galaxy is eating well below its Eddington Limit.
However, Professor Done says that all it would take for Sagittarius A* to become active would be for something to ‘dump a whole load of matter into it’.
If the amount of infalling matter is close enough to the Eddington Limit, it would trigger Sagittarius A* to become active and produce its own jet.
When will Sagittarius A* become active?
In 2.4 billion years, the Milky Way will collide with a dwarf galaxy known as the Large Magellanic Cloud. This will give the black hole enough energy to become active
Since all the black hole needs to become active is a sudden meal of matter, scientists think that it might flare back into activity at any moment.
In fact, between 2011 and 2014, scientists were hopeful that a cloud of gas called G2 might fall into the accretion disk and trigger a burst of activity, although it eventually escaped unscathed.
Dr Degenaar says: ‘Although G2 managed to escape a dramatic fate, many similar objects are passing by Sagittarius A* all the time; the centre of our Galaxy, where the black hole lives, is a densely populated neighbourhood.
‘It is thus perfectly possible that at some point there will be one or more objects falling into Sagittarius A*’s gravitational trap and spark fireworks.’
However, for Sagittarius A* to become active on a more permanent basis, it would need an even bigger supply of matter to feed on.
Scientists believe this will next come when the Milky Way collides with our neighbouring galaxy, the Large Magellanic Cloud (LMC).
Currently located 200,000 light-years from Earth, the LMC is the Milky Way’s satellite galaxy and has roughly one hundredth of our galaxy’s mass and may contain a smaller supermassive black hole.
However, in 2.4 billion years, the two galaxies will finally come together in a collision, which will result in the Milky Way swallowing its smaller neighbour.
When Sagittarius A* feeds off the Large Magellanic Cloud, and potentially the small black hole at its core, it will grow to become up to eight times as large. Pictured: A simulation of two supermassive black holes merging
Professor Carlos Frenk, an astronomer from Durham University who predicted the date of the collision, told Daily Mail: ‘The instability that this event will create will funnel gas, originally belonging to the LMC and to the disk of the Milky Way, into the centre of our galaxy.
‘Some of this will be accreted by our central black hole.’
According to Professor Frenk’s research, Sagittarius A* could become up to eight times more massive as it feeds on the sudden abundance of gas and dust.
As it feeds, the black hole will erupt into a prolonged period of activity, which will affect the entire galaxy.
The same process will likely happen again when the Milky Way undergoes a far more catastrophic collision with the Andromeda Galaxy in four to five billion years’ time.
What will happen to Earth when Sagittarius A* becomes active?
In the absolute worst-case scenario, the sudden awakening of Sagittarius A* could lead to total devastation.
Professor Frenk says: ‘Active Galactic Nucleus (AGN) activity is often associated with the generation of jets that produce gamma-rays.
When Sagittarius A* becomes active (artist’s impression), it will produce a ‘firehose’ of radiation known as a jet, which has the potential to strip planets of their atmosphere
Some black hole jets, such as this one within the galaxy Centaurus A, can move at a substantial fraction of light-speed and contain enough energy to disrupt star formation
‘If the jets were powerful enough and pointed in our direction, they could destroy the Earth’s ozone layer and cause mass extinctions.’
Luckily for us, this is not what is going to happen when we hit the LMC in a little over two billion years.
‘The Galactic AGN triggered by the merger with the LMC will not be powerful enough for this, so it is very unlikely to pose a serious danger to terrestrial life.
‘The merger will also eject stars from the Galactic disc into the Galactic halo, but, again, it is extremely unlikely that this will include the Solar system.’
Although the radiation released by Sagittarius A* would be very powerful, at 26,000 light-years away, Earth’s massive distance from the black hole will keep us safe.
Dr Joseph Michail, an astronomer from Harvard & Smithsonian Centre for Astrophysics who studies Sagittarius A*, says: ‘26,000 light-years might not seem like a huge distance in astronomical terms, but it’s substantial, and any radiation we received on Earth from it would be highly diluted by the distance.’
Additionally, since Earth is in the Milky Way’s disk, we are protected by huge volumes of gas and dust, which would filter out any lingering radiation.
Scientists believe that large solar flares and supernovae in the galactic neighbourhood would pose much more of a threat to Earth than the black hole.
Luckily, Earth will be protected by the 26,000 light-years of gas and dust between our solar system and Sagittarius A*. This means that astronomers will be the only ones to notice the change from Earth
In fact, scientists are quite hopeful that Sagittarius will eventually become active again because this will make it much easier to study from Earth.
What will happen to the Milky Way?
The most significant effects of Sagittarius A* waking up would actually be seen in the wider Milky Way.
Professor Done says: ‘We’ve started to understand that black holes have very powerful winds.
‘If you have a whole load of material piling on and there’s a whole load of radiation, that radiation can actually blow matter away.’
This wave of radiation from the black hole outflow ‘smacks’ into the rest of the galaxy, scattering clouds of dust.
In the past of our own galaxy, researchers believe that Sagittarius A* produced such a powerful outburst that it ejected material 25,000 light-years above and below the galactic plane.
Scientists call the remnants of this outburst the ‘Fermi Bubbles’, which were only found about 15 years ago.
The activation of Sagittarius A* will have big effects on the Milky Way. The last time it was intensely active, the black hole jet shot matter 25,000 light-years into space to form the Fermi Bubbles (pictured)
This wave of radiation prevents star formation in some parts of the galaxy, and speeds it up in others as the wave of dust is crushed together at the edges.
This, in turn, changes the flow of dust into the centre of the galaxy and moderates the power of the black hole’s activation in a continuous feedback loop.
Professor Done says that scientists are beginning to understand that there is an intimate relation between the evolution of the galaxy and its black hole.
Professor Done says: ‘There’s something in that way that galaxies grow that is controlled by the growth of the black hole in their centre.’
