A huge eruption lit up the skies on our planet two million years ago - creating a fuzzy ball of light around the size and brightness of the moon.
The eruption - a huge blast from the black hole at the centre of our galaxy - would have been witnessed by our ancestors, homo erectus, on the plains of Africa.
Scientists have found the first evidence of this “huge explosion” of radiation from the supermassive black hole - four million times the mass of our sun - a faint glow in a cloud of gas millions of miles from the black hole itself.
Scientists describe the supermassive black hole - Saggitarius A* - as a “dormant volcano”.
Blasts of energy can jet out of black holes when stars and gas clouds are “consumed” by the black hole - and scientists warn that there are “lots and lots” circling our galactic centre.
One in particular could erupt next year.
[Solar sails to "carry mankind to the stars"]
"They have been monitoring a cloud and predict that it will fall into the black hole at some point in the next year; however, the amount of material will be far less than the event that illuminated the stream," said Greg Madsen, astronomer at the University of Cambridge.
"It will be much fainter and will pose no threat to Earth, but several powerful telescopes will be poised and ready to watch what happens."
Astronomers have long suspected that a large explosion occurred millions of years ago - but have never been able to prove it.
The evidence comes from a lacy filament of gas, mostly hydrogen, called the Magellanic Stream. This trails behind our galaxy’s two small companion galaxies, the Large and Small Magellanic Clouds.
“For twenty years we’ve seen this odd glow from the Magellanic Stream,” said Professor Joss Bland-Hawthorn, of the University of Sydney, who led a team studying this problem.
“We didn’t understand the cause. Then suddenly we realized it must be the mark, the fossil record, of a huge outburst of energy from the center of our galaxy.”
“It’s been long suspected that our galactic center might have sporadically flared up in the past. These observations are a highly suggestive ‘smoking gun’,” said Martin Rees, Astronomer Royal.
The team gives its arguments in a paper accepted for publication in The Astrophysical Journal. Professor Bland-Hawthorn will speak about the work at the Galaxy Zoo meeting in Sydney, Australia, on 24 September.
The galaxy’s supermassive black hole has been known for decades. It’s orbited by a swarm of stars whose paths let us measure the black hole’s mass: four million times the mass of the Sun.
The region around the black hole, called Sagittarius A* [“A-star”], pours out radio waves, infrared, X-rays and gamma rays.
Flickers of radiation rise up when small clouds of gas fall onto the hot disk of matter that swirls around the black hole.
But evidence has been building of a real cataclysm in the past. Infrared and X-ray satellites have seen a powerful ‘wind’ (outflow) of material from this central region. Antimatter boiling out has left its signature. And there are the ‘Fermi bubbles’ -- two huge hot bubbles of gas billowing out from the galactic center, seen in gamma-rays and radio waves.
“All this points to a huge explosion at the center of our galaxy,” said team member Dr. Philip Maloney of the University of Colorado in Boulder, USA.
Scientists studying the galactic center came together at a workshop at Stanford University in California earlier this year.
While at the workshop, Professor Bland-Hawthorn realized the Stream could be holding the memory of the galactic center’s past.
Struck by the fiery breath of Sagittarius A*, the Stream is emitting light, much as particles from the Sun hit our atmosphere and trigger the colored glows of the aurorae -- the Northern and Southern Lights.
The brightest glow in the Stream comes from the region nearest the galactic center.
Geometry, the amount of energy from the original flare from Sagittarius A*, the time the flare would take to travel to the Magellanic Stream, the rate at which the Stream would have cooled over time -- “it all fits together, it all adds up,” says team member Dr. Greg Madsen of the University of Cambridge in the UK.
Scientists say that such flares can and will happen again.
“There are lots of stars and gas clouds that could fall onto the hot disk around the black hole,” says Professor Bland-Hawthorn. “There’s a gas cloud called G2 that we think will fall in next year. It’s small, but we’re looking forward to the fireworks!”