Down on Earth around 3.5 million years ago, humanity was starting to take its earliest forms in some regions of Africa. At the same time, the sky was bursting with radiation from an explosive flare that took place in the center of the Milky Way.

An international team of astronomers recently found evidence of the explosion, the impact of which extended across 200,000 light-years and released a flash of energy that shone out into space through the two poles of our galaxy. And while humans probably couldn’t see this flare, if there are any intelligent beings in the rest of the galaxy, they may have caught a glimpse.

The study, published on Sunday in The Astrophysical Journal, is based on observations made by the Hubble Space Telescope that caught the afterglow of the flare.

Gerald Cecil, Ph.D., professor of physics and astronomy at the University of North Carolina and co-author of the study, believes that the flare was caused by gas and shattered stars falling into the supermassive black hole at the center of the Milky Way.

“We then see this glowing light,” Cecil tells Inverse. “Although we don’t have a direct view of the explosion in the center, we see its reflective light.”

The new study is based on a 2013 discovery of the Magellanic Stream, a stream of gas clouds that extends over the Milky Way, as shown in the video above. Light from distant quasars, massive and extremely bright cosmic objects, passed through the stream on its way to Earth. The composition of the arc, which removed some parts of the quasar light, led the team to suspect that something was heating and lighting up the gas stream.

What the team managed to observe was the fading remnant of the explosion, which was much brighter when it occurred 3.5 million years ago.

“The fire is over, and we’re looking at the glowing coals,” Cecil says. “A lightbulb burning out.”

The researchers estimate that the blast may have lasted for 300,000 years but is no longer active today based on mathematical models of how the heated gas de-energized.

“The supermassive black hole in the Milky Way is always flickering, if an asteroid fell in instead of dust, you might get a flare that lasts for hours,” Cecil says.

The flare event disrupted the ionizing radiation field over the South Galactic Hemisphere of the Milky Way. Bland-Hawthorne, et al/ASTRO 3D

The new findings suggest that our galaxy is more active than we had initially thought it to be. The Milky Way is known to have some low level activity but, according to Cecil, this new research suggests that it was perhaps a million times more active only a few million years ago than what astronomers believed in the past.

“Other galaxies have this behavior,” Cecil says. “It puts the Milky Way into the mainstream.”

Considering that this event took place only a couple of million years ago, an insignificant amount of time in the life of a galaxy, means that it may be one of many such episodes that have occurred over the lifetime of the Milky Way.

“If we start thinking about the cumulative effects of those radiation, we have to incorporate them in the models of what’s going on in the Milky Way,” Cecil says. “That hasn’t been done reliably.”

The new study casts the Milky Way in a new, perhaps brighter, light, as well as the activity of the black hole at its center, if it is in fact the culprit behind this relatively recent explosion. The team behind the research hopes to get a better understanding of the duration of the flare, and whether it flared back up again during that period or continued to fade gradually.

Another aspect of the findings that Cecil points out is that this galactic event may have been observed by other inhabitants of the Milky Way, which he strongly believes exist, and essentially “synchronized everybody’s clock” to a point of reference in the galaxy’s timescale.

“Here is an event that’s galaxy wide, for wherever they are in the galaxy,” Cecil says. “There was an event, one that everyone could see.”

Abstract: There is compelling evidence for a highly energetic Seyfert explosion (10^56−57 erg) that occurred in the Galactic Centre a few million years ago. The clearest indications are the x-ray/γ-ray ‘10 kpc bubbles’ identified by the Rosat and Fermi satellites. In an earlier paper, we suggested another manifestation of this nuclear activity, i.e. elevated Hα emission along a section of the Magellanic Stream due to a burst (or flare) of ionizing radiation from Sgr A*. We now provide further evidence for a powerful flare event: UV absorption line ratios (in particular CIV/CII, Si IV/Si II) observed by the Hubble Space Telescope reveal that some Stream clouds towards both galactic poles are highly ionized by a source capable of producing ionization energies up to at least 50 eV. We show how these are clouds caught in a beam of bipolar, radiative ‘ionization cones’ from a Seyfert nucleus associated with Sgr A*. In our model, the biconic axis is tilted by about 15° from the South Galactic Pole with an opening angle of roughly 60°. For the Stream at such large Galactic distances (D >∼ 75 kpc), nuclear activity is a plausible explanation for all of the observed signatures: elevated Hα emission and H ionization fraction (x^^e >∼ 0.5), enhanced CIV/CII and Si IV/Si II ratios, and high CIV and Si IV column densities. Wind-driven ‘shock cones’ are ruled out because the Fermi bubbles lose their momentum and energy to the Galactic corona long before reaching the Stream. Our time-dependent Galactic ionization model (stellar populations, hot coronal gas, cloud-halo interaction) is too weak to explain the Stream’s ionization. Instead, the nuclear flare event must have had a radiative UV luminosity close to the Eddington limit (fE ≈ 0.1 − 1). Our time-dependent Seyfert flare models adequately explain the observations and indicate the Seyfert flare event took place To = 3.5 ± 1 Myr ago. The timing estimates are consistent with the mechanical timescales needed to explain the x-ray/γ-ray bubbles in leptonic jet/wind models (≈ 2 − 8 Myr).