The mysterious brightening of a distant galaxy has been believed to be caused by material falling into a huge black hole.
And scientists believe it to be 1 million times the size of the sun.
For decades, nothing seemed special about this galaxy in the Virgo constellation.
But everything changed at the end of 2019 when astronomers noticed a surprising surge in its brightness that still continues today.
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Researchers now think they're witnessing something never seen before, with the black hole at the galaxy’s core putting on an extreme light show as it consumes large amounts of material.
'We discovered this source at the moment it started to show these variations in luminosity,' said Dr Paula Sánchez-Sáez, a staff astronomer at the European Southern Observatory headquarters in Garching, Germany. 'It’s the first time we’ve seen this in real time.'
The galaxy known as SDSS1335+0728 is 300m light years away. It caught attention in December 2019 when the Zwicky Transient Facility in California recorded a sudden rise in its brightness.
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This alert led to a wave of new observations and fact-checking of old data from telescopes to learn more about the galaxy and its past behaviour.
The astronomers found that the galaxy had recently doubled in brightness in mid-infrared wavelengths, become four times brighter in the ultraviolet, and at least 10 times brighter in the X-ray range.
Although the exact cause of the sudden brightening is unclear, researchers think it might be the creation of an 'active galactic nucleus,' where a massive black hole at the galaxy's centre starts actively consuming surrounding material.
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Active galactic nuclei emit a wide range of light as gas around the black hole heats up and glows. Dust particles around it absorb some wavelengths and re-radiate others.
But the mystery continues as this may not be the only possibility.
The team has not ruled out a 'tidal disruption event' which is a fancy term for a star being ripped apart after getting too close to a black hole.
Tidal disruption events usually brighten a galaxy for no more than a few hundred days, but more research is needed to cross this out.
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'With the data we have at the moment, it’s impossible to disentangle which of these scenarios is real,' said Sánchez-Sáez. 'We need to keep monitoring the source.'