|The strange green glow of Hanny’s Voorwerp.
Photo: Dan Smith; Peter Herbert; Matt Jarvis, via ING
There’s a strange green blob in the middle of the constellation Leo Minor. Unless you have a good telescope, you won’t be able to see it. Although it’s the size of a small galaxy, at about 700 million light years away from Earth it’s so small it was only noticed for the first time in 2007.
The Dutch school teacher Hanny van Arkel was looking through pictures for the Galaxy Zoo project when she spotted a bright green shape that looks like a bit like small letter ‘g’. Nicknamed ‘Hanny’s voorwerp’ (Hanny’s object in Dutch), its colour mystified astronomers. At 16 light years across, it might have been a galaxy if it contained any stars. By analysing the light, the researchers found it was instead a massive cloud made of gas and dust.
But why is it glowing green?
One hypothesis was that it was a ‘light echo’ – a reflection of light cast by a neighbouring galaxy called IC 2497. But the galaxy seems quiet now, meaning it would have emitted a sudden blast of radiation. This radiation ionised the gas just as radiation from our Sun can bump into the gases of our upper atmosphere and create beautiful glowing auroras. Unfortunately nobody could explain why this burst of radiation occurred. Now, a team of astronomers based in the Netherlands have come up with a better idea.
It’s now understood that many galaxies contain supermassive black holes near their centre. For example, at the core of our Milky Way there is one called Sagittarius A* with a mass 4 million times that of the Sun. While they are hard to see directly, there are ways of detecting their effect on surrounding matter. If matter falls into a black hole in the centre of IC 2497, a cone of radiation would blast out of the galaxy right at Hanny’s voorwerp, making it glow green.
The problem was nobody noticed IC 2497’s black hole before; the galaxy’s centre was hidden behind a veil of dark galactic dust clouds. Only by carefully looking at the light coming from IC 2497 could the team of researchers conclude that there was a huge black hole lurking out of sight.
Just imagine what other mysteries are still hidden in the universe, waiting to be found.