KentuckyFC writes "In 2007, a Dutch school teacher named Hanny van Arkel discovered a huge blob of green-glowing gas while combing though images to classify galaxies. Hanny's Voorwerp (meaning Hanny's object in Dutch) is astounding because astronomers have never seen anything like it. Although galactic in scale, it is clearly not a galaxy because it does not contain any stars. That raises an obvious question: what is causing the gas to glow? Now a new survey of the region of sky seems to have solved the problem. The Voorwerp lies close to a spiral galaxy which astronomers now say hides a massive black hole at its center. The infall of matter into the black hole generates a cone of radiation emitted in a specific direction. The great cloud of gas that is Hanny's Voorwerp just happens to be in the firing line, ionizing the gas and causing it to glow green. That lays to rest an earlier theory that the cloud was reflecting an echo of light from a short galactic flare up that occurred 10,000 years ago. It also explains why Voorwerps are so rare: these radiation cones are highly directional so only occasionally do unlucky gas clouds get caught in the crossfire."
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