Butterfly in the night sky, 1,200 years ago

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A Christmas cracker written in the stars? It might look like that. In fact it's an example of a "butterfly" nebula: a star which is throwing out incredibly hot, ionised gas at speeds of 720,000 miles per hour.

This picture, taken by the orbiting Hubble Space Telescope, is of a star known simply as M2-9, 2,100 light-years away in the constellation Ophiucus - relatively close compared to our galaxy, the Milky Way, which is 200,000 light years across.

The explosion we are seeing now actually started happening just 1,200 years ago, according to estimates by astronomers, so the plumes visible now have been growing for 900 years.

But what is actually happening? According to studies from the ground, M2-9 is one of two stars which orbit each other very closely - so closely that one of them is probably pulling gas from the surface of its partner, or possibly engulfing it.

Some gas is captured by the central star's gravity, but some more is thrown out as a thin, dense disk which surrounds both stars and extends well into space.

This false-colour image shows oxygen atoms as red dots, nitrogen ions as green, and ionised oxygen as blue. But why does it look like a Christmas cracker, or a jet exhaust? Scientist modelling gas flows have found that the high-speed wind from one of the stars rams into the surrounding disk, which serves as a nozzle.

The wind is then deflected at right angles, to form the pair of jets seen here. Much the same process takes place in a jet engine: the burning, expanding gases are deflected by the engine walls through a nozzle to form long jets of high-speed hot air.

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