Getty Has the Milky Way been dead for billions of years?

An “alive” galaxy is one that continues to produce gas, and thus also creates new stars. Once the stars die, the gasses are released back to help produce new stars. They are also typically in an spiral shape, like the Milky Way. But another type of galaxy is more football shape, with the stars orbiting in a more free-flowing motion which gives it a rounder appearance, otherwise known as elliptical galaxies. This is due to the fact that they have stopped producing gasses and stars and have lost their flat looking shape.

It had previously been thought that blue galaxies are a sign that they are relatively young and that gasses are being produced at a rapid rate, and red galaxies signify that they are near to the end of their life. However, as technology advanced and we were able to see hundreds of thousands of galaxies in our universe, scientists noted that many of the red galaxies were still spiralling and had not lost their shape – meaning that signs of young and old were conflicting. Kevin Schawinski, Assistant Professor of Galaxy & Black Hole Astrophysics, Swiss Federal Institute of Technology Zurich, explains in the Conversation: “The crossroads of galaxy evolution is a place called the ‘green valley.’

Getty An elliptical galaxy which has lost its shape

“This may sound scenic, but refers to the population between the blue star-forming galaxies (the ‘blue cloud’) and the red, passively evolving galaxies (the ‘red sequence’). “Galaxies with ‘green’ or intermediate colours should be those galaxies in which star formation is in the process of turning off, but which still have some on-going star formation – indicating the process only shut down a short while ago, perhaps a few hundred million years.” He argues that spiralling galaxies – like our own – are just dying at a slower rate and that the Milky Way could have begun this process a billion years ago, leaving it in a zombie-like state.

Getty Red galaxies were believed to be dying

He and his team “found the slowly dying ones are the spirals and the rapidly dying ones are the ellipticals. There must be two fundamentally different evolutionary pathways that lead to quenching in galaxies. “When we explored these two scenarios – dying slowly, and dying quickly – it became obvious that these two pathways have to be tied to the gas supply that fuels star formation in the first place. “Imagine a spiral galaxy like our own Milky Way merrily converting gas to stars as new gas keeps flowing in.

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“Then something happens that turns off that supply of fresh outside gas: perhaps the galaxy fell into a massive cluster of galaxies where the hot intra-cluster gas cuts off fresh gas from the outside, or perhaps the dark matter halo of the galaxy grew so much that gas falling into it gets shock heated to such a high temperature that it cannot cool down within the age of the universe. In any case, the spiral galaxy is now left with just the gas it has in its reservoir. “The Milky Way is just at the edge, ready to tumble into the green valley. It’s entirely possible that the Milky Way galaxy is a zombie, having died a billion years ago.”