Stars like blowing bubbles. Especially when they get old. TT Cygni here, is no exception. Except that bubbles like this one are rather a lot more lethal than the kind you’d blow with soapy water.
An ageing giant, TT Cygni is a carbon star. Billions of years worth of nuclear fusion have built up massive amounts of carbon inside this venerable beast. Carbon which is slowly, slowly being dredged up to the star’s outer layers and boiled away into space. The result is this vast bubble in the image below, roughly half a light year in diameter, which has been gradually expanding for the past 6000 years. In this image, taken with a radio telescope array, you can see the edges of the bubble as a ring around the central star.
The thing is, three of the most common elements in old stars like these are carbon, nitrogen and oxygen. An elderly star has had plenty of time to build up large amounts of these through stellar fusion. These combine happily into molecules (often with hydrogen). Two of the most prolific ones are well known to us on Earth, because they’re extremely poisonous! Cyanide (HCN) and carbon monoxide (CO) often make up a huge amount of the bubbles (more properly referred to as “shells”) around carbon stars. In the case of TT Cygni, the ring you can see in this image is actually carbon monoxide.
When you mention carbon monoxide, people usually give you a slightly worried look. Understandably so, after all the talk of it being “the silent killer” in public information campaigns. While it’s definitely not something you’d want in your home, carbon monoxide is actually the second most common molecule in the Universe — and extremely useful it is, to radio astronomers.
Pictures like this one can be taken because CO is a polar molecule. In other words, it holds slight positive and negative charges at either end of it. Conveniently, polar molecules like CO and HCN emit microwaves when they rotate, which can be picked up by radio telescopes. Microwaves which go straight through things like big dust clouds which would otherwise get in the way. As a result, radio astronomy is a useful way of looking at things which would be otherwise invisible to us, such as the core of the Milky Way.
Poison, perhaps. But useful poison all the same.