Ancient Explosion Left a Bubble in Space express
In radio sky data, like on old photographic film, two bright gas streaks have emerged. They move at different speeds but hang near the same dim star. The star and the streaks turned out to be inside a huge empty bubble in the interstellar gas. This bubble is like a blown soap bubble left over from an ancient supernova explosion.
The gas streaks are most likely remnants of a planetary nebula — the shell shed by a dying star. Usually such shells quickly dissipate into the surrounding gas. Here they survived because they expanded into the nearly empty space of the bubble. Over 300,000 years, the gas cooled, became neutral, and began emitting radio waves at a wavelength of 21 centimeters.
So an ancient explosion inadvertently preserved for us the traces of another, more modest stellar death. The bubble acted as a museum display case: without it, the gas would have long since mixed with the interstellar medium.
Radio waves at a wavelength of 21 centimeters are emitted by neutral hydrogen — the most abundant element in the universe. It is through this radiation that astronomers see cold gas invisible to ordinary telescopes.