Free Astronomy Magazine March-April 2015

SPACE CHRONICLES times the mass of the Sun.) The team who found this massive pair actually set out to try to solve a different problem. They wanted to find out how some stars produce such strangely shaped and asym- metric nebulae late in their lives. One of the objects they studied was the unusual planetary nebula known as Henize 2-428. (Planetary nebulae have nothing to do with planets. The name arose in the eighteenth century as some of these objects resembled the discs of the distant planets when seen through small telescopes.) “When we looked at this object’s central star with ESO’s Very Large Telescope, we found not just one but a pair of stars at the heart of this strangely lop- sided glowing cloud,” says co- author Henri Boffin from ESO. This supports the theory that double central stars may ex- plain the odd shapes of some of these nebu- lae, but an even more interest- ing result was to come. "Further obser- vations made with telescopes in the Canary Islands allowed us to determine the orbit of the two stars and deduce both the masses of the two stars and their sepa- ration. This was when the big- gest surprise was revealed," reports Romano Corradi, another of the study's authors and researcher at the Instituto de Astrofísica de Cana- rias (Tenerife, IAC). They found that each of the stars has a mass slightly less than that of the Sun and that they orbit each other every four hours. They are sufficiently close to one another that, according to the Einstein’s theory of general relativity, they will grow closer and closer, spiral- ling in due to the emission of gravi- tational waves, before eventually merging into a single star within the next 700 million years. The resulting star will be so massive that nothing can then prevent it T his picture shows the sky around the planetary nebula Henize 2-428. The object itself is visible as a small curving nebula at the centre of the picture, surrounded by huge number of faint stars in the Milky Way. This picture was created from images in the Digitized Sky Survey 2. [ESO/Digitized Sky Survey 2] from collapsing in on itself and sub- sequently exploding as a superno- va. "Until now, the formation of su- pernovae Type I a by the merging of two white dwarfs was purely the- oretical," explains David Jones, co- author and ESO Fellow at the time the data were obtained. “The pair of stars in Henize 2-428 is the real thing!” "It's an extremely enigmatic sys- tem," concludes Santander-García. "It will have important repercus- sions for the study of supernovae Type I a, which are widely used to measure astronomical distances and were key to the discovery that the expansion of the Universe is acceler- ating due to dark energy." n