30-year-old mystery of planetary nebulae solved

Astronomers have resolved a 30-year-old argument, ruling out a one-size-fits-all mechanism for shaping some of the most beautiful objects in space -- the planetary nebulae.

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Planetary nebulae have nothing to do with planets, but are enlarged glowing objects that is stars late in their lives which have shed much of their gas into space. Some appear as round blobs and can look like an eye, an ant or an hourglass. The question is, what forms these shapes? There have been two ideas. One is that a planetary nebula arises from a single star in the late stage of its life. The second idea is two stars are involved with the aged one providing the gas. Now, an international team, led by Brent Miszalski of the Anglo-Australian Observatory, has clearly ruled it out as a general mechanism for shaping majority of planetary nebulae, the Astronomy & Astrophysics journal reported. In their research, the team drew on a number of catalogues of planetary nebulae, particularly those derived from the Macquarie/AAO/Strasbourg H-alpha surveys, to amass a collection of hundreds of possible central stars in nebulae. Using the OGLE-III photometric survey, they then looked for periodic variability in the light of the central star, which could be a sign that the central star was, in fact, a binary two stars orbiting each other. After refining their sample by knocking out dubious objects, the team was left with a set of about 300 planetary nebulae, of which 21 showed evidence for a central binary. This more than doubled the number of planetary nebulae known to have central binaries. The astronomers concluded that close binaries occur in 10-20 per cent of planetary nebulae. Their figure agrees with, and provides the first independent support for, an estimate of 10-15 per cent published in 2000. The team also found that the periods of the detected binaries were much shorter than had been predicted. 'Why, we don't know yet. All we can say is that the existing models don't well match what we've found. 'Binarity is not a precondition for the formation of planetary nebulae and close binaries do not play a dominant role in the shaping of nebular morphologies,' Miszalski said.