Planetary nebulae have nothing to do with planets. They are shells of gas and dust thrown off by dying stars but they got their name from the fact they often appear as planet-like disks.
Famous examples visible with amateur telescopes include the Ring Nebula (M57) in the constellation of Lyra and the Helix Nebula (NGC 7293) – also nicknamed the Eye of God – in Aquarius.
Now a completely new type of planetary nebula has been discovered around much heavier stars in two of the Milky Way’s companion galaxies, the Magellanic Clouds. They have been dubbed Super Planetary Nebulae because of their size.
The new heavyweights were detected by a team of astronomers from Australia and the United States, led by Associate Professor Miroslav Filipović of the University of Western Sydney, using radio telescopes of Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO).
Fifteen objects broadcasting strong natural radio signals were found to match distant planetary nebulae already observed with optical telescopes.
Planetary nebulae are usually found around stars comparable in size to the Sun or smaller. However, the radio data shows that the newly discovered central stars are up to eight times the mass, or “weight”, of the Sun. What is more, the nebular material surrounding them appears to be as much as 2.6 times the Sun’s own mass.
The scientists, who report their results in the journal Monthly Notices of the Royal Astronomical Society, say the new population may be a long-predicted class of shells around heavier stars.
They also suggest that the detections of these new objects may help to solve the so called “missing mass problem” – the absence of planetary nebulae around central stars that were originally one to eight times the mass of the Sun. Until now, most known planetary nebulae have been found to have central stars and surrounding nebulae with only about 0.6 and 0.3 times the mass of the Sun. None have previously been detected around more massive stars.
Filipović said: “This came as a shock to us as no one expected to detect these object at radio wavelengths and with the present generation of radio telescopes. We have been holding up our findings for some three years until we were 100 per cent sure that they are indeed planetary nebulae”.
Some of the 15 newly discovered planetary nebulae in the Magellanic Clouds are three times more luminous then any of their Milky Way cousins. But to see them in greater detail astronomers will need the power of a huge radio telescope being planned for the future – the Square Kilometre Array set to be sited in the deserts of Western Australia.
Picture: An optical image from the 0.6-m University of Michigan/CTIO Curtis Schmidt telescope of the brightest radio planetary nebula in the Small Magellanic Cloud, JD 04. The inset box shows a portion of this image overlaid with radio contours from the Australia Telescope Compact Array. (Optical images are courtesy of the Magellanic Cloud Emission Line Survey (MCELS) team).
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