“Planetary nebulae” have nothing to do with planets. Instead, the old-fashioned name refers to Sun-like stars in a late, brief, phase of their lives.
The core of the star becomes hotter than before. The star is surrounded by gas it has previously ejected: the extra heat of the core now makes this gas glow. This glowing gas is a "planetary nebula".
This unique phase of a star’s life lasts a few tens of thousands of years—a tiny fraction of its total life of perhaps several billion years. Despite this brevity, planetary nebulae are a crucial stage of stellar evolution. Astronomers use them to study how matter from stars is recycled back into the gas between the stars, from which new stars form.
Stars up to eight times the mass of the Sun become planetary nebulae. (More massive stars explode as supernovae.)
The known number of planetary nebulae falls well short of the number that astronomers think exist. Based on the number of stars that give rise to them, there should be up to 35 000 planetary nebulae in our Galaxy. However, finding them is difficult.
Planetary nebulae are a good tracer of galactic structure and star-formation, indicating the presence of stars even in places where mainstream stars are too sparse and faint to detect.
Planetary nebulae are relatively bright when imaged in narrow bands of optical light centred on specific emission lines. This survey used H-alpha, an emission line in the red end of the spectrum that is produced by hydrogen atoms. Using H-alpha is a very good way to detect ionised gas—gas where electrons have been knocked off the atoms—which is present in planetary nebulae.
Unusually in this digital age, the survey was made on photographic plates. It was the last photographic survey to be carried out with the UK Schmidt telescope but the first to be solely available to the astronomical community in digital form via the Web, and the first in a very narrow optical band centred on an emission line of considerable astrophysical interest.
The survey was made using a filter that lets through only H-alpha. No suitable filter previously existed, so the researchers commissioned a special large one of very high quality.
As a recorder they used Kodak Tech Pan film, which is very uniform and fine-grained, and was well-matched to the H-alpha wavelengths they wanted to pick up.
The researchers used two techniques to increase the amount of detail visible in each image. One was to “stack” images: that is, to take a number of images of the same piece of sky, digitise them and combine them. This increases the contrast in the image and makes faint objects more visible. This technique was employed in the study of the Large Magellanic Cloud.
The second technique was to compare the narrow-band images with ones of the same piece of sky imaged in a broad red part of the spectrum, but with “continuum emission” rather than just the single H-alpha emission line, which lies in the red part of the spectrum. Comparing the two images made it easier to find point sources of H-alpha—sources that could be very compact planetary nebulae.
Once planetary nebulae candidates had been spotted, they were confirmed and studied with other telescopes: the Anglo-Australian Telescope at Siding Spring Observatory, the Very Large Telescope of the European Southern Observatory in Chile, the Gemini North telescope in Hawai’i, and a few smaller 2-m class telescopes in South Africa and Australia.
Previous surveys have tried to cover either small areas of sky with high sensitivity or high resolution, or very large areas of sky with low resolution. This one covered a large area at very high spatial resolution and good sensitivity. It has created the least-biased, most homogenous sample of planetary nebulae ever put together.
These follow-up observations were to obtain spectra of the objects, to allow them to be more definitely identified as planetary nebulae and to reveal unusual spectral signatures and properties. Some exciting individual objects were found, including a bizarre late-stage star with a planetary nebula around it.
The survey results are published in two papers in the Monthly Notices of the Royal Astronomical Society in November and December 2006.
Dr Quentin Parker, Anglo-Australian Observatory and Macquarie University
Reid, Warren A.; Parker, Quentin A. “A new population of planetary nebulae discovered in the Large Magellanic Cloud - II. Complete PN catalogue.” Monthly Notices of the Royal Astronomical Society, Volume 373, Issue 2, pp. 521-550. (2006 December).
Parker, Quentin A.; Acker, A.; Frew, D. J.; Hartley, M.; Peyaud, A. E. J.; Ochsenbein, F.; Phillipps, S.; Russeil, D.; Beaulieu, S. F.; Cohen, M.; Kppen, J.; Miszalski, B.; Morgan, D. H.; Morris, R. A. H.; Pierce, M. J.; Vaughan, A. E. “The Macquarie/AAO/Strasbourg Halpha Planetary Nebula Catalogue: MASH.” Monthly Notices of the Royal Astronomical Society, Volume 373, Issue 1, pp. 79-94. (2006 November).