Measuring planetary nebulae

Astronomers have undertaken a study to measure the distances to planetary nebulae with more accuracy than ever before.

A collage showing 22 planetary nebulae arranged in order of physical size. Credit: ESA/Hubble & NASA, ESO, Ivan Bojicic, David Frew, Quentin Parker

A collage showing 22 planetary nebulae arranged in order of physical size. Image Credit: ESA/Hubble & NASA, ESO, Ivan Bojicic, David Frew, Quentin Parker

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The measurement of the distances from Earth to planetary nebulae is a longstanding problem in astronomy, but a team of researchers are using new data to provide more accurate results than ever before.

The solution uses an estimate of the dimming of the nebula caused by interstellar gas and dust, the nebula’s size as projected on the night sky and a measurement of its brightness.

This creates what is called a ‘surface-brightness relation’.

The team behind this latest study tested the method using over 300 planetary nebulae whose distances had also been verified using other means.

“The basic technique is not new but what marks out this work from what has gone before is the use of the most up-to-date and reliable measurements of all three of those crucial properties,” says co-author Professor Quentin Parker of the University of Hong Kong.

Planetary nebulae do not actually have anything to do with planets.

Their name was created by early astronomers who saw them as glowing disc-like objects when observing them through their telescopes.

In fact, planetary nebulae are the final stages of smaller stars ejecting their material into space, leaving behind a dense core.

This core is extremely hot, causing the expanding cloud of gas to glow in different colours.

This creates a beautiful effect and, as a result, planetary nebulae are a favourite sight among astronomers.

Dr David Frew, lead author on the paper, says:

“For many decades, measuring distances to galactic planetary nebulae has been a serious, almost intractable problem because of the extremely diverse nature of the nebulae themselves and their central stars.

But finding those distances is crucial if we want to understand their true nature and physical properties.”

The team worked with updated information on the nebulae and analysed current nebulae catalogues to remove doubles that have caused problems for other astronomers attempting to measure the distances.

This approach has produced calculations of distances to planetary nebulae up to five times more accurately than previous calculations, the team said.

“Our new scale is the first to accurately determine distances for the very faintest planetaries” says Dr Frew.

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“Since the largest nebulae are the most common, getting their distances right is a crucial step.”