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A collage showing 22 individual planetary nebulae all at various distances from Earth. They are artistically arranged in approximate order of physical size. The scale bar represents five light years.

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A new technique to estimate distances between Earth and planetary nebulae accurately will give them a more meaningful place in astronomical research.

Planetary nebulae potentially offer a wealth of information about dying stars, but astronomers have been unable to tap into that wealth fully because ascertaining their distance from Earth has been problematic.

Based on the culmination of 10 years of research work, a team of three astronomers in HKU’s Department of Physics – Dr David Frew, Professor Quentin Parker and Dr Ivan Bojicic – has come up with a more accurate technique to measure that distance. The implications of their new method, could enable a new era in our ability to study and understand the fascinating final stages of the lives of low- and mid-mass stars.

Planetary nebulae (or PNe) have long held the imagination with their distinct colours and complicated shapes, which range from soap bubbles, to butterflies. The name planetary is actually inaccurate, but came about because early astronomers such as William Herschel in the 1780s thought they resembled planets when in fact they are from dying stars. It wasn’t until the 1860s that the first spectroscope revealed they were not solid objects but gas. They are interesting to astronomers because they also offer a brief glimpse into the history of stars’ lives, but their scientific potential has been considerably reduced because it was so difficult to estimate their distances.

“Planetary nebulae are valuable tools for astronomers seeking a better understanding of how stars evolve and die, and of the nuclear processes acting in old stars and producing elements such as carbon and nitrogen,” said Dr Frew. “We want to know how and what elements are being recycled back into the Galaxy.

“Over the history of the Milky Way, the gas between the stars has become richer in heavy elements over time. If it hadn’t we humans wouldn’t be here – we’re mainly carbon, which wasn’t produced in the Big Bang. Material that once was in planetary nebulae is almost certainly in you and me.

“Measuring distances to Galactic planetary nebulae has been an intractable problem for many decades, because of the highly diverse nature of both the nebulae themselves and their central stars. However, knowing their distance is crucial to understanding their true nature and physical characteristics.”

The solution the team has come up with is a robust statistical distance indicator, using three sets of data. First, the size of the object in the sky taken from the latest high resolution surveys; second, an accurate measurement of how bright the object is in the red hydrogen-alpha emission line; and third, an estimate of the dimming toward the nebula caused by so-called interstellar-reddening. From these quantities, an intrinsic radius is calculated, which when combined with the angular size, yields the distance directly.

“The resulting surface brightness to radius relation has been calibrated using more than 300 planetary nebulae whose accurate distances have been determined via independent and reliable means – such as trigonometric parallax measurements of their central stars,” said Dr Frew. “It is not the technique that is new but the development of more accurate and reliable measurements means that this method is far more reliable than what has gone before.”

Dr David Frew

"Over the history of the Milky Way, the gas between the stars has become richer in heavy elements over time. If it hadn’t we humans wouldn’t be here – we’re mainly carbon, which wasn’t produced in the Big Bang. Material that once was in planetary nebulae is almost certainly in you and me."

Dr David Frew

The full version of this article was originally published in Bulletin. Please click here to view this HKU publication.

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