Questions in the Void

Certainly not empty space.  Published by Manchester University Press 1980.  Cover illustration shows a set of molecular peak emission intensity contours superimposed on a Lick Observatory photograph of the HII regions of NGC1977 and Orion A.

Certainly not empty space. Published by Manchester University Press 1980. Cover illustration shows a set of molecular peak emission intensity contours superimposed on a Lick Observatory photograph of the HII regions of NGC1977 and Orion A.

Boyajian’s Star continues to be something of a mystery.  The fluctuations of light coming from that star, around 1500 light years away, are very hard to explain according to any known natural phenomena.  And as I mentioned in a previous webpost – The Lonely Universe: Think Big – there’s been a tendency to explain it all, probably incorrectly, through artificial alien megastructures orbiting the star.  But we should only turn to such exotic reasons when all else fails.

Unusual stellar light curves, showing variations in a star’s brightness over time, can normally be explained fairly easily.  Brief stellar flares, or the more regular dips caused by orbiting planets, are common.  It’s the more erratic behaviour that requires further thought and investigation.

The light curve of another star – J1407, around 400 light years away – caused something of a puzzle, as explained by Matthew Kenworthy, associate professor of astronomy at Leiden Observatory in the Netherlands, in Rings of a Super Saturn (Scientific American, January 2016).  Fluctuations in 2007 seemed to indicate a star which flickered in an unpredictable way for a while, then dimmed almost completely over a week before returning to normality.  Various explanations were tried out, but the most likely cause was a giant planet with a vast ring system some 200 times larger than Saturn’s.

But Boyajian’s Star is something else again.  The extreme variability of its light curve could not be produced by any planetary system.  In Strange News from Another Star (Scientific American, May 2017), Kimberley Cartier and Jason T Wright of Pennsylvania State University describe the seemingly random dips in brightness for periods ranging from hours to weeks, with the star dimming by up to 20 per cent at times.  Boyajian’s Star has also been steadily darkening over the past century.

The authors rehearse the possible explanations, but none seem very satisfactory.  The star would warm any disk of gas and dust surrounding it, or any swarm of comets large enough to cause such effects, resulting in a detectable radiation of heat.  But there isn’t any.  Variations in the star itself might be changing its intrinsic brightness, but no star like this one has behaved like that before.

But they do suggest more plausible hypotheses – perhaps the true cause lies not in the vicinity of the star, but in interstellar space between it and us.  Possibly a swarm of dense clouds of gas and dust, or even a black hole with a disc of material of varying density.  This prompted me to look back at one of my old textbooks – The Physics of the Interstellar Medium by JE Dyson and DA Williams of the University of Manchester, with its first edition published in 1980.  They describe how spectroscopy measures the emission or absorption of radiation by atoms, ions and molecules in interstellar gas.

The microscopic processes involved not only tell us about the temperatures and densities of the regions concerned, but also actually control those properties.  The authors admitted that, at least at the time, there was “no general agreement regarding the detailed importance and interplay of the many physical process at work in interstellar space”, but even so those processes were well understood, as were the gas dynamics which determine the more dramatic events occurring between the stars.  Things have moved on further again in the intervening forty years or so.

With such detailed understanding of the interstellar medium, it seems unlikely that much doubt can remain for long on the question of whether the fluctuations of Boyajian’s Star arise in that quarter.  Are we back with vast alien artefacts orbiting around it?  Perhaps we should bear in mind the words of Conan Doyle’s great detective Sherlock Holmes in the 1890 novel The Sign of Four: “when you have eliminated the impossible, whatever remains, however improbable, must be the truth.”

Richard Hayes, Assistant Editor (Odyssey)

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