The Milky Way’s young runaways
A dozen youthful stars are circling the Milky Way much farther out than we think they should be
wo and a half years ago, the Gaia collaboration released the second tranche of data from their mission to map the nearest billion stars. Ever since, astronomers have been picking through the treasure trove of data it delivered, publishing several papers a day using Gaia’s measurements, and the flood of discoveries shows no sign of abating.
This month’s paper adds some fast moving and unusual stars to the haul. Roberto Raddi in Barcelona and colleagues in Germany have sifted through the Gaia data to find 12 stars speeding through a region where they don’t belong, far from the Milky Way’s disc.
Though we generally picture the Milky Way as a thin disc with a central bulge – think of Patrick Moore’s description of the Galaxy as ‘two fried eggs, clapped back to back’ – there are stars that exist above and below the disc, travelling on their own unusual orbits around the Galactic centre. Mostly, these stars have ended up there after long and eventful lives, but the unusual thing about the stars studied by Raddi and his colleagues is that they are young – young enough that we’d like an explanation for why they’re so far from the densely populated disc where they must presumably have been born.
T“The unusual thing about the runaway stars is that they are young, so why are they so far from the disc where they were born?”
The outsiders
The stars in question were first identified in the Gaia database, and then their properties determined during a run on the New Technology Telescope in Chile. The runaway stars are between two and four solar masses, and the assumption is that most formed in the dense environment of a young open cluster. In such a cluster, occasional random interactions between the jostling stars may lead to the expulsion of one of the cluster’s members. Alternatively, if one of the stars in a binary system goes supernova the surviving star may receive an
The Sky at Night almighty kick, being thrown out of the cluster and out of the disc of the Galaxy. Given their speeds and positions, this sort of origin story makes sense for nine of the stars.
Three of the stars, though, are moving too fast for either of those mechanisms to be responsible. Our understanding of the physics involved in these mechanisms means that it turns out the maximum speed that can be reached is 450km/s, but three must have exceeded that. These stars, rushing away from the disc, present a true mystery – it’s not at all clear how they came to be, and further work on more speculative possibilities for their origin is needed. Perhaps, somehow, they formed in the thin gas that surrounds the main disc.
However they set off on their current trajectories, the fate of these unusual stars is clear; 11 of the intrepid 12 will remain part of the Milky Way family, existing on orbits that take them high above and then far below our Galactic disc. One of the cohort, though, has a grander fate – 2207-4329 will join a select band of intergalactic wanders, spending the rest of its life out in the cosmic gloom.
As a science correspondent for BBC News I get to cover lots of cool stories, but this year there’s been only one story in town as far as the newsroom has been concerned. Don’t get me wrong, the COVID-19 pandemic is arguably the biggest news story of our generation and as a journalist I’m glad to have had the opportunity to contribute to the BBC’s coverage, but it’s not as fun or uplifting as the topics that are my normal fare.
Then, in early September, I received a call from Steve Crabtree, The Sky at Night’s executive producer. He told me that the programme had “an incredible story” that would be of interest to our news platforms. I remember joking to Steve that unless it was the discovery of alien life it wouldn’t get on because of the airtime COVID-19 was taking. There was a short pause on the line before Steve told me that he couldn’t tell me what it was just now – only that it was a big story.
When Steve was eventually able to pass on the details, it was one of those jaw-dropping moments;
I had to ask him to repeat what he had said, just in case I had got hold of the wrong end of the stick.
Signs of alien life?
Clearly it wasn’t the definitive discovery of alien life – but it was the next best thing. And it was an enigma which only added to the story’s charm. Researchers had a biosignature from our neighbouring planet Venus, one so hellishly hot and acidic that it’s named ‘Earth’s evil twin’. It was thought to be one of the last places that life could exist – then comes the phrase that science journalists love to use – “until now”! The researchers had discovered a gas called phosphene in the Venusian atmosphere, something that can be produced by living organisms on Earth.
What’s more, the gas was found to be short lived – so if there really are super-indestructible organisms in the planet’s clouds, the critters are still there pumping the stuff out.
The research team had double-checked their findings using another telescope. They had carried
out simulations and could not find an alternative explanation. Everyone’s sensible side assumes that there has to be another process going on to produce the phosphene, but why be sensible when there was scope in this discovery for a scientifically sanctioned flight of fancy, especially in the context of the miserable year we’ve had with the pandemic.
Senior editors in my department are lovely people, but they can sometimes be quite serious and austere when talking about the news. After I told them about the Venus story, many of them became quite giddy with excitement when discussing it in their editorial meetings.
The Sky At Night’s programme about the Venus discovery (‘Life Beyond Venus’, aired in November) got the science across beautifully. The natural enthusiasm of the scientist co-presenters Chris Lintott and Maggie Aderin-Pocock shone through, and the balance of the possibility of life and scepticism was just right. I loved the way Maggie’s short pieces punctuated Chris’s relaxed walkabout of discovery; between them they told a wonderful story – wonderfully!
For a moment they enabled us to put aside our worldly concerns and turn our thoughts to higher things, to make us feel the sense of awe and wonder that The Sky At Night so often brings.