The Week in Space and Physics: Gamma Ray Bursts
The Week in Space and Physics: Gamma Ray Bursts
On the most powerful explosion ever seen, Wolf-Rayet stars, the James Webb's calibration woes and a bull's-eye for DART
Few things in nature come close to the power of a gamma ray burst. They detonate with the force of ten billion Suns; an eruption of such magnitude that perhaps only the Big Bang has ever outshone them. They are, fortunately, extremely rare: lighting up, in every known case, galaxies billions of light years distant.
So when, on October 9, telescopes spotted a surge of gamma waves racing through the Solar System, astronomers were quick to pay attention. The surge soon turned into a tidal wave. For more than ten hours the energy kept on coming, hitting levels unprecedented in the history of astronomical observations.
This, we soon realised, was the strongest gamma ray burst ever seen and quite possibly the largest explosion ever detected by humanity. Remarkably, for something that originated two billion light years away, it even seems to have disrupted the upper layers of our atmosphere. Long wave radio systems recorded a sudden disturbance as the wave smashed into our ionosphere.
Fortunately it is also likely to be one of the most watched in history. An array of gamma ray and X-ray telescopes across the solar system turned to observe it, capturing details about the burst and its afterglow. That should reveal much about the origin of both this eruption and of gamma ray bursts in more general.
Since they are so energetic, bursts like this must come from truly catastrophic events. It seems likely, too, that their energy is concentrated in some way. Models suggest that beams of gamma rays shoot out from the eruption, rather than exploding out equally in every direction. That would mean we only detect a fraction of the true number – only catching those where the beam happens to pass over Earth.
Theories suggest they are intimately linked to black holes. Indeed, most, we think, come from dying stars collapsing into black holes: the burst of energy coming from the terrible force of this implosion. Some speculate that Wolf-Rayet stars – of which a few hundred exist in our galaxy – are responsible, creating the bursts in their final moments.
That raises a slightly worrying possibility. Should such a burst ever occur close to Earth – within a few thousand light years, say – the effects on our planet would be devastating. The intensity of the radiation would strip the ozone layer and fill the air with noxious smog, weakening the Sun’s energy for decades.
Some signs, indeed, suggest something like this may well have happened before. Four hundred and fifty million years ago half of all life on Earth suddenly perished. The cause of this catastrophe is hard to explain – but certain features of it look eerily like a powerful gamma ray burst striking the planet.
Rings and Dancing Stars
Wolf-Rayet stars are rare and fascinating things. Unlike most other stars they burn helium, not hydrogen, and as result glow at enormous temperatures. Indeed one of them – Wolf-Rayet 102 – is the hottest known star in the universe, glowing at over two hundred thousand degrees Kelvin. Even those that are slightly cooler are still intensely bright: outshining the Sun a thousand times over.
Perhaps natural, then, that a Wolf-Rayet star should capture the attention of the James Webb Space Telescope. Astronomers recently directed it to examine WR-140, one partner of a binary system lying some five thousand light years away.
The result is an unusual image: something that looks more like the artificial distortions of a camera lens than a real photograph of a star. In the centre of the image is the star itself, shining with an intense brightness. But extending far around it are rings; ripples of gas stretching far away into space.
These rings, the researchers found, are made of carbon and silicon dust. They likely come from the outer layers of the star: a kind of sooty residue left over as it burns helium and other heavier elements. At regular intervals – roughly once every eight years – something blows this dust away from the star: forming the sequence of rings seen by the Webb.
Quite why WR-140 throws out dust at such regular intervals is unknown. Not all Wolf-Rayet stars behave like this: though all emit a dusty residue as they burn, they more often emit it as a continuous stream. But WR-140 is somewhat of a special case: it has a partner star. As the two stars dance around each other they seem to be shaping the rings; leaving cosmic ripples in their wake.
The image also confirms the power of the James Webb telescope. Past observations of the star revealed just two rings around the pair of stars. Yet the new data shows at least seventeen rings, stretching billions of kilometres across space. More such rings are probably there – faint echoes of the long lasting dance between two stars.
James Webb Calibration Woes
As soon as the James Webb started full-time operations, scientists raced to analyse its data and publish papers. Dramatic claims were quickly made: of ancient stars, distant galaxies and the early cosmos. Some went so far as to cast doubt on the Big Bang Theory itself; finding hints that stars and galaxies formed far earlier than seems possible.
As this newsletter warned at the time, those claims now look in doubt. The telescope, like all new instruments, needs to be calibrated before its results can be fully trusted. In their haste to publish, many astronomers seem to have overlooked this detail – and, as it now emerges, their results need to be rethought in the light of better calibration data.
At the heart of matters is the Webb’s Near Infrared Camera, an instrument vital for exploring the earliest stars and galaxies. Telescope operators didn’t have time to fully calibrate this instrument before handing it over to scientists. They thus made a bit of a guess on some of the parameters involved.
After having more time to correct things, the telescope’s operators have released new calibration values. Yet, according to an article in Nature, these values are significantly different to the earlier guesses. That casts a lot of the recent papers and claims in doubt, and means astronomers must go over their numbers again.
It’s not clear yet whether this is enough to resolve questions about the Big Bang and the first galaxies. But it does highlight the danger of making big claims so soon after the launch of the telescope.
Bull’s-eye for DART
At the end of September DART, a NASA spacecraft, smashed into the asteroid Dimorphos. The resulting explosion was impressive; leaving a ten thousand kilometre long trail of debris in its wake. Now analysis by NASA confirms the impact also moved the asteroid: shifting its orbit around the nearby asteroid Didymos.
NASA had hoped the impact would slightly accelerate this orbit by pushing Dimorphos closer to Didymos. The change was expected to be small: calculations suggested Dimorphos would complete each orbit seventy seconds faster than it had before the impact. In the end, however, telescopes found DART was far more successful: shortening Dimorphos’ orbit by over thirty minutes.
All this demonstrates the potential of asteroid redirection. If – or when – we find one heading towards Earth, we now have the option of using something like DART to move it onto a safer path. That can only be good news.