We are all stardust – the result of elements created in old stars, dispersed, and re-accumulated
into planets as the solar system formed 4 and a half billion years ago.
Tantalisingly some of those original dust grains survived the birth of the solar system,
and can be found intact, inside meteorites. And this week, a hint of the deep pre-history
of those grains was revealed thanks to some extraordinary chemical detective work by Philipp
Heck of the Field Museum in Chicago. These are real stardust grains, so the carbon
in our bodies has the same origin and that's why we are so interested in those stardust
grains, because they are wonderful tracers for the carbon.
They are tough crystals, that survived processing in the interstellar medium, in the early solar
system. That was a turbulent time, 4.6 billion years
ago, then all the way through the present day, 50 years ago, when the meteorite Murchison
fell in Australia, and until the moment we actually zapped them with a laser in the lab,
to extract the neon to date them, that's when they ended their life essentially.
So the sun was born and the planets were born, is it about 4.54 thousand million years ago? But you're
saying you can actually date how long these grains had existed before then? Yes exactly.
So this is a relative age, an age before the start of the solar system and we come up with
this age range of 4.6 to 7 billion years ago. So some of those have been floating around
for an awful long time waiting for something to happen.
I guess, you know, if we are stardust then one of the questions is can you tell anything
from these grains apart from their age, what kind of star they came from?
Yeah, so that's, one of the first things we did, we determined the varieties of carbon
and silicon in these grains, the isotopes, and matched them to the compositions of stars.
and interestingly that was already known before - that these grains are perfect match to carbon
stars. These are evolved types of stars that are about the mass of the sun, a little bit
more massive, and have the same carbon isotopic composition. Many of the same types of carbon
atoms are produced in these stars than the ones that we find in these dust grains.
So we can already match them with star types. Obviously these stars don't exist any more,
because the end of their life, they produce that dust and they basically became dormant.
But they are essentially, not only the parent stars of the grains that wes studied, they
are parent stars of the sun - they're our parent stars. They made the carbon we are
made out of. So that's why we think it's so interesting. The tracer of the building blocks
of life actually. It's funny that the headline is how old - this
is the oldest, pristine material that's been found on the earth. But weirdly, for me actually,
the funny ones are the ones formed just before the solar system formed, because presumably,
those ones only just made it in time. They were really last minute Charlies.
Exactly, they just made it in time. Many others that we don't have now in our collections,
they didn't make it in time. They're maybe floating around or have been destroyed later.
Those young grains, just were produced before the start of the solar system. They tell us
an interesting story about our galaxy. We find many more young grains than we would
expect if stars would form at a constant rate in our galaxy. We find so many grains that
we basically have to explain it by an episode of enhanced star formation. We did a deep
dive into the astronomical literature and found that astronomers previously found independently
evidence for such a episode of a star baby boom, seven billion years ago, and that perfectly
matches with our ages. The seven billion year star baby boom essentially made 50% more stars
than normal. And at the end of their lives, about 2 to 2 and a half billion years later,
that's just before the solar system started, they produced the dust that we now see.
So we can learn something about the star formation history of our galaxy and actually have evidence
now from these samples that we can touch, that the star formation wasn't constant over
time and was pretty bumpy, bursting, you can call it. And there were more bursts in the
past, that we currently cannot access, but hope to do so with more ages. We already started
separating more pre-solid grains from other meteorites, to get many more ages.
Because we have only a few grains which are really old that would basically allow us to
look deeper into time, into the galactic history. Philipp Heck, who's study was published by
the National Academy of Sciences this week (January 2020).
Now the carbon and silicon in Philipp’s grains are relatively light elements that
make up much of the Earth, and which came as he said from lighter Carbon stars. But
we also have much heavier elements, many of which we use in our technology. Iron for example.
Or zinc. And these are sprayed out at light speed from supernova explosions like the one
we are anticipating from the supergiant star Betelgeuse. Some of this atomic spray flies
through our atmosphere – and the place to intercept them is high over the South Pole.
That’s where Brian Rauch is right now, flying his complex detector on a drifting balloon.
Nerve-wracking work. You know, you never know what you're going
to get when you watch a balloon. The success record is pretty good with it, but it doesn't
always stay up for you. And that's what happened to us last season. It lasted six hours before
it came down. This season we've had a wonderful balloon, that's been up for over 30 days.
It's a crab chute, you don't know how the winds in the stratosphere are going to behave.
And it's just tracking round and round the continent in the winds at a several kilometre
height? It mostly just follows a more or less circular
pattern around the pole. That can get offset or do strange things. We've had a payload
wander all the way down to the South Pole before.
So that's the practicalities of launching your device. To put it crudely, you are just
prospecting for heavy elements up there, is that right? Yeah, that's very good. Yeah we're
looking for elements that are energetic particles, going almost the speed of light, from outside
our solar system. We're looking at those heavier than iron.
These are things that are not made in fusion during a star's normal life, but that are
made in neutron capture processes that... Well they do happen slowly during a massive
star's life, but they also happen rapidly when it dies.
So what are you hoping to learn with SuperTiger about these particles? Presumably it's
partly to do with their energy, but their mass, I guess, which ones are arriving here,
but not their direction. People can't wait to figure out where these
rays are coming from, and you can get clues on that from their composition.
We are looking to, one, confirm our model for the source of the rays, but also
to deduce which elements are made in massive star processes, and to figure out which ones
might be coming from binary neutron star masses, which have in the last two years, been touted
as a very likely source of the very heaviest elements. So there's probably a transition
between the mass of stars and the binary neutron stars. And we're trying to probe where that
happens. You have to be pretty patient, it sounds like,
to sit there and wait for this stuff to come in. It must be worth it?
Well, we like to think so, I mean others might dispute that of course.
I won't! Thank you, you're very generous.
But it does seem to surprise people that what we're doing is more just a patient collection
of a data set and then finally, after we have it, then we have to go through a lot of analysis
to pick out the pattern that tells us the story. It's hard to explain, you know, it's
so much easier with astronomy where you can show people pictures of what you're observing,
and explain it. Whereas with us, we have to collect the data and make graphs and little
kinds of analysis. And then we come up, finally, with numbers that are mostly ratios. But it
does give us a story about where the matter that makes all of us, comes from.
And I do find that very compelling. Brian Rauch of Washington University, St Louis,
on a line from Antarctica, where he’s looking for today’s star dust.