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Earthquakes are a reminder that there’s way more
going on under our feet than you might think.
But it’s hard to know exactly what it’s like inside our planet
since we can’t just, like, dig a big hole and look.
On the bright side, earthquakes themselves give us a way to sort of see inside the Earth,
because the properties of seismic waves say a lot about the materials they pass through.
And that’s not just the case for Earth!
We’ve detected seismic activity all around the solar system,
moonquakes, marsquakes, venusquakes,
and we can use those tremors to learn about the interiors of those places, too.
But the most dramatic quakes we know of actually happen on stars.
Certain types of stars can have quakes a lot like planets,
and they can give us rare clues about what it’s like inside those stars.
On top of that, starquakes may be behind at least 3 unsolved mysteries of the universe.
The fact that starquakes happen at all might sound weird,
since stars are just balls of gas and plasma.
They don’t exactly have tectonic plates or a solid crust to rattle around.
Except, waves of energy can roll through some stars, a lot like seismic waves.
And certain stars actually do form a sort of crust.
Neutron stars are basically a giant ball of neutrons,
and their gravity is so intense that it forces the surface material into a solid, rigid crust.
Beneath that crust there’s a wobbly layer of what’s called nuclear pasta,
which may be the most elastic and robust material in the universe;
and that’s the real name for it.
So these features of neutron stars are weirdly analogous to the Earth’s crust and mantle:
There is a slightly squishier, more mobile layer underneath a relatively brittle one.
Which means that the basic principles that drive earthquakes
should also work on a neutron star!
And that just might help explain something astronomers so far have no explanation for.
Fast radio bursts, or FRBs, are short pulses of radio waves
that seem to be tied to neutron stars.
Sometimes, we randomly see these pulses in one location
and they never come back, but on rare occasions,
we’ve also detected repeaters: FRBs that come from the same source more than once.
And that’s awesome: More bursts means more data.
One repeating FRB in particular is giving us some especially intriguing data.
We’ve detected around 300 bursts from this one source over eight years,
and when you put together a plot with all of the different energies from those bursts,
it looks a lot like the distribution of energies associated with earthquakes in one area.
Even the timing looks familiar;
the timing between bursts looks like the timing you’d see
between an earthquake and its aftershocks.
So, this thing is looking suspiciously earthquake-y.
At this point, we don’t have enough data to say for sure that this FRB is caused
by a neutron star with starquakes, let alone that all FRBs are produced this way.
But it gives astronomers a promising direction to explore.
And that is not the only mystery they’re investigating in neutron stars.
Certain spinning neutron stars, called pulsars, have occasional, unexplained glitches.
Pulsars get their name because they appear to pulse:
As they spin around, electromagnetic jets shooting out from their poles
come in and out of our line of sight.
Since their rotation rate is essentially constant,
those bursts of radiation are spaced super evenly.
If we were to see any change in their timing, we’d expect a gradual slowing down,
because those jets carry energy out of the system,
so there’s less energy remaining to keep up that rotation speed.
So imagine astronomers’ surprise when they saw pulsars that briefly started flashing faster.
That’s a pulsar glitch.
A glitch because it’s not supposed to happen, unless there is more to this story.
And that’s how astronomers began to wonder if starquakes were involved.
See, as quakes rattle through a star, its insides get all disheveled,
and its mass slightly redistributes itself.
When that happens, the center of mass moves.
And if the center of mass of a rotating object moves closer to the axis of rotation,
it’s gonna spin faster just like an ice skater.
Try it for yourself in your swivelly desk chair!
Start spinning around, and then pull your arms and legs in, and get real dizzy.
So that’s exactly what some astronomers think is happening in pulsars.
They think quakes are moving the center of mass and making these stars spin faster.
Like with FRBs, this is not confirmed, so scientists will keep looking into it.
In the meantime, asteroseismologists have their hands full with another mystery:
something called the Blazhko effect.
Got lots of good names in today’s episode.
The Blazhko effect happens in a type of variable star called RR Lyraes,
whose brightness goes up and down on a regular cycle.
And the star that gave the group its name, RR Lyrae,
is one of the best known examples of the effect.
RR Lyrae’s brightness goes up and down, with regular peaks and troughs,
about once every 13 and a half hours.
That happens thanks to waves of hot, charged gas moving through it.
But if you look at those brightness oscillations over a very long period of time,
you will notice that the peaks themselves rise and fall.
That is the Blazhko effect. And we don’t really know how it happens,
but something seismic might be at play here.
RR Lyrae stars don’t have a crust like neutron stars do,
so they don’t have the same kind of starquakes,
but seismic waves could still be shaking things up.
For instance, if the waves of heated gas
move through different layers of the star at different rates,
they could periodically line up in a way that might amplify peaks.
And that could potentially account for that second cycle of peaks and troughs
that no one knows how to explain.
And even though none of these mysteries are solved,
they all show us that shaking up a star is a good way to learn about it,
and that starquakes hold a surprising number of clues
to the internal workings of celestial objects.
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