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Twinkle twinkle little star how I wonder about your interior structure and dynamical properties
Believe it or not we can now map the interiors of stars by
Listening to their harmonies as they vibrate with seismic waves
Stars are among the best understood objects in astrophysics
This is impressive given the fact that they are impossibly distant opaque balls of fiery plasma
yet mathematical models emerged in the early
1900's that describe the balance between the gravitational crush and the outward flow of energy from the fusion reactions in the core
these equations of hydrostatic equilibrium
Allow us to calculate things like the density and temperature of the core the way energy flows to the surface
And even the lifespan of stars these models are largely built around what little we can learn from the light
We receive directly from the surface of stars
But how do we test these models if we can never see beneath those surfaces?
Well we may not see light from beneath the stellar surface
But another type of wave travels freely through stars. I'm talking about
seismic waves see stars have a
dynamical complexity far exceeding the simplest predictions of the equations describing stellar structure
they Resonate vast waves reflect around the stellar interior setting up global oscillations
natural resonant frequencies that carry information about stars impenetrable interiors
Stars ring like bells and although we can't hear this resonant vibration directly
We can see its effect in the changing brightness and the motion of the stellar surface
the fast-growing field of astro
Seismology uses these oscillations to probe the interiors of the distant stars
When we try to understand other stars we always start with our Sun, while the distance does are
infinitesimal points of light - even our best telescopes the surface of the Sun can be resolved in incredible detail
The effects of seismic activity can be mapped across its surface
understanding asteroseismology starts with understanding helioseismology
actually back up
understanding helioseismology starts with regular old seismology on earth
Geo seismology on earth seismic waves are generated by earthquakes and can travel around the planet as longitudinal
pressure or P waves
Transverse shear or S Waves and surface waves which are a mixture of P And S
Waves stars also support P waves and these are true sound waves that echo around their interiors
Because stars are fluid rather than solid they don't support shear waves however they do support two types of gravity waves
Now these are not gravitational waves
gravity waves result from the restoration of gravitational equilibrium
When some material is moved from its preferred depth buoyancy forces try to push it back into place
In stars these waves occur below the surface G Waves
and on the surface F waves the latter are
Closely analogous to ocean surface waves on the earth however
It's the pressure waves the P waves that really dominate in stars like the Sun
These acoustic waves are generated by turbulence just below the surface of a star
just as seismic waves on earth are created by earthquakes just below the surface in the lithosphere
They start as traveling waves that can move throughout the stars in a structure however
Just as a single tap con set an entire bell ringing, a single traveling wave feeds its energy into
Standing pressure waves that cause the entire star to vibrate
These P mode oscillations follow the rules of spherical harmonics
Taking the form of regular patterns of density oscillations
Throughout the star the distribution of the pan depends on the frequency or the mode much like on the skin of a drum
Many modes vibrate at once overlapping in a complex structure of resonance
the strongest oscillations in the Sun are in the 2 to 4 mhz range
These are the suns
5-minute oscillations, so
What does this look like?
well for the Sun we can map these oscillations in two ways
changes in brightness and changes in velocity
brightness of spectral lines in the sun's atmosphere
Can change by around 1 part per million over the course of an oscillation?
At the same time gas moves vertically in and out during the same oscillation reaching velocities of 0.1 m/s
this can be detected in the Doppler shift of spectral lines
and because many different modes overlap the complex overlapping effects of these oscillations are
separated using Fourier analysis
We've spoken in depth about Fourier analysis in our recent episode on understanding the uncertainty principle
So I won't go into too much detail here, in short the many overlapping modes form complex oscillations on the surface of the star
But these can be deconstructed into simple sinusoidal oscillations each of which corresponds to an individual mode
resonating throughout the star
Hélio and asteroseismology are all about determining and modeling the resonant modes within a star
See the nature of these modes depends on the internal structure specifically on how the speed of sound
Changes throughout the star, which in turn depends on the stratification of temperature density and Composition
The internal rotation of the star is also a key factor
Helioseismology has allowed us to verify and improve the models of the sun's internal structure
It's also revealed new things for example that the inner radiative zone rotates almost like a solid ball
while the outer convective zone rotates at different speeds depending on latitude
this differential rotation
Powers the sun's magnetic field and is also responsible for twisting that magnetic field to drive the sunspot cycle
Helioseismology has also allowed us to measure the composition of the core
Which tells us how much of the sun's hydrogen fuel source has already been burned into helium
In this way it's been revealed that our Sun is currently around halfway through, its 10 billion year life span
Which is consistent with h dating of the oldest meteorites?
observations of the Sun surface are relatively easy
seismological studies of distant stars
asteroseismology Is much more difficult, they're too far away to resolve their surfaces
So we only see the global effects of their oscillations.
The Doppler shifts due to local gas moving are completely washed out
however,
There is global flickering tiny changes in overall brightness that can allow us to figure out the strongest resonant modes
Those modes allow us to determine fundamental properties like radius mass density and surface gravity in red giant stars
Asteroseismology has been used to determine the fusion activity in their dying cause
Allowing us to learn just how close these stars are to their last flicker
But astro seismology really is hard to do to see that faint flickering we have to go to space
The Canadian MOST and the French COROT satellites, pioneered this work while KEPLER does asteroseismology?
mere as side gig to finding alien planets
future planet hunting satellites like TESS and PLATO will continue this work with higher precision and for many more stars
Most stellar seismology is focused on learning about the average global structure of stars
But at least for the Sun, It's possible to learn about current local events that are hidden from our view
For example we can map the currents of plasma and density fluctuations as
they shift beneath the solar surface
In helioseismic holography,
The visible wave field so the distribution of Doppler velocities across the visible surface of the Sun
is used to infer the current state of the standing waves throughout the Sun, that includes the far side of the Sun
In fact Helioseismic calligraphy is capable of detecting
sunspots long before the Sun rotates them into visibility and this can give us advanced warning of
potentially dangerous solar activity
stars seen
There harmony ISM a hidden in the flickering of their light and in the subtle in/out of their services
Woven into that music is knowledge of their mysterious depths and of their pasts and futures
We're only now learning to decipher the complex overlay of tones in stellar oscillations.
We're learning the lyrics
Twinkle twinkle little star and in doing so you give up your secrets because the science of asteroseismology
Can now translate the messages of stars twinkling at us from across space-time.
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Contribute and today especially to fires our sword for your exceptionally generous Big Bang level contribution
You are keeping the studio lights on and the camera running seriously
Thanks, because space time the podcast just wouldn't be as cool so before the break
We didn't episode on what would happen to the earth if we were hit by a gamma-ray burst beam
You guys had a lot to say about it
Demetria liran gave a shout-out to the excellent kurzgesagt
episode on the gamma ray burst apocalypse he points out that they say the Earth's surface would be incinerated by such an event
So the results actually depend very much on the distance to the bursts a DOB within a few light-years
Would be directly devastating to life
No need to wait for the follow-up ozone problems, but the chances of a GRB going off that close are exactly zero
Because there are no stars that would possibly explode that way for hundreds of light years
Now it may. Eventually happen as the Sun wanders the galaxy and encounters new neighbors
But it's still spectacularly unlikely. Dimitri Also asks, whether being in a different spot in our orbit can save us from a gamma-ray burst
Well the answer is no
Nowhere is safe
Geo beams from exploding stars are estimated to have jets with conical opening angles of between 2 and 20 degrees
But even if a super focused one degree gamma-ray burst hit us from a single light year away
That jet would have diverged to something like
10 times the size of our solar system by the time it reaches us it would hit the whole solar system equally
Luna asks whether building underground cities would help against a gamma-ray burst, well
I'm sure they'd protect us against the follow-up effects of increased UV
But so does hats and sunscreen however, none of these would protect the rest of the biosphere on which we're still rather dependent
Felix scneider asks how electromagnetic radiation can be focused by a magnetic field
Now this is an excellent question
As Felix realizes light is not electrically charged and so isn't affected by em fields
in fact the magnetic field of a gamma-ray burst
focuses charged particles electrons and the nuclei of the exploding star
those particles can then fire photons in our direction in a couple of different possible ways
One is synchrotron radiation
The charged particles spiral around the axial magnetic fields and emit photons as they do
The other is inverse Compton scattering
particles in the jet bumpy two existing photons perhaps synchrotron photons and scatter them to higher energies and
Preferentially in the direction of the flow, in both cases photons are emitted in different directions
but an effect called relativistic beaming massively amplifies our perceived brightness of the light emitted in the same direction as the
near light speed charged particles of the jet
Upcycled electronics suggest that I spoil the Ordovician Silurian extinction script from PBS eons
Ridiculous if I'd stolen it from eons it would have been really good check it out PBS eons
It's really good