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.

[MUSIC PLAYING] 11 00:00:28,340 --> 00:00:30,230 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 1900s that described 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 life span 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.

You 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 asteroseismology 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 stars are infinitesimal points of light to 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-- geoseismology.

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 can 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 pattern 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 two to four millihertz range.

These are the sun's 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 one 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 meters per second.

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 a recent episode on understanding the uncertainty principle, so I won't go to 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.

Helio- and asteroseismology are all about determining and modeling the resident 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.

Its 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 lifespan, which is consistent with age dating of the oldest meteorites.

Observations of the sun's surface are relatively easy.

Seismological studies of distance 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 cores, allowing us to learn just how close these stars are to their last flicker.

But asteroseismology 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 as a 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 holography is capable of detecting sunspots long before the sun rotates them into visibility.

And this can give us advance warning of potentially dangerous solar activity.

Stars sing.

Their harmonies are hidden in the flickering of their light and in the subtle in and out breathing of their services.

Woven into that music is knowledge of their mysterious depths and of the past and futures.

We're only now learning to decipher the complex overlay of tones in stellar oscillations.

We are 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.