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In a sense, stars are the fundamental  building blocks of the universe. I mean, galaxies are made of stars,  planetary systems form around stars, and virtually every atom that isn’t  hydrogen or helium was born inside a star.

Given all that, it’s not  surprising that astronomers want to know as much as  possible about these objects. But that is easier said than done. There’s only one star scientists  can study in detail: our Sun.

And, even then, we can only  look at it from the outside. Fortunately, astronomers have devised a clever way to peek inside stars without actually going there. It’s called asteroseismology, and it takes advantage of the last  sense you might expect: sound.

Now we often think of sound  as something that is heard. But audible sound is just a special case  of a more general phenomenon: vibration. So, a birdsong isn’t that different  from something like an earthquake.

One makes vibrations in the air, the  other in the earth under our feet. But, in both cases, we can get  information about something we cannot see. Like, we might know a singing bird is outside  the window, even if the curtains are closed.

And, while earthquakes happen deep underground,  we can feel those effects on the surface. Scientists have taken that one step  further with the field of seismology. By measuring how earthquake vibrations  bounce around inside the planet, they’ve been able to figure out  the Earth’s internal structure   without ever actually seeing it.

Which sounds exactly like what astronomers  are looking for in studying stars! There’s only one catch, uh, in  space, no one can hear you scream, or, you know, hear anything at all. Because space is a vacuum, and  there’s no medium for sound waves or any other vibrational waves to travel through.

Fortunately, stars make it possible  to almost “see” the sound inside them, because movement on a star’s surface  reflects what’s going on inside it. A key process here is convection, where pockets of material inside a star  heat up, become less dense, and rise. When they reach the surface, they lose  their heat to space in the form of light.

Then, as they cool and get  denser, they sink down again, making way for the next pocket of  rising gas in an endless cycle. This circular motion creates vibrations  that ripple across the surface of the star, or, as we might call it, sound. And these sound waves cause tiny  changes in how the surface emits light, creating little flickers  that astronomers can observe.

One way researchers have put asteroseismology  to work is figuring out what a star is burning. For most of their life, stars fuse  hydrogen into helium in their core. Then, after that hydrogen is exhausted,  they puff up to become red giants.

During the red giant phase,  they create energy in two ways. First, they burn hydrogen  in a shell outside the core. Then, they burn the helium  that’s built up inside the core.

Distinguishing the hydrogen-burning  and helium-burning phases from the outside has generally  been really difficult, because if you just look at the stars,  there’s not much of an obvious difference. But here’s the thing: The helium core is  much denser than the shell of hydrogen   around it. And density has  a huge effect on vibrations.

So, if the red giant hasn’t started  burning its helium core yet, we’ll see different vibrations  on the star’s surface. In a 2011 paper, researchers were able  to use NASA’s Kepler Space Telescope to identify those different seismic  waves in around 400 nearby stars, and classify them as either  helium-burning or hydrogen-burning. And so now, being able to separate  these phases of a star’s life will help astronomers understand what other  properties are different in the two groups, and what else changes in a red  giant over the course of its life.

Beyond that, asteroseismology is also  helping pin down the age of stars in general. Knowing a random, individual star’s  age can answer all kinds of questions, like knowing whether it’s been around  long enough to have habitable planets. But traditionally, it’s been  almost impossible to do.

You can usually date a cluster of stars by  plotting out their masses and temperatures, and comparing those to other  star clusters and models. But that doesn’t work for a single, lone star. There’s just not enough data, and  although there are some trends, a star looks about the same  for virtually its entire life.

Except in 2008, a pair of astronomers proposed a way scientists might one  day be able to overcome that. See, as they age, stars undergo changes in  size, density, temperature, and composition, depending on what elements they’re burning,  fusing together, and also some other stuff. Those changes can affect a  star’s internal structure, which, in turn, affects the way  vibrations travel through the star.

So, if astronomers could study the seismology  of a group of stars whose ages they do know, such as those in a star cluster,   they could calibrate a model  linking seismic activity and age. Then, they could use that model  to study some random star, and calculate its age to within 10-20%; a huge improvement over the  35-40% margin we have now. That would open the door to all kinds of  studies, including how star systems have evolved, how stars change as they age, and of course, whether they could have any  exoplanets that could support life.

Asteroseismology illustrates how  clever astronomers have to get when they can’t reach out and  touch what they want to study. It’s a good reminder that, in science, even seemingly-random details like  the bubbling of a star’s surface can be the gateway to a deeper  understanding of what’s really going on. Thanks for watching this episode of SciShow Space!

If you want to learn more about stars,  we recommend watching this episode about how the first stars changed the universe,  and not just because they were bright. [♪ OUTRO].