Asteroseismology: using the natural music of the stars


Interviewer: Andy Tootell (Ideas Lab)
Guest:  Professor Bill Chaplin
Recorded: 20/03/2011
Broadcast: 02/04/2011

Intro VO: Welcome to the Ideas Lab Predictor Podcast from the University of Birmingham. In each edition we hear from an expert in a different field, who gives us insider information on key trends, upcoming events, and what they think the near future holds.

Andy: Hello. Yes, we are recording this over a lunchtime but no, that wasn’t the sound of my stomach growling for a sandwich. It was in fact the sound of a star, which is the science of Asteroseismology. Joining me today is Professor Bill Chaplin, Professor of Astrophysics at the University of Birmingham’s School of Physics and Astronomy, who will hopefully tell me a little bit more. Hello, Bill. 

Bill: Hello.

Andy: So, asteroseismology - the sounds of stars. How does it work?

Bill:  Asteroseismology is a tool that we can use to study stars. What we use is the natural music of the stars in essence. Stars are able to make sound naturally in their interiors and that sound gets trapped, just like the sound in a musical instrument. So if you think about if you play an oboe for example or another wind instrument, with an oboe what you do is you blow over the reed at the end of the instrument, that makes the reed vibrate, it creates changes in pressure in the air and that’s just sound waves and the sound is trapped in the musical instrument, in the body of the instrument. It makes the instrument resonate and so we hear the nice crisp tones of a musical instrument with someone playing it.  In the case of a star the sound is made naturally in the very outermost layers of stars like the sun by turbulence and that sound is trapped and it makes the star resonate. Now we can’t of course hear the stars resonating. What we do instead of actually listening to the stars, we do that indirectly. As the star resonates, because of the sound trapped inside it, it gently breathes in and out and as it breathes in the gas that makes up the star is compressed and it gets hotter and the star gets brighter and then as the star relaxes and breathes out it gets cooler and a little bit dimmer.  So what we’re doing is we’re coding the information on the sound inside the star into changes in the star’s brightness and the sound that we heard earlier, what we’d done there was to take, if you like, a trace of the brightness of the star varying, to convert that to sound and that’s what we heard. Stars resonate naturally at frequencies that are much much too low for the human ear to hear so we have to speed it up, but when we do that we then have a tool and a way to actually figure out what’s happening inside a star. 

Andy: Now you’re using data from the NASA Kepler Mission which is a telescope that was launched in 2009 with the primary objective of searching for exoplanets which are planets outside of our own solar system.  How is your research helping with the Kepler Mission?

Bill: As you said, Kepler’s main goal is to detect actually planets in the habitable zones of stars like our own sun. 

Andy: Known as the ‘Goldilocks Zone’ I was reading earlier before this. 

Bill: Yeah, that’s right. So the Goldilocks Zone, because it’s those regions are not too hot, not too cold, but just right for the planets to potentially have liquid water on their surfaces.  Not only is Kepler a great mission, you know, obviously because that was its primary goal in terms of finding exoplanets but it’s also a great mission for studying stars and it really is revolutionising our ability to do what we call stellar astrophysics to understand stars and to help place our own star, the sun, in a clearer context.  And the reason why asteroseismology is so exciting in this context is that it enables us to measure the basic properties of the stars, so the size, mass, age, to levels of precision and accuracy that are extremely hard to achieve by other means, and also as well that information becomes particularly important when we find stars that have planets going around them.  The method that Kepler is using to detect exoplanets is called the 'Transit Method' and what we detect is as a planet passes in front of the star it blocks some of the light from the star and so we are looking for, or seeking to detect, a miniscule dimming, really really tiny, something like 80 parts per million, so really really tiny, as a planet like the earth passes in front of a star like the sun.  Now when we measure the size of the dip, the fractional amount of light that’s been blocked, that gives us the size of the planet relative to the size of the star, but to know in absolute terms how big the planet is, for example is it the same size as the Earth, we need to know precisely and accurately how big the star is.  And that we can do from asteroseismology much much better than is usually doable with, if you like, traditional techniques of astronomy.  So let’s say we’ve done that and we’ve found an Earth-sized planet, the next thing we want to know is, is the planet in the habital zone, possibly of the star, and that depends upon the intrinsic brightness of the star which in turn depends on the size, the radius.  So again, we need that very precise asteroseismic radius. So let’s say we’ve ticked both those boxes, we’ve found an Earth-sized planet in the habital zone, we might then want to know how old is the planet because we might then start to speculate on perhaps maybe life has had a chance to evolve. So there what we do is we use the idea that the seismology measuring the sound waves is giving us information on the core, the beating heartbeat if you like of the star and from that we can then say how old the star is, which sets an upper limit then on the age of the planet. So we can then say OK, we have a planet like the Earth in the habital zone of a star and maybe the star is of a similar age to the Earth so one can then say hmm, OK, maybe in the future – this is something we’ll come to – maybe in the future we might be able to detect biosignatures from the atmosphere of the planet. So that’s why asteroseismology is so very important and why I’m working along with other colleagues around the world, working with the members of the Kepler team who are working on the exoplanets. We’re now working together, there are very strong synergies between the two areas of research.

Andy: NASA’s Kepler funding is due to be slashed next year in 2013 from $19.6 million to $13.6 million and then potentially cut entirely from 2014.   What does the future hold for your research and for this project?

Bill: Kepler at the moment is being reviewed by NASA. I think it’s very unlikely that the mission will be stopped. It’s been a tremendous success, so fingers crossed the mission will be extended from its baseline of three and a half years to seven years.  So Kepler is going to hopefully give us then seven years of data which is a huge treasure trove for the astrophysics community but also as well there are other missions and other ground-based projects, so there’s the BiSON Project which we use to look at the sun; for the stars there’s a planned network of ground-based observatories called the Stellar Observations Network Group (SONG) which will be run by our colleagues in Aarhus, in Denmark, and that’s an attempt to have a ground-based network of observatories which will look at the very brightest stars in the sky. And then also there are future planned missions as well. In the US there is a mission on the books called TESS and it will be doing a sort of similar job to Kepler but on brighter stars, and also as well in Europe there are prospective missions that may hopefully get selected – one called PLATO and also as well another mission called ECHO which is looking to actually detect signatures from the atmospheres of the exoplanets and that’s sort of the first step if you like towards ultimately being able to maybe detect biomarkers in the atmosphere.  So for example maybe detecting signatures of ozone or other signatures in the atmospheres of these planets.  Those are observations that are very very challenging so that is not only driving, you know, originality in terms of the theory but also data analysis, but also as well technology. 

Andy: That’s almost mind-boggling in its scope.  It’s absolutely fascinating and long may your research continue and thank you very much for joining me today. 

Bill: Thank you. 

Outro VO: This podcast and others in the series are available on the Ideas Lab website: On the website, you can find out how to e-mail us with comments, questions or suggestions for future topics for the podcast. There's also information on the free support Ideas Lab has to offer to TV and radio producers, new media producers and journalists. The interviewer and producer for the Ideas Lab Predictor Podcast was Andy Tootell.