Date: November 21st, 2012
Title: Astronomy Word of the Week : Helioseismology
Podcaster: Dr. Christopher Crockett
Organization: United States Naval Observatory
Description: Like a ringing bell, our Sun pulses and vibrates, driven by the nuclear engines far below. Studying the pattern of these oscillations reveals much about the interior of the Sun and is a crucial test of theories of stellar evolution. The astronomy word of the week is “helioseismology”.
Bio: Dr. Christopher Crockett is an astronomer at the United States Naval Observatory in Flagstaff, Arizona. His research involves searching for planets around very young stars (“only” a few million years old). It is hoped that the results from this research will help constrain models of planet formation and lead to a better understanding of where, when, and how often planets form. Chris is also passionate about astronomy outreach and education and will talk for hours about the Universe if you let him.
Today’s Sponsor: This episode of 365 days of Astronomy is sponsored by iTelescope.net – Expanding your horizons in astronomy today. The premier on-demand telescope network, at dark sky sites in Spain, New Mexico and Siding Spring, Australia.
Additional sponsorship for this episode of 365 days of astronomy was provided by Clear Skies Observing Guides, a Modern Day Celestial Handbook. www.clearskies.eu..Clear skies observing guides, or CSOG, is a new concept in visual amateur astronomy. The observing guides contain thousands of objects to observe through amateur telescopes, with matching tours for GOTO telescopes and matching AstroPlanner plan-files. CSOG allows you to target deep-sky objects and carbon stars you never observed before, night after night. Wishing astronomers around the world: Clear skies..! ”
Listen to this.
And to this.
And now to this.
Can you tell the difference? Can you identify what made these noises? Not surprising if you can. Like nearly everyone, you’re using sound to describe what an object is like.
You don’t need special training or mathematical tools. You’re simply drawing on experience. You know the difference between a trumpet, a violin, and a bell.
All instruments derive their unique sounds from their structure. Their shape and composition produces a distinctive pattern of vibrations across their surfaces. Those vibrations in turn disturb the air around them which we eventually hear as music.
But what if you could reverse the process? What if, given a vibration pattern, you were able to deduce the makeup of the source? This is the essence of helioseismology—using ripples on the sun’s surface to understand the interior of our star.
Astronomers suffer from a major limitation: everything we learn about stars has to come from the light emitted at their surfaces. We can’t dissect a star in a laboratory or grow a new one under controlled conditions. This might seem to severely restrict what we can know. But the pulsations dancing across the face of the Sun betray many of the secrets held within!
Solar astronomers turn to a trick used by geologists for mapping the interior of the Earth. Whenever there is an earthquake, waves scatter throughout the interior of our planet. The different wave motions and frequencies bend as they pass through layers of varying densities. As the waves refract through our planet, the shocks register at seismic stations around the globe. By bringing together the amplitude and timing information from all these monitoring devices, geologists are able to piece together a model of the Earth’s interior. Earthquakes are nature’s way of taking a three-dimensional snapshot of the inside of our planet.
While the Sun doesn’t experience anything like seismic activity, deep convective currents—driven by enormous energy gradients emanating from the solar core—perturb the outer layers of plasma. When these waves hit the Sun’s surface, they bounce back inwards. As the reflections pass through regions of varying temperature and composition, they bend. Waves of different frequencies bend by different amounts. Eventually, these reflections resurface elsewhere before diving back down once again. The end result is a complex interaction of reflecting and refracting waves driving intricate motions on the surface of the Sun.
In essence, the nuclear engine at the Sun’s core makes our star ring like a bell. We can’t hear these sounds—at least, not directly. Sound can’t travel through space and, even if it could, the solar frequencies are much too low for human ears. However, if we multiply the frequencies up to the range of human hearing, then the Sun might sound something like this:
Rather than listen to the Sun, solar astronomers use Doppler shifts from the moving gas to make detailed velocity maps of the Sun’s surface. Through complex mathematical techniques that can isolate the individual wave frequencies, researchers can use computer models to reconstruct the internal makeup of the Sun. Temperatures, pressures, gas currents, even chemical composition reveal themselves through subtle solar oscillations.
Helioseismology has become an indispensable tool for checking our understanding of how the Sun works. In this way, astronomers can combine the results of helioseismology with measurements of the Sun’s temperature, spectrum, neutrino emissions, mass, rotation, and many other variables to produce a consistent, detailed picture of the inner workings of our star.
One way that this has helped is to confirm the age of the Sun. The 4.6 billion year lifetime of our Sun comes predominately from radiometric dating of meteorites and rocks on the Earth. However, a star burning for 4.6 billion years should have built up a certain amount of helium in its interior as a byproduct of hydrogen fusion. Seismic studies of the Sun’s surface confirms the internal abundance of helium and provides another column of support for the accepted age of our solar system.
The lessons learned from helioseismology allow us to apply these techniques to far more distant suns as well. The more general term “astroseismology” refers to measuring the ripples on stars hundreds of light years away! While we can’t get nearly the same amount of detail as we can from our Sun, this has become a fundamental tool for understanding how star’s differ from one another. Ages, mechanical structure, rotation speeds—all reveal themselves in the ringing of stars across the cosmos.
A trumpet, a violin, a bell, and the Sun. Their unique sounds tell us something about how each is made. Ripples echoing across our Sun help flesh out the long history of our star and, in turn, our solar system. And now we’re reaching across the Galaxy to hear the songs from more distant systems. The stars are singing. And we are listening!
End of podcast:
365 Days of Astronomy
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