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-------------------- Sun’s violent surface:
----------------- - 1732 - Listening to the Heartbeat of the Stars.
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- Most everyone is familiar with seismology. The science used to study the interior of the Earth down to its iron core. An earthquake on the surface sends a shockwave through the earth and seismologists detect the reflections and measure the speed of the wave traveling through the different layers of the interior. With this data science can determine the composition of the Earth’ interior down 4,000 miles to its center. Calculating the movement of layers also allows science to study Earth’s magnetic field.
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- Now, most everyone does not realize that astronomers do the same thing in studying the interior of the Sun. But, wait, shockwaves and sound waves do not travel thought space. Right, but light waves do. Astronomers can measure the tones and hum of the Sun monitoring the slight changes in the amplitude and frequencies of light waves. By monitoring a specific frequency and detecting the oscillations created by eruptions and disturbances on the surface science can learn the composition of the Sun’s interior.
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- The astronomer’s version of a seismograph is a photometer, an instrument that measures the slightest changes in brightness over time. By detecting brightness over a wide range of frequencies astronomers can trace out a “ power density spectrum.”
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- The Sun and all the stars have different and unique spectrums. A Cepheid Variable star may have one fundamental note. Other stars may ring at dozens of frequencies, with periods of hours. Our Sun vibrates in millions of frequencies with periods of minutes. Each fundamental note comes and goes in a few days. Our Sun is not just ringing it is humming.
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- Our Sun is close enough that these small changes in brightness due to the modulated waves on its surface can be accurately measured But, what about stars that are millions of lightyears away and just a pin point of light? To measure light changes here we must detect changes in a few parts per million, 1 / 1,000,000 To detect these small signals the telescopes must be away from the background noise of Earth. And, they must be outside our turbulent atmosphere.
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- Spacecraft have been put in orbit to do this 2003, MOST, 2006 CoRoT, 2009 Kepler. Kepler does not orbit the Earth. It orbits the Sun along with us, trailing a million miles behind us. Kepler has 42 CCD’s, 94,600,000 pixels on a 37 inch telescope.
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- Kepler was designed to detect planets orbiting 160,000 stars. Kepler stares at one spot in the sky, the Constellations Cygnus and Lyra. It can detect dips in the star’s brightness when a planet passes in front of the star. The planet must be monitored for at least 3 ½ years to get repeat dips, called transits, that define its orbit and diameter.
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- Astronomers soon realized that Kepler’s precision could not just find planets. It could be a seismograph for the parent star’s interior. It can measure the star’s diameter which is needed to calculate the diameters of the transiting planets. It can measure the mass and the age of the stars.
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- Measuring the light oscillations on a Red Giant star astronomers learned that the star’s core spins 10 times faster than its surface.
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- Stars ring because the gas near the surface gets pushed and pulled out of equilibrium. “Pulled” happens when a binary star’s gravity changes with eccentric orbits. The star’s tides can be measured. And the tides create repetitive ringing. Astronomers refer to these as “ heartbeat stars”.
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- It is new science: names like space photometry, asteroseismology, exoplanet science.
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- Analyzing the data can tell astronomers how much hydrogen is left in the star’s core. What is the age of the star and how much longer it will live. Our star is 4.6 billion years old and will live for another 5 billion years before collapsing into a White Dwarf star.
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- Stars are in a delicate balance between gravity collapsing them to the center and gas pressure expanding them into space. Upset this equilibrium slightly and you create oscillations around a stable, balanced, state. Ringing! Certain resonant frequencies travel at different speeds depending on temperature and composition of the medium. Astronomers are now studying stars the way seismologists have studied Earth. Stay tuned, there is more to learn.
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-Other reviews available:
- #33 Kepler’s 3 laws in astronomy, Isaac Newton’s biography.
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- #1070 Kepler gathering light from 1 trillion stars in 100 square degrees of sky.
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- #1251 Kepler estimates there are 1,280,000 Earth-like planets in the Milky Way.
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- #1717 Kepler has identified 1,000 exoplanets.
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----- 707-536-3272 ---------------------- Thursday, January 29, 2015 -----
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