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David Simmons
2001-Nov-07, 05:50 PM
The link describes the 3-D anatomy of a sunspot. Sunspot (http://news.bbc.co.uk/hi/english/sci/tech/newsid_1641000/1641599.stm)

And this one uses some esoteric chemistry to analyze the age of a comet. Comet (http://news.bbc.co.uk/hi/english/sci/tech/newsid_1639000/1639356.stm)

MongotheGreat
2001-Nov-08, 06:49 AM
I'm curious as to exactly how they can measure the speed of a sound wave through the sun. Some kind of space seismograph? But then it would have to be attached to the sun somwehow. So what kind of changes in the light coming from the surface of the sun allows them to see sound waves?

David Simmons
2001-Nov-08, 01:13 PM
On 2001-11-08 01:49, MongotheGreat wrote:
I'm curious as to exactly how they can measure the speed of a sound wave through the sun.



Me too. I guess you would have to get a description of this Michelson Doppler Imager seismograph thing to find out.

Any real scientist-astronomers out there know how this thing works? /phpBB/images/smiles/icon_confused.gif

<font size=-1>[ This Message was edited by: David Simmons on 2001-11-08 08:14 ]</font>

ToSeek
2001-Nov-08, 02:23 PM
On 2001-11-08 01:49, MongotheGreat wrote:
I'm curious as to exactly how they can measure the speed of a sound wave through the sun. Some kind of space seismograph? But then it would have to be attached to the sun somwehow. So what kind of changes in the light coming from the surface of the sun allows them to see sound waves?



Here is a (somewhat technical) description of the instrument:
http://soi.stanford.edu/science/obs_prog.html

My impression is that the instrument can detect slight shifts in frequency from points on the sun moving up and down and therefore can detect sound waves moving through those points. It's not really spelled out, though, so I could be wrong.

ToSeek
2001-Nov-08, 02:29 PM
Okay, an explanation I can understand:

Helioseismology (http://soi.stanford.edu/results/heliowhat.html)

An excerpt:


On the Sun's surface, the waves appear as up and down oscillations of the gases, observed as Doppler shifts of spectrum lines. If one assumes that a typical visible solar spectrum line has a wavelength of about 600 nanometers and a width of about 10 picometers, then a velocity of 1 meter per second shifts the line about 0.002 picometers [Harvey, 1995, pp. 34]. In helioseismology, individual oscillation modes have amplitudes of no more than about 0.1 meters per second. Therefore the observational goal is to measure shifts of a spectrum line to an accuracy of parts per million of its width.

David Simmons
2001-Nov-08, 03:43 PM
On 2001-11-08 09:29, ToSeek wrote:
Okay, an explanation I can understand:

Helioseismology (http://soi.stanford.edu/results/heliowhat.html)

An excerpt:


On the Sun's surface, the waves appear as up and down oscillations of the gases, observed as Doppler shifts of spectrum lines. If one assumes that a typical visible solar spectrum line has a wavelength of about 600 nanometers and a width of about 10 picometers, then a velocity of 1 meter per second shifts the line about 0.002 picometers [Harvey, 1995, pp. 34]. In helioseismology, individual oscillation modes have amplitudes of no more than about 0.1 meters per second. Therefore the observational goal is to measure shifts of a spectrum line to an accuracy of parts per million of its width.


Yeah, I finally did the right thing and typed "Michelson Doppler Imager" in the Yahoo! search box and came up with the same web site you reference.

Unfortunately, a lot of the info assumes quite an extensive scientific background in this particular area.

But if you pore over it long enough it sort of clears up. That's "sort of clears up."

frenchy
2001-Nov-08, 07:42 PM
There are several ways to detect oscillations or pulsations in a star. There are three instruments on the SOHO spacecraft which use different techniques (the MDI/SOI mentionned above which gets a Doppler image of the Sun, the GOLF experiment sampling the line shape of a specific sodium line, and VIRGO which follows variations of the solar irradiance (luminosity, brilliance)).

Once you identify the modes of ocscillation,
their frequency, amplitudes, separations in
frequency, temporal variations and other features, you can reconstruct the structure of the body through which the sound waves travel, i.e. you get the sound speed at different points in the star. The math gets a bit hairy at that stage.

Basically the sound wave for each mode travels in a different path in the Sun. Comparing them to each other in various combinations allows you to get the sound speed in a specific region.

The more distinct oscillation modes the better but not too much that they start piling up on top of each other.

Hope that helps a bit.

Kaptain K
2001-Nov-08, 09:59 PM
The math gets a bit hairy at that stage.
You think you could understate that a little more? /phpBB/images/smiles/icon_confused.gif