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ProfileNathan
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Message 954 - Posted: 7 Dec 2007, 18:05:06 UTC
Last modified: 7 Dec 2007, 18:09:34 UTC

So in the very near future (as soon as Travis/Joe get the binaries updated again) there's going to be a change in the WUs. Namely they are going to be longer and I wanted to give you guys an idea as to why.

First of all, it's imperative to understand that it's immensely difficult to determine the distance to a star. This is because we can only measure how bright something is: this is know as the apparent magnitude. This wouldn't be a problem but all stars are not the same and so a really bright star that's really far away may look the same as a really dim star that's really close. So we developed a scale of the intrinsic brightness of stars. So we say how bright would a star be if it was a fixed distance away: we call this the star's absolute magnitude. Only if we have both the apparent and absolute magnitude can we then determine the distance to the object, and even then there is a large amount of error.



This figure is an HR diagram. They come in many differing forms but always show the temperature/color (x-axis) versus brightness/absolute magnitude (y-axis). So if you make a plot of all stars within a galaxy, etc., you will get something that looks like this. All stars traverse this plot throughout their life-cycles and so if you look at a population of stars you will see stars at all stages and thereby fill the diagram. If you follow the above link you can see more about what types of stars fall where and gain a little bit better understand of what you're looking at. Now if you divide up the the color/temperature correctly you are able to determine what type of star is within that range. This becomes very useful.

Right now with our algorithm we are looking at what's known as F-turnoff stars. These are the F stars that fall right at the point that the stars "turn off" the main sequence and start to move up the giant branch. And I've approximated this on the figure above with the green lines, so we are only looking at the stars between those lines. You can see that all stars we are looking at are not at the same y-value (absolute magnitude) but actually differ quite greatly over the small range in color.

The current WUs have been assuming a fixed value for the absolute magnitude and calculating distance for all stars using this value. As you can see, this is a rough estimate at best and not very good. The updated WUs will be assuming a Gaussian distribution of stars with a maximum at that fixed value. This then says that most of the stars will have this fixed value but theres some chance it's spread out from the value. Therefore, when we combine this with our algorithm we get a much more accurate representation.

Now, you may be thinking "that's great but why is it making the WUs take longer" well that's because we have to numerically integrate this Gaussian distribution and so we have to actually do about 30 times as many calculations as before. You shouldn't see this large of an increase in time as we've decreased some of the other values as well, but on average a WU should run about 4 times longer now. This may increase a little more later, too, if we see we need to be more accurate.

Hope this helps in letting you know what's going on, and let me know if you have any questions/comments.
~Nate~
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Profile[B^S] Acmefrog
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Message 961 - Posted: 7 Dec 2007, 23:06:40 UTC

Thanks for the update and letting us know where we are headed.

Before I forget, will there be a break or downtime between the switch in WUs? If there is can someone post when it may be so that we do not have our machines running on empty?
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Message 963 - Posted: 8 Dec 2007, 0:04:45 UTC - in response to Message 961.  

Thanks for the update and letting us know where we are headed.

Before I forget, will there be a break or downtime between the switch in WUs? If there is can someone post when it may be so that we do not have our machines running on empty?

Also, how will the work units be designated, so we can see which are the longer ones?

Thanks, it is nice to see the research advancing! :)
Regards,
Bob P.
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Message 971 - Posted: 9 Dec 2007, 5:52:58 UTC

Thanks for explaining in easy language.
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ProfileNathan
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Message 999 - Posted: 10 Dec 2007, 20:59:56 UTC

There should be no break between the change, it should be nothing more than an update of the binaries.
I don't know about the WU designation, I'll have to talk to Travis about that.

We will have the ability to run both convolved and unconvolved searches, however, and it will be of interest to see how much of a time increase there is based on how many points we use, etc.


~Nate~
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