Why Does My Normalized Star Flux Data Show a Bimodal Distribution?

In summary, the conversation is about a problem in analyzing frequency histogram data for Tycho photometry. The data has been converted to flux and normalized using a formula, but the plotted data shows a bimodal distribution instead of a normal distribution. The person is seeking advice on what could be causing this and mentions considering making an appointment with a statistician for help.
  • #1
big man
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Homework Statement


I'm absolutely desperate for help on this. The image supplied below is an image of a frequency histogram for the V-band Tycho photometry data (from the Tycho Epoch Photometry Annex). What I've done is this:

1. I've converted the V-band magnitudes into flux so that it is no longer on a log scale.

In the Hipparcos mission there were a number of transits for each star (70-200). So that means I have 70-200 photometric measurements for each star.

2. I've now calculated the mean of all the flux values for those transits of a particular star as well as the standard deviation. This process is repeated for each star.

3. I have then normalised the data by using the following formula:

Z=(X-µ)/σ

where X is the original flux value, µ is the mean of the star’s dataset and σ is the standard deviation of the star’s dataset.

4. The resultant 'normalised' data has then plotted using Matlab (seen below).

The problem is that I'm expecting something that is at least close to a normal distribution. But instead I have this strange bi-modal function that is more narrow than a typical normal distribution.

What could cause this bi-modal effect? I have no idea at all. Well the only thought I've had is that maybe this approach fails for fainter stars. I was also thinking of making an appointment with a statistician at my university, but then I'm not sure if he would necessarily know (if it happens to be some astronomical issue).

I thought I'd try here first because there are some people here who do astronomy and might know something that I'm missing.

Any advice on this would be extremely appreciated.
Thanks








http://img521.imageshack.us/img521/4174/untitledqg6.jpg
 
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  • #2
The distribution appears to be bimodal, but appearances can be deceiving. There are several tests for normality. Have you tried any of these to determine whether you must reject the hypothesis that the distribution is normal?

Make the appointment with the statistician.
 

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