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

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SUMMARY

The discussion centers on the unexpected bimodal distribution observed in normalized V-band Tycho photometry data. The user converted V-band magnitudes into flux, calculated the mean and standard deviation for multiple photometric measurements per star, and applied normalization using the formula Z=(X-µ)/σ. Despite these steps, the resulting distribution is narrower and bimodal rather than normal. The user speculates that the issue may arise from the treatment of fainter stars and considers consulting a statistician for further insights.

PREREQUISITES
  • Understanding of V-band photometry and flux conversion
  • Familiarity with statistical normalization techniques
  • Proficiency in using Matlab for data visualization
  • Knowledge of statistical tests for normality
NEXT STEPS
  • Research statistical tests for normality, such as the Shapiro-Wilk test or Kolmogorov-Smirnov test
  • Explore advanced data normalization techniques in astrophysics
  • Learn about the impact of measurement errors on photometric data
  • Consult resources on bimodal distributions and their implications in astronomical data analysis
USEFUL FOR

Astronomers, data analysts, and statisticians interested in photometric data analysis and the implications of distribution shapes in astronomical datasets.

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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








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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.