Quiet Stars (examples)
Thanks very much for your help with this project. At last count, roughly 50,000 light curves had been sorted at planethunters.org. Many of you have requested more examples about how to classify stellar variability, so we’ll start with the easiest case. All of the light curves below are examples of quiet stars. Random variations in brightness occur because of photon noise (similar to shot noise in electronics). The number of photons that are collected are small enough that there random fluctuations that have nothing to do with the actual brightness of the star. Photon noise (or Poisson noise) produces scatter, but the data remain in a nearly featureless band of points.
If you look closely at the light curve data for these quiet stars, you will see light gray error bars associated with each data point. In any physical measurement, the error bar simply captures our ignorance about the true value of the measurement. In the Kepler light curves, the brightness is represented as a discrete dot, however, any and all points along an error bar are equally correct values for that particular brightness measurement.
In the quiet light curves above, should any of those low points be flagged as possible transits? Probably not. A deviant point or two can still just be noise. A true transit event should have a series of low brightness points that last for the time it takes the planet to cross in front of its stars (i.e., a few to several hours, represented by a few to several data points). Low dips that repeat are also good indicators of a transit, however some of the most exciting transits (from planets in wider, more habitable orbits) will only occur once per month (for example, a true analog of our Earth would just transit once per year).
The quiet light curves above may seem like duds, but they are an extremely important aspect of research for this project. Stars that do not vary in brightness are particularly important objects for exoplanet searches with other techniques. The work that you’re doing will feed into our understanding for the next generation instruments and space missions that could be built to detect planets.
Happy Holidays to All! Debra Fischer
7 responses to “Quiet Stars (examples)”
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- August 5, 2014 -
I think this is a good explanation how quiet stars look. I hope their will be similiar article about variable stars and transits, which are most confusing.
I think it’s also worth mentioning that the range of y-axis values on the quiet stars is very small (most of the datapoint in the graphs shown here are between 1.0075-1.0085, 1.0078-1.0082, 1.0076-1.0084, and 1.0075-1.0085), even though it looks like the datapoints are varying a lot.
Your explanation of electronic and other noise makes sense…ist that also what is going on with stars like these?
I have noticed that in the light curves of many stars that have no obvious transits or periodic behavior, there are gaps above the scattered data of the plot where the gray lines indicating the uncertainty in the data do not extend above the scattered mass of the data points to the left and right. What one sees is a dark gap in the overall gray background of the data points to the left and right of the gap.
What I wonder is whether these are in fact transits where the reduction in light is not enough to drop several points below the light curve and say, here is a transit event. Is this what one should expect from a small planet transiting a star large enough so that the light loss from the transit is small compared with the noise level?
Should we mark these events?
Hi Clif – sorry for the late reply. I think you’re idea is a good way to find the smallest radii planets. If there is a dip in the brightness, then you’ll see it at the top of the light curve as well as at the bottom.
This definately helps! At least I know now that I misidentified some photon noise as transits when I first joined this.