Light Curves
Light Curves
Wondering what a light curve is? Sadly, it's not a cool weapon from Star Wars (although I imagine Jedi would find them useful for fighting around corners). Light curves are a fundamental tool for variable star astronomy. They are relatively simple and easy to grasp. They are simply a graph of brightness (Y axis) vs. time (X axis). Brightness increases as you go up the graph and time advances as you move to the right.
Here is a light curve of epsilon Aurigae from its last eclipse:
This light curve shows that the star began at a brightness of magnitude 3 in 1982. Around mid-year it began to rapidly dim until it reached brightness of magnitude 3.8 by the end of the year. It remained there until the beginning of 1984 when it began a slower climb back to normal brightness. By the middle of 1984, it was almost back to normal brightness.
That light curve is idealized, in that it was processed to only include the best observations to make it clear. Now let's look a more complicated, real-world light curve. Here is a recent light curve of the bright star Betelgeuse (a.k.a Alpha Ori to astronomers and one of our ten training stars) made with the AAVSO's online light curve generator:
Notice that there are many observations made on the same dates, but they don't agree! That's because we're all human (politicians excepted) and so will make different estimates of a star's brightness. We chose this star as an example because it is very bright and very red. This makes it extra hard for humans to make consistent observations. We call this variation in brightness estimates scatter.
But fear not, we can address the problem with basic statistics! Below is the same light curve, but this time with a red line drawn through it:
The red line reflects the average observation made at that time. More precisely, we calculated an average brightness of the star in 30 day increments. Then the light curve generator drew a line between the average points. The verticle red bars you see along the line is a 1-sigma standard deviation error bars (a.k.a. "uncertainty"). It's a statistical value that provides an idea of how much you can trust the red line. (It means there is a 66% chance that the real data falls within that horizontal error - a common benchmark scientists use.)
A good rule of thumb is to see if you can draw a horizontal and straight line between the error bars. If you can, it means there is no real variation in the data. If you can't, then the variation you see is more likely than not to be real. Can you draw a straight, horizontal line between the error bars in that light curve? Try it by holding a sheet of paper up to your monitor.
Notice in the first set of data in the light curve that you cannot draw such a line. But you can in the second set! That is because the second set is noisier (i.e. has more scatter). There are lots of things you can do to try to lower the error bars in that data, but we'll save that for future discussions. But the best thing we can do is gather more high quality data. This is why we need more observers in the Citizen Sky project. The more we have, the more we can beat down those error bars. It's also important to take your time to make a quality observation.
So as observations of epsilon Aurigae and the other 10 stars begin to pour in, you'll be seeing lots of updated light curves. Post questions you have here and we'll help you answer them. We also have a more detailed description of light curves and basic analysis in Chapter 11 (PDF) of our online Variable Star Astronomy curriculum.
