Eclipsing binaries, binary stars where the orbit causes one to pass in front of the other from our view, are the main way astronomers measure the sizes of stars. When a star passes in front of another star it will dim a little just like when a planet transits a star. The amount of time it takes for the brightness to reach its minimum and the amount of time it stays at the minimum tell us about the sizes of the two stars.
Lab Tools
For this lab we will use the Eclipsing Binary Simulator developed at the University of Nebraska – Lincoln. In the upper left box of the simulator we see the perspective from Earth. The upper right box shows the light curve as a function of phase which is just the fraction of the stars orbit. This is normalized to be 1.0 when there is no eclipsing. The bottom left panel allows you to control the System Orientation and the Animation and Visualization Controls. Here you can set the longitude and inclination of the binaries orbit. Note that at most inclinations the binary will not be an eclipsing binary. You can also start and stop the animation and control its speed. You can click off the lock on perspective from Earth, which will let you drag the orbit so you can see it from a top down view without effecting the light curve. The right bottom panel controls the properties of the stars and their orbit. There are a number of preset choices and you can control the mass, size and temperature of each star as well as the separation between the stars and the eccentricity of their orbit.
Assignment
Let us start with preset Example 1. This is the case of two identical stars. What feature of the light curve can be used to measure the size of the star? Now switch to preset Example 2. In this case what features of the light curve can be used to measure the size of the smaller star? How about the bigger star? Move on the preset Example 3. What is different about this light curve? What star property causes the difference? Now let’s try preset Example 6, notice we skipped Example 4 and 5. What is different about the light curve now? What is causing the difference, you can change the inclination of the orbit to help you see what it is? Finally let’s look at Example 7. Change the first stars temperature to 5000K and the orbit eccentricity to 0.3. Now look at the light curve. Can you explain why it looks like this? This is actual a more realistic example then the previous ones which are simplified to make it easier to see what is going on.
Questions
- In preset Example 1 what is the minimum inclination where you can still see this configuration as an eclipsing binary?
- What effect does changing the star’s mass have on the orbit?
- What effect does changing the separation have on the light curve?
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