From: Frank Reed
Date: 2021 Dec 10, 10:15 -0800
Before I forget, to see an example of the issue with the proper motion of Rigil Kentaurus (alpha Centauri), try this: open Stellarium, go to Rigil K. and zoom in until you can see that it is a double star. Now fix the app's attention on that star by tapping space bar on your keyboard. You may also want to turn off the atmosphere and ground. Open the date and time selection tool and increment or decrement the year. You can do this by putting the cursor on the year and then hitting "+" or "-" on the keyboard. Hold the key down to run through a few centuries. If the app handled this system properly, it might manage with two options: either do the orbital motion of A and B around their common center of mass, which would be the best choice, or keep the two stars at a fixed separation and have them both follow the correct center of mass proper motion. Instead the stars have independent proper motions, which are presumably the values derived from the original Hipparcos analysis, and they behave as if they are gravitationally unbound. The two stars in the system separate at a rate of about 0.8 minutes of arc per century. This is not good.