The star’s motion
For this activity we will use the interactive application to show the movement of the star.
Activity _unit4.4.4 Activity 6 The star’s motion
This application is zoomed in on the centre of mass, so the star’s orbit appears larger on the screen and the planet is too far away from the star to be visible within the views shown. The direction to the planet is indicated with a green line. The direction to the hypothetical observer (not you) is indicated with a purple line.
Click on the play button and the application will start. Notice that the line from the star to the (unseen) planet always passes through the centre of mass of the system.
Set the orbital inclination to the value i = 90° exactly, using the arrow keys on the keyboard to amend the values if necessary. Using the play/pause button, halt the application when the star is moving exactly towards the observer. What is the value of t (see the slider position) when this is the case?
Answer
The value is t = 0.25 give or take however much uncertainty your clicking led to. There should be a right-angle (exactly 90°) between the green and purple lines, with the star at the bottom of the circle shown in the 2D graphics window. At this instant, the star is moving horizontally to the right in the 2D window, exactly towards the observer.
At this instant, what direction is the planet moving in?
Answer
The planet is moving exactly away from the observer, to the left in the 2D window.
Using the slider, change the value of the orbital inclination to i = 70°. Set the application to play.
What is the definition of radial velocity?
Answer
Radial velocity is the motion towards or away from the observer.
Using the play/pause button, halt the application when the star has maximum radial velocity away from the observer. What is the value of t (see the slider position) when this is the case?
Answer
The value is t = 0.75 give or take however much uncertainty your clicking led to. There should be a right-angle (exactly 90°) between the green and purple lines, with the star at the top of the circle shown in the 2D graphics window. At this instant, the star is moving horizontally to the left in the 2D window. This is as close at it will get to moving exactly away from the observer, who is positioned off to the right and above the 2D graphics.