FIGURE 11. Alpha Centauri A virtual parallax measurement

Figure 11 is a screen grab from the running applet. To try this virtual experiment, select the ATLAS icon to start the applet.

See the list of other virtual experiments.

VIRTUAL EXPERIMENT 3
Calculating the Distance to Alpha Centauri A using Virtual Parallax Measurements

• Calculate the distance to Alpha Centauri A using virtual parallax measurements.
• Understand that parallax can only be when the visible Field of View is small.
• Evaluate the relationship between field of view and accuracy in making parallax measurements.

• Set the target star to Alpha Centauri A using the select star pulldown menu.
• Set the Field of View to 45 degrees.
• Set the operation mode to parallax measurement.
• Progressively change the Field of View, making it smaller and smaller.
• Eventually, you will see the elliptical pattern Alpha Centauri appears to sweep out over the course of a year. In effect this is a multiple exposure taken at one-month intervals. See Figure 8.
• Calculate the parallax angle by using to mouse clicks. Make the first mouse click at the centre of the ellipse and the other at the end of the major axis.
• Record the parallax angle returned by the simulator.
• Given equation 1, calculate the distance to Alpha Centauri A using the reciprocal of the parallax angle for measurements taken at several fields of view.

• Alpha Centauri is a binary star with components A and B.
• Alpha Centauri A and B cannot be separated when the Field of View is large.
• Between about 1 degree and 40 arc seconds, Alpha Centauri A and B can be resolved separately.
• Parallax effects can be seen for Alpha Centauri A between about 1arc minute and 2 arc seconds.
• Measuring parallax angle accurately requires very small fields of view.

In Figure 11, the simulated parallax angle is measured to be 0.742 arc seconds. Using Equation 3, the distance to Alpha Centauri A is therefore:

The virtual experiment produced a result of 4.4 ly, consistent with the value listed in Appendix 4 of Kaufmann and Freedman (1999).

This virtual experiment may be repeated for target stars other than Alpha Centauri A.