Parallax Demos and Exercises

Parallax Simulator

Take a look at the Parallax Simulator at the University of British Columbia Astronomy 311 Website. To get to the simulations page, click on the blue object below the picture of the Earth in the left-hand frame, and from there, select the "Parallax" Demo. Try to use the demo to answer the following questions.

1. First run the simulation with the "nearby star" in its default position. Notice that the position of the star on the sky as viewed from Earth shifts back and forth. How would you expect the size of the shift to change if you moved the nearby star closer to, and farther from the Earth? Once you think you know how the size of the shift will change, test out your guess by sliding the nearby star closer and farther from its starting position and rerunning the simulation.
2. Notice that the stars labeled as "Distant stars" also shift about as the Earth orbits the Sun. Why is this?
3. When astronomers measure the parallax of a nearby star, they actually measure the shift of the nearby star, with respect to other stars. Knowing this, what stars would be the best to pick as comparison stars? How would you actually go about picking out these stars from the many stars present on your images of the sky?
4. Notice that picking a comparison star that is too nearby to measure the parallax of another nearby star will lead you to determine an incorrect parallax. Would you expect this incorrect parallax angle to be smaller or larger than the true parallax angle? Test your prediction using the parallax simulator.

The Hipparcos Parallax Catalog

The Hipparcos Satellite measured accurate parallaxes for 120,000 stars, down to an apparent magnitude of approximately V=12.4. Take a look at their tables of the 150 most nearby stars in the Hipparcos Catalog, and the 150 most luminous stars in the Hipparcos Catalog. Use these tables to try and answer the following questions. ( Note: The parallax angles given in these tables are in milliarcseconds (i.e. 1/1000's of an arcsecond.)

The most Nearby Stars

1. Look at the list of the 150 most nearby stars. What is the distance from Earth of the 150th nearest star? How long does it take the light emitted by that star to reach the Earth?
2. Are most of the 150 nearest stars more or less luminous than the Sun? What fraction of the stars on the list are less luminous? What does this imply about the luminosity of the Sun compared to that of most stars within our galaxy? You will need to find the absolute V magnitude of the Sun from your textbook or another source to answer this question.
3. Is it possible that there are stars more nearby the Sun than the 150th most nearby star on the Hipparcos list? If so, what properties must a star have to allow it to be that close but not be detected by Hipparcos?

The most Luminous Stars

1. Now look at the table of the 150 most luminous stars. How many of these stars are more luminous than the Sun?
2. How many stars on the most luminous list are also on the most nearby list?
3. Can you tell from the data given in the table if any particular star on the most luminous list is a main sequence or a red giant star? If not, what additional, specific, piece (or pieces) of information would you need in order to make this determination?
4. Is it possible that Hipparcos observed stars that are actually more luminous than some or all of the stars given in the table, but that doesn't itself appear in the table? If so, what properties must the star have that would lead to it not being tabulated?

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