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.
- 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.
- Notice that the stars labeled as "Distant stars" also shift about as
the Earth orbits the Sun. Why is this?
- 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?
- 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
- 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?
- 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.
- 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
- Now look at the table of the 150 most luminous stars. How many of these
stars are more luminous than the Sun?
- How many stars on the most luminous list are also on the most nearby
list?
- 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?
- 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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