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Distance Between Stars
Dana Desonie, Ph.D.
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Printed: August 8, 2012
AUTHORS
Dana Desonie, Ph.D.
www.ck12.org
C ONCEPT
Concept 1. Distance Between Stars
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Distance Between Stars
• Define parallax and explain how astronomers use parallax to measure distances to stars.
How far is that star?
How can you measure the distance of an object that is too far away to measure? What if you don’t know the size of
the object or the size or distance of any other objects like it? That is the problem facing astronomers when they try
to measure the distances to stars.
Parallax
Distances to stars that are relatively close to us can be measured using parallax. Parallax is an apparent shift in
position that takes place when the position of the observer changes.
To see an example of parallax, try holding your finger about 1 foot (30 cm) in front of your eyes. Now, while
focusing on your finger, close one eye and then the other. Alternate back and forth between eyes, and pay attention
to how your finger appears to move. The shift in position of your finger is an example of parallax. Now try moving
your finger closer to your eyes, and repeat the experiment. Do you notice any difference? The closer your finger is
to your eyes, the greater the position changes because of parallax.
As Figure 1.1 shows, astronomers use this same principle to measure the distance to stars. Instead of a finger, they
focus on a star, and instead of switching back and forth between eyes, they switch between the biggest possible
differences in observing position. To do this, an astronomer first looks at the star from one position and notes where
the star is relative to more distant stars. Now where will the astronomer go to make an observation the greatest
possible distance from the first observation? In six months, after Earth moves from one side of its orbit around the
Sun to the other side, the astronomer looks at the star again. This time parallax causes the star to appear in a different
position relative to more distant stars. From the size of this shift, astronomers can calculate the distance to the star.
For more about parallax, visit http://starchild.gsfc.nasa.gov/docs/StarChild/questions/parallax.html and http://imagi
ne.gsfc.nasa.gov/YBA/HTCas-size/parallax1-more.html.
A parallax exercise is seen here: http://www.astro.ubc.ca/ scharein/a311/Sim/new-parallax/Parallax.html.
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FIGURE 1.1
Parallax is used to measure the distance to stars that are relatively nearby.
Other Methods
Even with the most precise instruments available, parallax is too small to measure the distance to stars that are more
than a few hundred light years away. For these more distant stars, astronomers must use more indirect methods of
determining distance. Most of these methods involve determining how bright the star they are looking at really is.
For example, if the star has properties similar to the Sun, then it should be about as bright as the Sun. The astronomer
compares the observed brightness to the expected brightness.
Vocabulary
• parallax: A method used by astronomers to calculate the distance to nearby stars, using the apparent shift
relative to distant stars.
Summary
• Parallax is the apparent shift in position of an object due to a change in the position of an observer.
• Parallax is useful for determining distances of stars that are a few hundred light years from Earth.
• Brightness is used to determine the distances of stars that are further away.
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Concept 1. Distance Between Stars
Practice
Use these resources to answer the questions that follow.
http://www.astro.ubc.ca/ scharein/a311/Sim/new-parallax/Parallax.html
1. What is parallax?
2. Describe what occurs when you run the animation.
http://www.universetoday.com/47182/stellar-parallax/
3. What is the farthest apart two locations on Earth’s orbit can be?
4. What has produced the most accurate parallax measurements to date?
5. What will Gaia measure?
Review
1. Why is parallax only good for measuring the distances of stars that are no more than a few hundred light years
away?
2. Explain the process that you would use to determine the distance to a star that was about 100 light years away.
3. How do astronomers determine the distance to a star that is further than a few hundred light years away?
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