Finding Proper Motion Pairs
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How does Proper Pair Work?

The Proper Pair web tool allows you to search the sky to find true visual binaries by comparing the star's proper motions (also called proper motion pairs). The data used for the web tool came from the Hipparcos Catalog. The Hipparcos satellite, launched by the European Space Agency in 1989, measured the parallaxes and proper motions for nearly 120,000 stars in the Solar Neighborhood. For two stars to be considered a proper motion pair, they must be moving in the same direction at the same speed. The also must be the same distance from Earth. These stars are bound together by mutual gravitational attraction as they orbit the center of the galaxy.

To begin using the Proper Pair web tool, navigate to the ProperPair, web tool and click the Proper Pair button in the lower right corner. If you have any questions while using the Proper Pair web tool, you read the tool's FAQ by clicking the FAQ button, or navigate back to this tutorial by clicking the Tutorial button.

ProperPair will display a map of the Sky. Move the mouse over the map to view constellations, and click on the one you want to select, then select a sky region within the constellation.

Once you have selected a constellation and a particular sky region, the web tool will display all of the Hipparcos stars in the selected region. The size of each dot correlates with the apparent brightness (the brightness as the star appears from Earth) of the star the dot represents, and the color of the dot represents the temperature of the star. The hottest stars are violet and the coolest stars are red, with the sequence violet, blue, white, yellow, orange, and red decreasing in temperature. Each star also has an arrow associated with it pointing in the direction which the star is moving. The length of the arrow indicates how fast it is moving in that direction. Clicking on two stars will bring up three additional boxes. Within two of the boxes each star is shown with its arrow. The direction of the arrows illustrates the direction in which the star is moving. The length of the arrow illustrates how fast it is moving. The third box shows the two previous boxes superimposed for comparison of the two stars' motion. Note that the distance and proper motion arrows are shown in different shades of green - you can match the colors of the arrows to the green borders around the individual star boxes.

How do you determine which of these stars are proper motion pairs? Imagine cars on the highway. At first glance they all seem to be going in the same direction, but after watching them for long enough you notice that they aren’t all going the same direction. Some exit the highway, and some enter onto the highway. What if you wanted to determine which cars were traveling together as a caravan? How would you know? First off they would all be in the same area. Secondly, they would all be going the same direction. Thirdly, they would all be going the same speed. If any one of those variables were not the same, the caravan would not be a caravan anymore!

Proper motion pairs are similar. As proper motion pairs orbit the galaxy, they are at the same distance from Earth, they are moving the same speed, and they are moving in the same direction. When using the Proper Pair web tool, you will look for pairs of stars which are at the same distance and moving in the same direction with the same speed. In the above picture, the two stars are moving in two different directions. Thus, they are not a proper motion pair. Let's look at some other examples of potential proper motion pairs.

EXAMPLE 1

Notice that these two stars are moving in the same direction but at different speeds. To be a proper motion pair, the stars must be moving in the same direction at the same speed!

EXAMPLE 2

Notice that these two stars are moving in the same direction and speed, but they are at drastically different distances. To be a proper motion pair, the stars must be moving in the same direction and speed and be at the same distance from Earth!

EXAMPLE 3

Notice that these two stars are moving in the same direction and speed, and they are at roughly the same distance. The two stars must be a proper motion pair!

If you find two stars at the same distance with proper motion arrows that agree in both direction and length, then you've found a visual binary!

For more information about each star, click on the "show info" button in the upper right corner. The stars' celestial coordinates, V magnitude, (B-V) color, parallax, and proper motions will be displayed. These values from the Hipparcos database are needed for activities associated with ProperPair.

 

Support from the National Science Foundation through the CCLI program is gratefully acknowledged.

 

 


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Last updated: 30 November 2008
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