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Tools of the Trade
Fritz Benedict
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Reference Stars

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Tools of the Trade

Simply by measuring the position of a star in the sky, astronomers can determine its distance, its motion through space, whether it has companions, and much more

The oldest technique for studying the stars is in many ways the simplest: plotting their positions in the sky. Known as astrometry, this technique can reveal a star’s distance, its motion through space, and the presence of unseen companions, and allow astronomers to plot the system’s orientation.

Hubble servincing mission

Astronauts Gregory Harbaugh (left) and Joseph Tanner replace one of the Fine Guidance Sensors during a 1997 servicing mission. Another of the three sensors was replaced during the final mission in 2009. [NASA]

Texas astronomer Fritz Benedict and his colleagues are using this technique to measure the distances to classes of stars that are an important first step in measuring the size of the universe. They’re also using astrometry to plot star systems with suspected planets, revealing the masses of the companions and providing more precise data for the theorists who model how stars and planets form.

The technology has advanced a bit over the centuries, of course. While early astronomers were limited to what they could see with their eyes alone, the Texas group is using precise sensors aboard Hubble Space Telescope, complemented with observations from a variety of ground-based telescopes.

Yet the technique remains basically the same: measuring the position of a star as precisely as possible.

A Little History

The modern practice of astrometry probably began with the second-century Greek astronomer Hipparchus, who compiled tables of star positions. While comparing his observations to those made centuries earlier, he discovered that some stars had moved a tiny bit across the sky. This change is called proper motion, and it measures a star’s true motion through the galaxy in relation to Earth.

Proper motion also tells us something about the distance to a star, because those with the fastest motion are generally the closest to us. Even so, even the closest stars are so far away that it is difficult to determine their distances. But beginning in the 19th century, astrometry provided a solution.

The technique is known as trigonometric parallax. Although it is difficult to implement because of the great distances involved, the concept itself is fairly simple.

To demonstrate it, hold a finger in front of your face. Look at it first with just your left eye, then with just your right. As you blink back and forth, the finger appears to shift a little bit against the background of more-distant objects. By measuring the distance between your eyes and the angle of the back-and-forth shift, you can determine the distance to your finger. That angle is known as parallax.

Astronomers apply the concept by looking at a star at different times during Earth’s year-long orbit around the Sun. Earth’s orbit is about 186 million miles (300 million km) wide. As Earth moves from one side of the Sun to the other, nearby stars appear to move back and forth a tiny bit compared to the background of more-distant stars.

Hubble fine guidance sensor example

An example shows a target star in the middle of this Hubble view with precisely catalogued reference stars numbered around it. [Benedict et al.]

In 1838 Prussian astronomer Friedrich Wilhelm Bessel became the first to use this technique when he measured the distance to the star 61 Cygni at about 11 light-years (3.5 parsecs).

Putting Astrometry to Work

Parallax works well for stars that are fairly close, but as you move farther into space, the angles become so small that they are difficult to measure. So far, astronomers have good measurements of stars within a few hundred light-years, and fair measurements for many more stars within a few thousand light-years.

Benedict’s group is measuring the distances to stars that are hundreds or thousands of light-years away, and expects to pinpoint them with an accuracy far better than any other measurements to date.

Hubble fine guidance sensor field of view

This image of the Pleiades star cluster shows the field of view of the Fine Guidance Sensors, which are the three curved green boxes. The yellow boxes show the view of other instruments. [NASA/ESA/STScI]

The group is also using astrometry to measure the back-and-forth “wobble” in a star’s motion induced by the gravitational tug of unseen companions. This tug causes the star to move a tiny amount as seen from Earth. Plotting this motion, and combining it with other information, reveals the mass of the unseen companion, telling us whether it is a star or a planet. It also reveals the distance between the star and the companion. Theorists use this information to refine models of how planets form and how the layouts of planetary systems change over time.

Sighting the Target

The principle instrument for these observations is one of Hubble Space Telescope’s Fine Guidance Sensors.

Fine Guidance Sensors
Each of Hubble's Fine Guidance Sensors is contained in a small cabinet that can be removed and replaced by spacewalking astronauts. Each cabinet measures about 5.5 x 4 x 2 feet, and on Earth weighs almost 500 pounds. Each sensor requires about 20 watts of power, which means you could operate the entire set on less energy than it takes to power a modern compact fluorescent light bulb.

The three sensors are like telescopes within a telescope. They find “reference stars” that are near Hubble’s target and keep the telescope pointed in that direction. Using the sensors, Hubble maintains a pointing accuracy of 0.007 arcseconds. That angle is the equivalent of an American dime as viewed from a distance of 400 miles.

Two sensors are required to keep the telescope pointed in the right direction, leaving the third available for scientific observations. The Texas-led team will compare the positions of its target stars to the reference stars around it many times over the two-year project, providing precise information on the motions of the target stars.

The two projects — measuring the distances to variable stars and plotting the geometries of star systems with project planets — have received 200 orbits to do their work. Hubble orbits Earth once every 90 minutes or so (although it cannot collect data for the full orbit), so the scientists will receive perhaps a couple of hundred hours of data.

Hubble, however, is only the “point man” for the project. Benedict and his colleagues combine Hubble observations with those made by many telescopes on the ground. The list includes the Hobby-Eberly Telescope and a 30-inch telescope at McDonald Observatory, 4-meter telescopes in Arizona and Chile, and several smaller instruments. The astronomers are also using data collected by previous studies that date back as early as the 1980s.

Because it is above Earth’s obscuring atmosphere, Hubble maps the positions of stars far more accurately than any ground-based telescope. But the ground-based telescopes provide a detailed dossier on each star, with information on the star’s temperature and composition as well as its motion. The combination of all these observations gives the astronomers a complete picture of what is happening in each star system.


Getting a Grip on Stars


Hubble floats free in space after astronauts completed the final service call on the telescope in May 2009. [NASA]