Located just off the tip of the Little Dipper in the night sky, Polaris, also known as the North Star, has been used by sailors for ages as a navigation tool.
But in the Space Age, Polaris has become more than just an indicator of the direction north. It is also a Cepheid variable star, meaning that it changes from bright to dim in a precise interval. Astronomers use the regularity of these stars to help determine large distances in the universe.
And until now, it had not been confirmed exactly what type of Cepheid variable Polaris is.
New measurements by an optical telescope array confirm previous theoretical calculations that Polaris is an overtone pulsator -- a type of irregularly pulsating Cepheid variable.
Cepheid variables change in brightness because they actually expand and contract. Most expand and contract evenly, like a spring being pulled apart with two hands.
But by measuring the diameter of the star with a powerful telescope array called an optical interferometer, a team of astronomers at the U.S. Naval Observatory outside of Flagstaff, Arizona determined that Polaris was too big to be an evenly expanding Cepheid.
The star was found to have a radius of about 46 times that of the sun, or about 20 million miles (32 million kilometers).
Instead, the team, led by the observatory's Dr. Tyler Nordgren, found that Polaris expanded unevenly, like a spring pulled apart by both hands and twisted in the middle. These observations confirmed a hypothesis offered by another astronomer team in 1997.
The findings were presented at a meeting of the American Astronomical Society in Rochester, New York.
Dr. Nordgren said that the type of telescope used was particularly well suited for measuring the angular diameter of Polaris, a fairly bright star. The array, called the Navy Prototype Optical Interferometer, combines several small optical telescopes. Connected to each other, their resolving capability is increased to that of a single 125-foot (38-meter) optical telescope. By contrast, the largest mirror in a single optical telescope is 33 feet (10 meters) in diameter, at the Keck Observatory in Hawaii.
The larger distance between the interferometer's mirrors allows the telescope to resolve, or discern, very small details on celestial objects. But because the telescope lacks a large mirror, it can only see very bright objects. In fact, the human eye can pick out stars that the telescope cannot.
"We have incredible resolving capability," Dr. Nordgren said. "With conventional telescopes, the stars still really look like points of light. With optical interferometers, of which the Navy type is one, you can begin to see a thickness and a size to these objects."
There are several optical interferometers in the Northern Hemisphere, Dr. Nordgren said. The Navy is interested in this method of optical astronomy because it can help make more accurate star maps, upon which sailors still rely for navigation.
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