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Better cosmic yardstick helps measure age, size of universe
Posted: October 4, 2000

Researchers using the testbed interferometer at Palomar Observatory have achieved the best-ever distance measurement to a type of star known as a Cepheid variable. The new results improve the "cosmic yardstick" used to infer the size and age of the universe.

In the September 28 issue of the British journal Nature, a group of astronomers from the California Institute of Technology, the Jet Propulsion Laboratory, and the Infrared Processing and Analysis Center announce that the distance to the star Zeta Geminorum in the Gemini constellation is 1,100 light years. The degree of accuracy in the measurement is about 13 percent, meaning that the star could be as close as 960 or as far away as 1,240 light-years. This represents an improvement of a factor of three over previous measurements.

The improvement is due to the use of the Palomar Testbed Interferometer, of which JPL engineer Mark Colavita is the principal investigator and codesigner.

Palomar Testbed Interferometer. Photo: Caltech
"This has been a bit of a Holy Grail in the field," says Benjamin Lane, a graduate student in Caltech's planetary science program and the lead author of the study. "The measurement of accurate distances to Cepheids is widely considered to be a principal limitation in determining the Hubble constant."

Cepheid variables for several decades have been an important link in the chain of measurements that allow astronomers to estimate the distances to the farthest objects in the universe -- and ultimately, the overall size and expansion rate of the universe itself.

Cepheid variables are stars that have very predictable relationships between their absolute brightness and the frequency with which they brighten up. A Cepheid is useful for measuring distances because, if it is known how bright the star really is, then it is a simple task to measure how bright it appears on Earth and then calculate the distance.

A good analogy is a light bulb shining at an unknown distance. If we are certain that only 100-watt light bulbs brighten once a day, and we observe that the light indeed brightens once daily, then we can calculate its distance by measuring the brightness of the light reaching us and comparing it to the known absolute brightness of a 100-watt light bulb.

"Zeta Geminorum is known to grow larger and smaller," says Lane. "We already knew this because we can see the Doppler effect." In other words, astronomers can measure a slight difference in light coming from the star because the surface of the star moves toward us and away from us as the star expands and contracts.

In the Nature study, the researchers couple this information with new data collected with the Palomar Testbed Interferometer. The interferometer combines the images from two 16-inch telescope mirrors in such a way that images are as sharp as they would be if the telescope mirror were 360 feet in diameter.

Data from the interferometer showed that Zeta Geminorum went through a change in angular size of about five hundred-millionths of a degree during its 10-day cycle.

"That's roughly the size of a basketball on the moon, as seen from Earth," says Colavita.

From previous Doppler measurements, the researchers already knew that the change in the star's diameter was about 4.2 million kilometers. By combining that information with the newly measured change in angular size, they were able to deduce the distance to the Cepheid.

The direct measurement of distance to Zeta Geminorum shows that the basic technique works, Lane says. "As a graduate student, it has been exciting to be at the leading edge of this field."

The Palomar Testbed Interferometer was designed and built by a team of researchers from the Jet Propulsion Laboratory in Pasadena led by Colavita and Michael Shao. Funded by NASA, the interferometer is located at the Palomar Observatory near the historic 200-inch Hale Telescope.

The device is intended as an engineering testbed for the interferometer that will soon link the 10-meter Keck Telescopes atop Mauna Kea in Hawaii.

The Keck Interferometer has been funded to find and study extrasolar planets. The Navy and the NSF are also funding the development of interferometers for astrometry and stellar astronomy.

"The current precision is a significant improvement over the previous determinations, but we expect to achieve distance measurements at the level of a few percent in the near future," says Shri Kulkarni, a professor of astronomy and planetary science at Caltech and a coauthor of the paper.

In addition to Lane and Kulkarni, the other authors are Marc Kuchner, a Caltech graduate student in astronomy; Andrew Boden of the Infrared Processing and Analysis Center (IPAC), and Michelle Creech-Eakman, a postdoctoral scholar at JPL.

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