Professor Arlo Londolt stands beside the telescope in the Landolt Astronomical Observatory at LSU. Photo credit: Jason Peak

A STELLAR CONSTANT
Astronomer steers career by the stars

Star light, star bright . . . If you wish to know how bright precisely, ask Arlo Landolt.

For more than fifty years, the eminent professor of physics and astronomy has made the careful measuring of stellar brightness and color the focus of his work. The result is a meticulous catalog of nearly 1000 well-documented stars: the deceptively humble cornerstone of modern astronomical research.

A star’s brightness, or magnitude, reflects its energy output once adjusted for distance. Its color relates to its temperature, just as iron in a forge glows white hot and cools to red. When quantified, these attributes reveal a star’s age, chemistry and stage of evolution. Taking their measure is the science of stellar photometry.

Landolt explains, “Light from a star comes through the telescope and falls on a photosensitive cathode that turns light into electricity. It emits electrons, and you count the electrons.”

He then adds quickly, “It’s not quite that simple.”

Landolt’s specialty is finding stars whose color and magnitude change little over time. Establishing this consistency can take years of repeated observation—a set of 200 stars may take a decade to complete. But once added to the list, or catalog, of constant stars, each one becomes a welcome guidepost in a very large and chaotic universe.

Stellar constants permit accurate charting and measurement of such diverse objects as extra-solar planets, black holes, star clusters and supernovae.

Landolt’s method of photometry is an extension of the work of American astronomer Harold L. Johnson (1921–1980), who pioneered the use of carefully selected filters to measure the intensity of light emitted by bright stars. The technique is known today as UVBRI photometry, in reference to its splitting of wavelengths from the ultraviolet to infrared.

“Light from a certain part of the electro-magnetic spectrum passes through [each] filter and onto the film, where its intensity can be measured,” said Landolt of the Johnson system.

“Different filters isolate different parts of the spectrum, telling how bright the star is at different wavelengths. A comparison of intensity at two different wavelengths is what gives us something we define as color.”

Today, instead of film, a photoelectric photometer captures the filtered light and measures its brightness with precision; but long before these modern methods, the human eye served as the essential instrument of astronomy.

“One of the brightness intensities I measure is called the v-magnitude, v for visual. The v-magnitude most closely matches what the human eye sees. It is possible to take this information and trace it all the way to the early astronomers like Hipparchus and Ptolemy. It’s a thread that ties all the measurements together down through history.”

For Landolt, the traditional methods of astronomy are as important as any current technology. He cites visual confirmation of instrument readings and travel to remote points of observation as parts of a scientific history quietly slipping away. “Computerization is making a big change. Originally we would go out to the dome and look through the telescope physically, search for the star and let the starlight fall on the detector, whatever kind we had. Now we sit in a room, and everything comes in by cable to a computer console. You type in coordinates of the star and the telescope automatically moves around. It isn’t as romantic as it used to be . . .”

Landolt praises such recent innovations as space-based instrumentation and digital data processing for allowing the quick measurement of millions of new stars. But the standards for establishing stellar constants are greater. Calibration must be exceptionally fine, for these cataloged stars will become, in effect, measuring devices of their own, upon which hundreds of later calculations depend.

“It well may be that astronomers will not need to interpret their data from the largest telescopes to the one percent level,” acknowledged Landolt. “However, the limits of interpretation [should] be scientific, not those of calibration.”

To safeguard precision in the field, Landolt keeps tabs on his catalog prospects. He travels regularly between two hemispheres and carefully repeats his measurements, as he says, “to make sure the stars are behaving.”

Arlo Landolt’s research in UVBRI star photometry has received recurrent funding since the mid-1960s by agencies including the U.S. Air Force and NASA. His work is currently supported by a three-year grant from the National Science Foundation.

...from the Autumn 2007 Issue