Learning from lights in the sky

Peter Kauffner

Most of us have to wait until the sun sets before we have the opportunity to view comet Hale-Bopp. But if the afternoon sky is clear, researchers in the University’s astronomy don’t wait for dark — they head out to the O’Brien Observatory in Marine on St. Croix, Minn., to take infrared observations of the comet.
As the brightest comet since Halley’s in 1910, Hale-Bopp is the object of intense study by astronomers. University researchers are focusing their efforts on observing the comet in infrared light. These comet watchers are taking measurements from both O’Brien and from the Mt. Lemmon Infrared Observatory in Arizona.
The comet reached its closest approach to the sun on April 1 and will be clearly visible in the northern hemisphere until about May 10, not to be seen again for another 4,200 years. Comets normally achieve peak brightness when they are closest to the sun.
“Comets are the ‘Rosetta Stone’ of the Solar System,” said astronomy professor Robert Gehrz.
Gehrz’s comet research is funded by grants from the National Aeronautics and Space Administration. “I’ve received about half a million dollars over 10 to 15 years,” he said.
The material that makes up comets was trapped early in the formation of the solar system, about 4.7 billion years ago. This material is released as the outer layers of a comet nucleus evaporate when a comet passes near the sun.
“By studying the chemicals that come out at different times we can work backward to make a model of the nucleus,” Gehrz said.
Solar radiation pushes the released material away from the sun, producing the tail that makes a comet visible.
“We have one detector and it measures how bright the comet is at different wavelengths. If you plot brightness versus wavelength you tell certain things from that,” said Chris Mason, an astronomy research assistant who is taking many of the Hale-Bopp observations.
The graph of the data is used to determine the chemical composition of the comet. Infrared radiation is a type of electromagnetic radiation that has a wavelength somewhat longer than that of visible light.
“What (Hale-Bopp’s graph) tells us is that there are no hydrocarbons in this comet,” Gehrz said.
This contradicts reports from other astronomers, who claim the comet is rich in hydrocarbons.
Some comets have hydrocarbons while others, such as Hale-Bopp, do not. The chemicals are essential to the formation of life. Why some comets have hydrocarbons and others do not remains a mystery to scientists.
The infrared data suggests Hale-Bopp consists primarily of silicates and carbons. Silicates are a salt based on the element silicon. On Earth, silicates are found in lighter rock, such as granite, while carbon is found in heavier rock, such as basalt.
The infrared observations are taken a detector attached to a telescope.
“The detector has a circular filter inside of it,” said Gehrz. “The filter is placed between the light coming from the telescope and a photometer.”
The filtering material is of varying thickness. A narrow beam of light passes from the telescope, through the filter, and finally to the photometer, a device that measures radiation intensity.
As the filter rotates, different wavelengths of infrared radiation reach the photometer. The intensity of radiation at various wavelengths is thus recorded electronically.
“The detector is inside a thermos bottle that holds liquid helium,” said Gehrz. “The detectors operate at four degrees above absolute zero.”
The detector needed for shorter infrared wavelengths requires even colder temperatures — one degree above absolute zero, just a tiny bit warmer than the coldest temperature possible.
“They have to pump at an enormous rate (to reduce the temperature of the helium from four degrees to one degree),” said Gehrz. “We use a pump that can handle about 500 liters a minute.”
That suction turns the detector into a potential bomb. “It’s a very dangerous process,” said Gehrz. “We hope the vacuum jacket on the outside is strong enough to contain the explosion. But if a person was standing nearby, the shrapnel could kill them.”
Although the University’s researchers have found some surprises, they haven’t found any evidence that would support the now-famous rumors of a Hale-Bopp UFO.
“I think these silly rumors of a spaceship following the comet came about in the following way,” said Gehrz. “An amateur who was not too knowledgeable took a picture of the comet. A star was showing through the tail. He didn’t think there should have been a star there. But it was because he didn’t have the right information to figure out what the sky should have been behind the comet.”