Four men and a Telescope

UNL's vision of the universe extends beyond telscope's limits.

written by Bob Sheldon

With four men and a 30-inch telescope, the astronomy component of the UNL Department of Physics and Astronomy does some pretty heavy stargazing.

Besides Kam-Ching Leung's contributions to binary star studies, Professors Edward Schmidt, Norman Simon and Donald Taylor also have also chalked up significant additions to astronomical knowledge.

Schmidt is cataloguing some 2,000 variable stars observable in the northern hemisphere under a $63,000 grant from the National Science Foundation.

This spring, a paper co-written by Taylor and Schmidt concerning phenomena in the Orion Nebula appeared in the Monthly Notices of the Royal Astronomical Society.

Simon, a theorist in the study of variable stars, forced physicists to re-calculate data involving a physical characteristic important in astronomical studies that Simon contended had to be wrong.

The paper by Schmidt and Taylor in the Royal Astronomical Society's publication resolves a long-puzzling aspect of the view from Earth of the Orion Nebula.

"We wanted to find out if the dark spots that are observed in the nebula were due to gases with less brightness within it or resulted from dust between the nebula and us, as we look at it from Earth," Taylor said. They used an area scanner developed by Taylor to look at different regions of the nebula's dust cloud. On the basis of their observations, the astronomers concluded the dark spots were caused by a dust cloud either between Earth and the nebula or in the nebula itself.

Although Schmidt and Simon both study variable stars, their approaches are different. Schmidt, the observer, has been gathering data for several years on these stars, whose brightness varies in intensity in periods ranging from a few hours up to a few years.

"I'm interested in finding out what's going on inside variable stars ... what makes them change their brightness," Schmidt said. He and Simon, the theorist, are in general agreement on the basic reason why variable stars periodically dim and then shine brighter. "Variable stars are like ordinary stars, except they oscillate," Schmidt said. "As they vibrate, they become hotter and cooler. They vibrate for many different reasons, and they represent stars in widely different stages of their evolution. Older stars tend to have shorter periods of variation in their brightness," Schmidt said. "The younger ones have longer periods." The particular kind of variable stars Schmidt and Simon are studying are pulsating variable stars. To Simon, those stars appear to be "breathing in and out, changing their brightness as they inhale and exhale."

The stars pulsate because photons, or tiny packets of light that can be regarded as eitller a wave or a particle, either pass easily through the star's outer surface or are stopped by the opacity of the outer layers, Simon said.

"When these pulsating stars are expanding, the surface matter of the star is opaque; able to block light which attempts to pass through it. These absorbed photons give the star an extra outward push. Later, as the outer envelope begins to contract, it becomes transparent, so that photons that formerly were stopped on the surface are able to pass through easily, and thus do not impede inward motion in the star." The precision with which these oscillations occur is akin to pushing a child on a swing, according to Simon. "If you want the swing to maintain its motion, you must push at the right moment."

Simon said that the process is complicated. "Opacity has more to do with temperature than density," he said, so only those stars where the temperature is just right will lose or gain in opacity enough to show variability in their brightness.

"The key to the process is helium, he said. "It turns out that the photon push will come at the right time in the cycle only in those stars in which helium atoms in the outer layers are on the verge of losing their second electrons. If the temperature iswithinarelatively narrow range, the first electron in a helium atom is already gone and the atom is barely hanging on to its second electron. If I raise the tempera ture a little bit, the second electron is history. If I lower the temperature, the atom will retain the second electron."

For Simon, gathering evidence about opacity and temperature in pulsating stars is an important way of obtaining information about stellar masses, a physical property that is much more difficult to obtain in stars that are stable. Such data is essential in verifying the theory of stellar evolution, which is largely dependent upon knowing the masses of comparable stars sprinkled throughout the universe.

"Unfortunately, it happened that masses obtained from double-mode pulsating stars didn't agree with what the theory of stellar evolution predicts," Simon said. "They weren't even close.

Many observers suspected that the result, based on double mode stars, was wrong. Simon had another idea. Ten years ago, he suggested that the problem was in the way opacities were calculated. The figure was based on radiation absorption experiments conducted at Los Alamos National Laboratory.

"I found that if I took the Los Alamos opacities and artificially made them larger, by a factor of two or three, and compared them with stars known to be in the temperature region of a few hundred thousand degrees, then everything shaped up.

Simon said his suggestion was rebuffed by the Los ALamos opacity group for years. But recently, two different groups have confirmed Simon's study. Opacities in the 100,000 to 300,000 degrees range are actually two to three times as high as they were thought to be.

Simon holds a grant from NASA to apply the new opacity results to various problems, including stellar pulsations. -- RES

Making Visual Contact With Distant Pulsar

Astronomers long have held that white dwarfs, black holes and neutron stars are the end products of stars with differ ent masses. Their presence in the universe ought to be verifiable through observation if their theory of the evolution of the universe is correct.

Neutron stars proved elusive for years until the discovery of pulsars in the late 1960s, and a UNL astronomer was among the first to make visual contact with one of these puzzling objects, which emit rapid pulses of light like beacons from the outer limits of the universe.

UNL Professor of Physics Don Taylor was at the University of Arizona as an assistant professor in 1967 when pulsars first were detected by radio astronomers. The radio signals showing up as sharp pulses of radio enerhy on their receivers were so strange that astronomers wondered if they might be signals being sent to Earth by LGMs (Little Green Men) from far away in the universe. More conservative speculation prevailed, however, as astronomers concluded that the signals were pulses of light being emitted from a far distant source rotating like the flashing light on a police car.

Taylor, a colleague, and a postdoctoral fellow at Arizona, using electronic equipment that was part of an area scanner that Taylor designed and built, located the pulsar through a 36-inch telescope operated by the University of Arizona. They found the pulsar right where they expected to: in the middle of the Crab Nebula, a super nova remnant from a star that blew up in1054 A.D.

"Actually we were just using the 36-inch telescope for a practice run," Taylor says. "We were going to use a 60-inch telescope owned by the Lunar and Planetary Laboratory at the university a couple of weeks later.

The historic accomplishment by Taylor and his colleagues was chronicled in Time magazine and other newspapers across the nation and within two weeks the discovery was dramatically recreated in a film, "The Violent Universe," produced by Nigel Calder for the BBC.

In any case, the pulsar is very likely a neutron star, one with a mass like our own sun but so small that it could fit comfortably between Lincoln and Omaha, and perhaps within the city limits of either city.

The neutron star pulsar is spinning very rapidly, emitting a very narrow beam of light, blinking on and off like a revolving searchlight as it revolves at the rate the pulsar is spinning.

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