Heart of the Star

Sumner Starrfield has his head above the clouds—not a bad place to be if you are a world-class astronomer. He has spent the past 30 years observing the great beyond and teaching the nuances of the heavens to students at Arizona State University.

Starrfield’s specialty is novas, exploding stars that most people know only from science fiction. The ASU astronomer observes and studies the theoretical cause and evolution of the classic nova outburst.

Starrfield remembers his arrival on campus in 1972 and his introduction during the College of Liberal Arts and Sciences faculty assembly.

“I was introduced as the university’s first astronomer, Sumner Starrfield. Of course, the entire faculty broke into laughter. That is how my ASU career began,” he says.

His work over the years has not gone unnoticed. In 2002, he was named an Arizona Regents Professor of Astronomy, one of the top honors a professor working for Arizona’s state universities can attain.

An unassuming man, Starrfield says his last name is only a coincidence with his chosen profession. He began his college career as a mathematics major at the University of California, Berkeley. He enrolled in his first astronomy class during his junior year. That’s where he first experienced the Big Bang theory—not the one that explains the creation of the universe—but the one that college students often experience on a more personal level.

The second “Big Bang” theory refers to that irrefutable moment when everything you have learned gels, and you realize for the first time that you are doing what you ought to be doing for the rest of your life–the one thing you really love.

“When I started doing undergraduate research and looking for things out there [in space] that no one had ever looked at before—the mysterious and the unknown—I was hooked,” Starrfield says. “I came to realize this was one area where someone like me could make a great contribution. Even as an undergraduate, I could do forefront research. That was one of the big clinchers for me in choosing this discipline.”

The genesis of Starrfield’s fascination with the night sky may have begun long before he set foot in a Berkeley classroom. He remembers visiting Griffith Observatory on Sunday afternoons as a child in his native Los Angeles.

Starrfield admits that watching the stars will keep him up at night. “I’m always scanning the sky,” he says. There’s a bumper sticker on his office door to prove it. He often spends his free time looking for that unknown stellar object.

As an astronomer, Starrfield can lay claim to many firsts. He was one of the first scientists to identify and explain the hottest known pulsating stars. These pulsating stars are strange, and rare; only 10 have been discovered since 1980.

Starrfield’s theoretical work covers many topics in nova studies. But he is most widely known in the scientific community for one specific achievement. Starrfield was the first to confirm that an explosion in gas falling onto the surface of a white dwarf star in a closed binary star system was the cause of the actual nova outburst.

A white dwarf is a type of star that contains about as much matter as the Sun, the star at the center of our solar system. But there is a difference. White dwarfs are made of matter jam-packed into a size comparable to the Earth. Most scientists think that white dwarfs are made primarily of carbon and oxygen.

Starrfield published continuously on this theory from 1970 to 1990. The work is important to scientists because it is leading to a better understanding of the ultimate fate of the Sun.

During the early morning hours of February 19, 1992, Peter Collins, an amateur astronomer in Boulder, Colo., discovered “V1974 Cygni 1992.” His discovery was very important. In fact, it is known among astronomers as “the most important Nova of the 20th Century.” Why? Because V1974 Cygni is the only nova to have been observed from its birth through to its death.

Within hours of Collins’ initial report, ASU astronomer Sumner Starrfield and colleague Steven Shore of Indiana University-South Bend began their own observations. They looked at the nova with the help of instruments aboard the International Ultraviolet Explorer satellite.

What Starrfield and Shore saw was important as well. They determined that that the explosion was just beginning.

“This nova answered many questions during its life. It raised more in death,” says Starrfield, who has dedicated a 30-year career to understanding the death of stars. He and other scientists will continue to observe V1974 Cygni well into this century. They hope the nova will supply answers to some of the puzzling questions it raised.

For starters, Starrfield and his team of researchers at ASU want to understand the evolution of the hydrogen fusion reactions taking place inside stars. They would also like to know the long-term effects of repeated explosions on the physical structure and evolution of the white dwarf star.

A nova binary star system consists of two stars orbiting one another under the mutual influence of gravity. The stars in the V1974 Cygni 1992 binary star system are located very close to one another. They are so close, in fact, that they revolve around each other in about two hours.

One of the two stars is much like our own Sun. The other star is very different. Scientists call it a white dwarf star. White dwarf stars contain about the same amount of material as our Sun. However, that material is mashed, compacted, and compressed into a volume the size of the Earth.

Astronomers know some things about white dwarfs. During an early stage of their life cycle, hydrogen gas inside the white dwarf undergoes nuclear reactions and fuses into helium, carbon, and oxygen. The density of the gas at the center of a white dwarf star is more than a million times that of water. As a result, a single matchbox full of white dwarf material would weigh many tons.

Because the stars of a binary system are so close to one another, the ultra-dense white dwarf star raises tides on the surface of its larger companion star. The process is similar to how our Moon causes on the oceans of the Earth. Unlike the Earth and the Moon, however, these tides cause hydrogen gas to flow, or fall, from the companion star onto the surface of the white dwarf.

As the falling hydrogen gas gradually collects on the surface of the white dwarf, Starrfield says the dwarf’s intense gravity compresses the gas, causing it to contract.

“Once an amount of gas 100 times more massive than the Earth accumulates on the white dwarf’s surface, then the density in the bottom layer becomes more than 10,000 grams per cubic centimeter. Comparatively, the density of water is one gram per cubic centimeter,” he explains.

The gas heats up as it is compressed. When the temperature reaches a few million degrees Kelvin (K), hydrogen fusion reactions are triggered in these layers. For comparison, consider that a fireplace fire is about 1,100 degrees K, the gas flame on your stove is about 1,900 degrees K, and molten lava is 2,000 degrees Kelvin.

Under these conditions, the hydrogen nuclei fuse into helium and release energy—the same reactions that power normal stars more massive in size than the sun. “The release of heat into the material causes it to become hotter. The fusion proceeds faster creating runaway thermonuclear reactions like those in a hydrogen bomb,” Starrfield explains.

Under normal conditions the gas would now expand and cool, effectively stopping the progression to a massive explosion. But conditions are different on a white dwarf.

“The material on a white dwarf behaves in a peculiar manner described by quantum mechanics,” Starrfield says. “It is packed together so tightly that the electrons are unable to interpenetrate. They become the source of pressure. The material, unlike an ordinary gas, heats up but cannot expand or cool.”

As a result, fusion reactions proceed faster and faster, heating the core layers to more than 30 million Kelvins. Violent mixing occurs between the surface layers and the core at these superheated temperatures. Mixing causes core gas to push to the surface. The conditions are now ripe for an explosion. Within minutes, the surface layers explode into space at speeds exceeding 3,000 miles per second.

“The layers become so bright that we can study nova explosions in distant galaxies,” says Starrfield.

Because the surface temperatures exceed 1 million Kelvins at maximum brightness, Starrfield has used X-ray satellites to study the earliest stages of nova explosions. X-ray studies showed that the nova outburst of V1974 Cygni lasted about 18 months.

“The life span of a nova depends on the mass of the white dwarf that hosts it,” says Starrfield. “A massive white dwarf compresses the accumulated gases more intensely. This causes fusion to begin early and consequently, the fuel runs out more quickly, causing the nova’s life to be brief.”

Starrfield and his colleagues used the Hubble Space Telescope to observe V1974 Cygni for the last time in September 1995. They were able to directly observe the white dwarf and confirm that the explosion was over.

The ASU astronomer and his colleagues now use the Multiple Mirror Telescope located on Mt. Hopkins in southeastern Arizona. They study the gas ejected into space and determine its chemical composition.

“To date, our results show that we are seeing core material from a white dwarf,” Starrfield says. “In no other astronomical object can we see core gases blown into space where they can be studied and provide data on stellar death.”—Lynette Summerill