WASHINGTON D.C. -- For about a month back in late 1999, there was an extra star in the night sky. Anyone who knew where to look could have spotted Nova Aquila, an aged stellar corpse that had just exploded -- or "gone nova," as scientists say.
For a time, the decrepit star, which had evolved into something called a fuel-burning white dwarf, put out more energy than our Sun -- as in 100,000 times more.
It took 100,000 years for the star to muster the energy and ingredients that fueled the explosion. For nearly two years, researchers have been trying to figure out what happened. The results, which included several surprises announced last week, fuel new mysteries associated with these rather common explosions that have been recorded, if not understood, for ages.
Going nova
Novae (the plural for nova) are usually, if not always, the result of an interaction between two stars.
In such a binary system, as they are called, an ancient and exhausted star called a white dwarf, which has used up all of its nuclear fuel, siphons hydrogen gas off its orbital companion, typically a much larger normal star. Every 100,000 years or so, enough gas builds up on the white dwarf to trigger a thermonuclear explosion.
Earthlings have been noting these cosmic-sized hydrogen bombs for at least 1,000 years.
Chinese astronomers noted more than 20 novae and supernovae dating as far back as the year 1006, for example.
In 1572, the astronomer Tycho Brahe saw a new star in the sky, in the constellation Cassiopeia. He wrote a book about it, "De Nova Stella," or "About the New Star." Since then, whenever a "new" star appeared in the sky, scientists called it a nova.
As early as the 1930s, astronomers realized that one class of these objects, supernovae, were something far more powerful.
So despite being widely rooted in the public consciousness -- perhaps helped by the public television program "Nova" -- novae don't get as much attention as their more explosive kin. But scientists are still studying them and still finding puzzles.
X-ray vision
Sumner Starrfield, an Arizona State Researcher, has been looking into these objects for decades, since before anyone realized that two stars were involved. And now he has a new tool. With the Chandra X-ray Observatory, Starrfield can examine the X-ray emissions of novae. X-rays, like visible light, are one type of electromagnetic radiation. But they are far more powerful.
When Starrfield and his colleagues used Chandra to observe the aftermath of Nova Aquila, planning to study the types of gases that it had tossed into space, they instead discovered a handful of unexpected phenomena, things they didn't even know they should look for.
As Nova Aquila wound down in brightness between April and October 2000, when the observations were made, its X-ray output was seen to pulsate over periods lasting roughly 40 minutes.
"We have never seen a nova perform in this way," Starrfield said last week at a Chandra Observatory symposium here. "We don't know what causes this."
A colleague of Starrfield's, Jeremy Drake of the Harvard Smithsonian Center for Astrophysics, speculated that the pulsations might represent the outer layers of the white dwarf expanding and shrinking.
But then came a stark spike of activity.
In one swift burst -- just 15 minutes long -- the X-ray output jumped to six times the previous levels. Again, an explanation will require further study. But in an interview, Starrfield said the spike could have been similar to flares on our Sun, when twisted magnetic fields snap and hurl hot gas into space. Or, he said, a blob of hot gas might have crashed onto the surface of the white dwarf.
More observations will be needed to determine whether such flares are typical of novae or if this one is uncommon, said R. Mark Wagner, a team member from the University of Arizona.
And more surprises
Nova Aquila had more than temper tantrums in store for its X-ray observers.
"At least as important, our Chandra observations told us that thermonuclear fusion reactions were still occurring on the surface layers of the white dwarf," said Robert D. Gehrz of the University of Minnesota. "Eight months after discovery, the explosion was not over."
The research team still has to analyze its data to explore the composition of the bubble of gas cast into space by the explosion. They estimate it contains as much mass as 30 Earths. The bubble has grown to envelope the entire binary star system -- not just the white dwarf, which is about the size of the Earth, but also the larger companion star, which is roughly the diameter of our Sun.
The gas is racing outward at 4 million mph (6.4 million kph) and will eventually combine with other material and help form new stars.
The researchers will search their data on the bubble for, among other things, aluminum. It is thought that some of the aluminum in our solar system came from ancient nova. Novae also play a role in creating much of the universe's nitrogen, a element crucial to the creation of life.
Though Nova Aquila was briefly 100,000 times brighter than our Sun, it didn't appear that bright on Earth because it is roughly 6,000 light-years away, in the constellation Aquila. Still, it was the brightest nova visible from the Northern Hemisphere since 1975.
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