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NSF Grant helps NMSU astronomer study strange nova

Something strange is happening in the constellation Cassiopeia and New Mexico State University astronomer Joni Johnson wants to know why.


The object of Johnson's interest is a nova that erupted in 1995. Novae -- stars that suddenly flare up and become thousands of times brighter than normal -- are dramatic but not uncommon events. What makes Nova Cassiopeiae 1995 so "weird," said Johnson, is that it's still unusually bright years later, whereas the typical nova settles back down to normal brightness in a matter of weeks.

"I'm trying to find out why it's different," the NMSU scientist said.

Johnson's research has attracted the attention of the National Science Foundation, which recently awarded her an $81,000 grant through its Professional Opportunities for Women in Research and Education (POWRE) program. She is one of 206 women nationwide to receive 1999 POWRE grants, which are intended in part to help overcome the under-representation of women in science and engineering. Only 39 grants were awarded in the category of mathematical and physical sciences, which includes astronomy.

The term "nova" comes from the Latin "nova stella," which means "new star." It's a misnomer that has stuck over the centuries.

"If you saw a nova explosion with the naked eye, you'd suddenly see a star where there wasn't one before," Johnson said. Early astronomers thought they were in fact new stars, but scientists now know they are eruptions of light from existing stars, caused by violent explosions.

Classical novae, like Nova Cassiopeiae 1995, are binary systems in which two stars -- a white dwarf and usually a red dwarf -- are bound by gravity. A red dwarf is a cool, small star that is still burning hydrogen at its core. A white dwarf is a much older, hotter star that has used up its hydrogen fuel.

"They are close enough that the white dwarf pulls material from the red dwarf as they orbit, and eventually enough mass is transferred to the surface of the white dwarf that you have a thermonuclear runaway," Johnson explained. "When this happens the brightness increases by a factor of 10,000 to 100,000."

With most novae, the brightness starts decreasing after a few days and is back to normal in a few weeks, she said. "The one I'm studying, called Nova Cas for short, went off in 1995 and still hasn't gone back to normal brightness," she said.

To find out why it's different, she uses the 3.5-meter telescope at Apache Point Observatory, high in the Sacramento Mountains near Cloudcroft, to look at the nova's spectrum -- its brightness as a function of the wavelength, or color, of the light. "The spectrum tells you what elements are in the object you're looking at," she said. "You can also use the spectrum to determine temperature and density of an object."

Surrounding Nova Cas 1995 is a bubble-like shell of material that extends beyond the two-star system, given off as a result of the thermonuclear runaway. The material is mostly hydrogen, Johnson said, but it also includes carbon, oxygen and nitrogen produced by the explosion.

"We use the spectrum as a fingerprint," she said. "The chemical makeup of this material will tell us what happened in the runaway."

Eventually this cast-off material becomes part of the intergalactic medium from which new stars and planets are formed. "That's how we came to be," Johnson said, explaining that scientists believe our solar system formed from material left over from a supernova explosion.

A supernova, as the name suggests, is an event that is bigger and more violent than a nova, usually signaling the death of a massive star.

The life cycles of stars are becoming better understood thanks to the work of researchers like Johnson. She began studying Nova Cas 1995 just a week after it was discovered in 1995, while she was at the University of Wisconsin at Madison, using its Pine Bluff Observatory. By observing the dramatic changes in the nova's spectrum over the years since, she and her colleagues have been putting together a detailed picture of the strangely behaving object.

Johnson is using part of her NSF grant to hire Jennifer Hoffman, an NMSU doctoral student in astronomy, as a research assistant.