Christine Nicholls

PhD candidate, 2008

We know more about these stars than ever before. Every day we're closer to solving the mystery.

Astronomical sleuthing is all in a day's work for Christine Nicholls, who is attempting to solve the mystery of Long Secondary Periods in red giant stars.

Nicholls, an ANU PhD student at Mount Stromlo Observatory in Canberra, is analysing the light and velocity variations of evolved stars in the Large Magellanic Cloud, a nearby galaxy. Red giants are stars in the later stages of their life, and are puffy, red, bright and cool - by stellar standards. They also pulsate, and lose mass through outflows known as stellar winds. 

"It's a pretty unstable time," she says. "Kind of like a star's mid-life crisis!". 

In time, our Sun will become a red giant, and show the same behaviour.

What's mysterious is when stars start multi-tasking. Stellar pulsations are easy to identify and understand, and in the simplest scenario the star appears to breathe in and out. Some red giants, however, show different - and slower - variations, at the same time as the normal pulsations. These Long Secondary Periods, as they are known, are still something of an enigma.

"We know they can't be the normal sort of stellar pulsation. Perhaps they're a different, more complex pulsation. Another possibility is that the Long Secondary Period is caused by a companion star, orbiting the first one."

By analysing all available data and testing the possible scenarios, Nicholls hopes to find an explanation that matches the Long Secondary Periods.

She's working with two forms of data: light and velocity variations. By monitoring a star frequently over several months or years, it's possible to build a picture of the variations present. The velocity curves she's working with were taken specifically for the project at the Very Large Telescope (VLT) in Chile, but the light curves were selected from the MACHO database. Many long-term astronomical monitoring projects, like the MACHO Project undertaken by an international collaboration at Mount Stromlo from 1992 to 1999, are still useful to astronomers around the world for their wealth of data.

From these two positions, Nicholls can study almost everything about her stars. So far she's derived stellar temperatures, radii, and many features such as velocity amplitudes and phase relations between the light and velocity curves. All of these, she maintains, give different points of view on what could be causing the elusive Long Secondary Periods.

"At this stage, we know more about these stars than ever before," she says. "Every day we're closer to solving the mystery."

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