Dances of the stars under scrutiny

Chandra X-ray Observatory Centre.

Chandra X-ray Observatory Centre.

This notion would remain unchallenged until the invention of the telescope centuries later, when changes in brightness that had been imperceptible to the unaided eye, could now be detected. By recording these changes — in the 17th century, they were painstakingly noted by hand — astronomers were able to monitor how the brightness of these stars changed with time: a flickering that sometimes took hours and in other examples years. And these almost imperceptible changes may be the Morse code that can help us understand the evolution of the universe and our own galaxy.

This is not an easy task. These stars became known as variable stars, but they do not vary in the same way or for the same reasons. Sometimes the changes in brightness are because of what is happening inside these huge shining balls of gas, such as when a star pulsates. At other times, it can be because of the environment around the star.

To the naked eye, the stars in the night sky appear to be single stars, a sprinkling of glitter against the black of night. But in the cold vastness of space, most of the stars are not single stars at all; sometimes two stars engage in an intricate dance around each other, gravitationally bound together. This is called a binary system and occasionally, during the course of their orbits, the two stars will pass in front of each other, blocking some of the light that reaches Earth. This is called an eclipsing binary star, and this phenomenon is vital to our understanding of stellar evolution. 

The star’s evolutionary path is determined by its mass, but it is difficult to determine the fundamental parameters of a star (such as the mass and radius) from Earth if the star is on its own. If the star is a part of an eclipsing binary system, we can track how the two stars move around each other, the shape and speed of their dance, and from this we can determine their individual masses. The distance between these dancing stars varies, but eclipsing contact binary stars are particularly interesting because they are so close that they touch each other.

They look a bit like peanuts in their shell — two giant, glowing separate nuts surrounded by a common shell and connected in the middle. Although we have a model for these stars, how they form and what they ultimately evolve into is still unknown. It does not help us that they are not static and their behaviour changes over time, and from peanut to peanut.

How do we identify eclipsing contact binary stars whose parameters, such as brightness and orbital period, are changing? We keep an eye on how their brightness changes over time. 

Picture an in-shell peanut sideways on, so that you see both ends as well as the neck in the middle. When an eclipsing contact binary is in this position in its orbit, the observer sees the light from both stars and the system is at its brightest. As the peanut revolves, the brightness dims as one star passes in front of the other, blocking the light of its mate. This stellar rotation can take anywhere from five hours to a day to complete. The time it takes is known as the orbital period of the system.

But sometimes this peak brightness is not constant. This is because the stars are not static; they are spinning collections of gas with magnetic fields. When a magnetic field line pokes through the surface of a star, a spot forms. This spot is cooler than the surrounding area and looks like a dark blemish on the surface of the star. Spots produce a measurable change in the brightness of system.

Sometimes the stellar dance takes a little shorter or longer to complete. Why this happens is a mystery: is it because one star is passing stellar matter on to its companion or is it something else, like another star that we can’t see? For eclipsing contact binary stars whose orbital period is changing, the difference between the old and new orbital period is in the region of 10 milliseconds a year. To put this in context, a blink of an eye takes about 200 milliseconds, which is 20 times longer than what we are trying to measure, so our measurements need to be incredibly accurate to detect these small changes. 

There are many unanswered questions, and it does not help that these stars live far longer than the lifespan of a human. This is why eclipsing binary stars that undergo changes over the course of a year are so important. Studying as many of them as we can with the use of cutting edge instruments such as the giant radio telescope the Square Kilometer Array and space-based telescopes may just help to shed some light on the mysteries of these stars. 

Patricia Skelton is a PhD candidate at the University of South Africa.

This publication is the culmination of a six-month-long Mail & Guardian project, called Science Voices, to teach postgraduate science students how to turn their academic writing into something the public can read and enjoy.