Feature

Twin Stars

Computer analysis and modelling of light variations from binary stars may lead to a better understanding of stellar processes.

By Tim Banks

Researchers at Carter Observatory and Victoria University are using sophisticated computer programs to model the changing light from twin stars as they rotate around each other. By comparing the modeled light variations with real observed variations, they are able to learn more about the structure of, and relationship between, binary stars.

A pair of stars revolving about a common centre of mass under the influence of their mutual gravitational attraction is called a binary star. In recent surveys of nearby sun-like stars, about half have been found to be twin stars.

Studies of these stars and the way they interact can tell astronomers a great deal about the component stars' absolute stellar masses, radii and densities. This information is useful in understanding how stars behave as they age, particularly since there is no direct means of measuring the mass of a single isolated star.

The effect of each star on its companion in a binary provides extra information that helps to fill in the extra details. For instance, the speed at which the stars orbit each other is directly dependent on their masses, and one can be calculated from the other.

Stars in close orbits can be distorted into pear shapes, as gravitational attraction between the two pulls matter from each star towards the other. Mass can be transferred as the stars age and, in some cases, the attraction is so great that a thin neck of matter joins the two stars.

Light Variations

As the stars rotate about each other, the brightness of the system as a whole changes over a period of time. When one star passes in front of the other, it can totally or partially eclipse it, lowering the brightness of the system. If the orbital plane is in our line of sight, these changes can be clearly observed.

When the brighter of the two stars is eclipsed in this fashion, a minimum, or period of dimming, is observed. The length of each minimum in these eclipsing binaries gives the relative sizes of the stars involved, and enables calculation of the relative stellar luminosities and the angle of their orbit relative to our line of sight.

The brightness variations during an entire orbital cycle form a light curve, which can be analysed to derive the system's photometric parameters, such as the orbital period. The eclipse duration, when compared to the times between two eclipses, indicates the stellar radii in units of the distance between the two stars.

The derivation of such physical parameters may be complicated by the effects of mass transfer and starspots. The latter are believed to be analogous to giant sunspots, the intense localised magnetic fields which appear as black blotches on the Sun's surface.

In the Sun's case, less than 1% of its surface is covered by sunspots, but binary stars appear to have as much as 30% coverage. These stars rotate many times faster than the Sun; the binary star CG Cygni spins some 50 times more rapidly. A rotating system, operating like a dynamo, is thought to be a significant mechanism in the production of strong stellar magnetic fields. Thus, it is not surprising that a quickly rotating star has more evidence of strong magnetic activity, such as starspots.

Starspots can give clues as to the internal structure of a star. We can't alter the strength of the Sun's magnetic field, but, by examining sun-like stars with different rotational speeds, it is hoped that we'll be able to gain an understanding of the Sun's interior. A better understanding could lead to our being able to predict the Sun's activity. This would be of obvious benefit, particularly in areas such as agriculture.

Computer Model

The computer programs developed by Carter Observatory and Victoria University researchers model the light variations of eclipsing binaries. Statistical measures show how close the fit is, and judicious adjustments to the theoretical parameters, such as stellar radii or masses, allow close representation.

The researchers have been using a similar program to model starspots. The models have shown that starspots prefer higher latitudes than the sunspots on our own single star. These stellar spots cluster into two longitudinal bands on opposite sides of the star. Interestingly, this banding has not been definitely found for the single star AB Doradus, suggesting that the presence of two stars in a binary system affects the distribution of starspots.

It is hoped that modeling starspot light curves over a long time period will result in identifying a complete activity sequence, similar to the Sun's sunspot cycle. The latter varies in intensity over an 11-year period.

Binary studies of white dwarf stars have revealed violent fusion explosions, which have provided information used in high-energy physics and studies of compressed matter.

Violent supernovae, the death throes of stars, can involve binaries, and the binary Cygnus X-1 remains the best candidate for the first observed black hole. Even the mass transfer between the stars can tell us things about fluid flow and hydrodynamics.

For these reasons, and many others, binary stars will remain an active research area for years to come.

Tim Banks is a PhD student at Victoria University, doing work at Carter Observatory.