Binary Stars – The Basics

Article by: Gavin Ramsay

Unlike our Sun, around half of all stars have another stellar companion. The closest stars to the Sun are two bright southern stars, alpha Centauri A and B, which orbit each other every 80 years and Proxima Centauri which orbits A and B every few tens of thousands of years – and is currently the closest star to the Sun.

The timescale it takes for two stars to orbit one another – the orbital period – can vary enormously. In some cases, if the orbital period is long and they are relatively nearby, telescopes can resolve the two stars. However, in most cases the binary star appears to be a single star when viewed through telescopes. So how do we know if a star is a binary?

One of the astronomer’s most useful tools is a spectrograph. This splits light from a star into a rainbow. Unlike the rainbow, which we sometimes see in bright showery days, spectrographs can resolve spectra from stars can resolve fine details which are called absorption lines. By taking a series of spectra of the same star it can sometimes reveal shifts in the lines caused by two stars orbiting around a common centre of gravity.

With multiple spectra of the same object astronomers can determine the orbital period. However, because we don’t know the orientation that the stars are orbiting (the inclination) we are unable to derive important quantities like the mass of each star.  In some binaries it just so happens that the inclination is such that once every orbit one star obscures the other star, causing an eclipse accompanied by an apparent dimming in the stars brightness.

In eclipsing binaries we know the inclination and through Kepler’s law we can then determine the mass of the two stars. This allows astronomers to test stellar models which predict the mass of stars of different types.

For those wanting more detail, it can be found HERE.

Most stars in binary systems are sufficiently far apart that they have no effect on the internal structure of the other star. However, there are some binaries which have a complex history. More massive stars evolve at a quicker rate than less massive stars. The more massive star will turn into a white dwarf once it has exhausted most of its hydrogen. White dwarfs are usually slightly less massive than the Sun, but have the same size as the Earth – they are dense objects. The resulting binary containing a white dwarf and a low mass normal star have a much shorter orbital period.

In accreting binaries the two stars are so close together that material from the low mass star gets transfered onto the white dwarf through what is called an accretion disc.

See image to see what an accretion disk might look like:

In dwarf novae the system brightens up every few weeks or months when material from the accretion disc is released onto the white dwarf. One well known dwarf nova visible in the summer months is SS Cyg.

More information on SS Cyg can be found HERE.

SS Cyg has an orbital period of 6.6 hrs. Other types of dwarf nova have even quicker periods of just over an hour. However, there are some binaries which are made up of two white dwarfs orbiting around each other. The fastest binary is called HM Cnc and it has an orbital period of 321 sec (5.4 mins). This was originally discovered as an X-ray source, but in 2002 myself and some colleagues identified the optical counterpart which was also varying in its brightness every 321 sec as well. Later, optical spectra showed that this was the orbital period. It is so compact that the entire binary could fit into the volume of Jupiter. It is predicted to emit copious amounts of gravitational waves and be easily detected by the ESA space mission LISA.