High-mass stars are rarely solitary: Binary stars are more common than thought

The sun sinks below the horizon; darkness falls above the Atacama Desert in Chile. A few spots are flickering in the sky, more and more emerge gradually, until the broad ribbon that is the Milky Way stretches across the firmament. There is no place in the world better suited for conducting astronomical observations than this one. Here, the RUB's astronomical observatory is located, and here, the team headed by Prof Dr Rolf Chini has made some truly amazing discoveries.

The RUB observatory is situated a mere 20 kilometres from the largest European observatory, the "Very Large Telescope," on Cerro Paranal. "This does pose the question what it is that we 'amateurs' will be able to accomplish there with our small instruments," says the Head of the Institute of Astronomy. The answer is: actually quite a lot. The RUB telescopes do not reach as high a resolution as the large telescopes. However, the astronomers can take their time observing the sky. "The average astronomer is assigned five to ten hours observation time per year on the large telescopes -- if he's lucky," explains Chini. This makes it impossible to keep an eye on celestial objects over the span of several days or weeks. But this is precisely what the RUB team has to do for their specific queries.

The astronomers from Bochum are interested in variable phenomena. They analyse how the brightness of stars changes over long stretches of time. "A few years ago, it began to emerge that high-mass stars, which are about one hundred times as heavy as our sun, preferably occur in the form of binary stars," tells us Rolf Chini. Together with his team, he has been systematically investigating this phenomenon. "I used to think: if you know how a single star works, you also know how a binary star works. However, this is wrong." The Bochum astronomers analysed all 800 high-mass stars that they were able to see from their location in Chile. More than 90 per cent turned out to be multiple systems, consisting of between two and four stars that orbit each other.

Why, though, has this never been picked up in the data of the world's largest telescopes? "Generally, those stars are so close to each other that they can't be distinguished as two discrete points," says Chini. Therefore, the astronomers from Bochum came up with a trick. They split the light of the stars into different wavelengths. A star's chemical composition determines at which wavelengths it emits light; this is referred to as spectral lines. An analysis of the spectral lines shows whether something that is supposed to be a single star is really composed of several stars.

The researchers took advantage of the fact that stars orbit each other in multiple systems, generating the Doppler effect. That effect is familiar to everyone who has ever listened to the siren of a passing ambulance. When the ambulance approaches, the sound is pitched higher than when the vehicle moves away. Something similar happens with light waves, too. When a star moves towards the observer, it emits light at shorter wavelengths, i.e. more blue, than would be the case with a star that does not move. When a star moves away, the emitted light is shifted towards longer wavelengths, that means into the red range of the spectrum.

Chini's team noticed in many of the stars they investigated that their spectral lines changed on a regular basis, shifting periodically between the blue and the red range. Thus, the apparent single star was really a binary where two stars orbited each other and, consequently, took turns with moving closer to and further away from Earth. In some systems, the astronomers discovered as many as three or four stellar partners. Based on those data, they calculated the orbital period of each multiple system, i.e. how long it takes the stars to orbit each other fully.

The statistical analysis indicated moreover: the heavier a star, the higher the probability that it is not alone. On the whole, star partners who have a similar mass occur together. According to Rolf Chini, this is no coincidence: "Why should a star of 50 solar masses capture a partner of likewise 50 solar masses? It would be much easier to attract a star of only one solar mass. Surely, the stars' formation process is what provides the explanation for this phenomenon." Chini assumes that high-mass stars emerge as twins: the celestial objects originate from gas and dust clouds which then become dense. In the final stage, the cloud apparently splits into two similar-sized parts.

Today, models do exist that explain this process. In fact, scientists have attempted in vain to come up with a theory explaining the formation of single high-mass stars. The RUB astronomers' observations have now given them a good reason for considering alternative models of star formation.

In order to conduct the measurements, a member of Chini's team was present at the observatory in Chile at all times. The RUB observatory is connected with Bochum via the Internet, and all telescopes can be essentially controlled from Germany. "Still, it would be impossible to do all this without having a person on site," he says. "We would require an infrastructure such as is in use in satellites, which would be infinitely expensive."

The director of the RUB observatory has always enjoyed watching the sky: "I spent the first money I'd ever earned myself on a small telescope from Tchibo, and I would sneak out at night to the fields." Today, he travels to the Atacama Desert four to six times a year, in order to attend to maintenance, repair and day-to-day operations. The infrared camera has to be filled with liquid nitrogen every day -- for cooling purposes. "Being in Chile is great," says Chini. "However, getting there is very stressful." From Germany, via Madrid, to Santiago de Chile, followed by a domestic flight, and finally a two-hour drive up the mountain in a 4x4 -- reaching the world's prime location for astronomical observations is not easy. However, a bred-in-the-bone astronomer will not let a little thing like that deter him.