Discovery of ‘triple star’ could revolutionize understanding of stellar evolution

A groundbreaking new discovery by scientists at the University of Leeds could transform the way astronomers understand some of the largest and most common stars in the Universe. The article, “Gaia Discovers Difference in Binarity of B and Be Stars on Small Scales: Evidence for Mass Transfer Causing the Be Phenomenon,” was published in the journal Monthly Notices of the Royal Astronomical Society.

Research by Ph.D. student Jonathan Dodd and Professor René Oudmaijer, from the University’s School of Physics and Astronomy, points to intriguing new evidence that massive Be stars – until now mainly thought to exist in double stars – could in fact be “triples”.

The remarkable discovery could revolutionize our understanding of the objects – a subset of B stars – that are considered an important “test bed” for developing theories about how stars evolve more generally.

These Be stars are surrounded by a characteristic disk made of gas – similar to Saturn’s rings in our solar system. And although Be stars have been known for around 150 years – having first been identified by renowned Italian astronomer Angelo Secchi in 1866 – until now, no one knew how they were formed.

So far, the consensus among astronomers says that the disks are formed by the rapid rotation of Be stars, and this may be caused by the stars interacting with another star in a binary system.

Triple systems

Dodd, corresponding author of the research, said: “The best reference point for this is that, if you’ve watched Star Wars, there are planets where they have two suns.”

But now, by analyzing data from the European Space Agency’s Gaia satellite, scientists say they have found evidence that these stars actually exist in triple systems – with three bodies interacting instead of just two.

Dodd added: “We look at the way stars move across the night sky, over longer periods, like 10 years, and shorter periods, like about six months. If a star moves in a straight line, we know there is only one star, but if there is more than one, we will see a slight wobble or, at best, a spiral.

“We applied this to the two groups of stars we are observing – the B stars and the Be stars – and what we found, confusingly, is that at first glance it appears that the Be stars have a lower companion rate than the Be stars. B. stars. That’s interesting because we expect them to have a higher rate.”

However, lead researcher Prof Oudmaijer said: “The fact that we are not seeing them could be because they are now too faint to be detected.”

Mass transference

The researchers then analyzed a different set of data, looking for companion stars that were more distant, and found that at these larger separations, the rate of companion stars is very similar between B and Be stars.

From this, they were able to infer that, in many cases, a third star is coming into play, forcing the companion closer to the Be star – close enough that mass can be transferred from one to the other and form the characteristic Be stellar disk. This could also explain why we no longer see these companions; they became too small and faint to be detected after the “vampire” star Be absorbed much of its mass.

The discovery could have huge impacts on other areas of astronomy – including our understanding of black holes, neutron stars and sources of gravitational waves.

Professor Oudmaijer said: “There is a revolution happening in physics at the moment around gravitational waves. We only observed these gravitational waves a few years ago and it turned out that they are due to the merger of black holes.

“We know that these enigmatic objects — black holes and neutron stars — exist, but we don’t know much about the stars that would become them. Our findings provide a clue to understanding these sources of gravitational waves.”

He added: “Over the last decade, astronomers have discovered that binarity is an extremely important element in stellar evolution. We are now moving further towards the idea that it is even more complex than that and that triple stars need be considered.”

“In fact,” said Oudmaijer, “triples have become the new binaries.”