Most of the biggest stars in the galaxy have smaller companions that are leeching their lifeblood from them like cosmic vampires, astronomers have discovered. Nearly three quarters of the very bright, high-mass stars that drive the evolution of galaxies possess a partner, a figure far higher than had been thought before.
Most of these pairs are also suffering powerful disruption, such as the transfer of material from the bigger star to the smaller, the violent act that caused scientists to draw the vampire analogy. A third of them are expected to end up becoming a single star.
The new findings, published in the journal Science, are the result of a study using the European Southern Observatory’s Very Large Telescope (VLT) in Chile’s Atacama Desert.
The target stars are unlike our much smaller Sun. Known as O-type stars, they have very high temperature, mass and brightness. They also live short but violent lives.
Astronomers first used a variety of ESO telescopes to look at 71 of what were thought to be giant single stars plus stars in binary systems in nearby clusters within the Milky Way.
By studying their light in greater detail than ever before, the team discovered that 75 per cent of all O-type stars exist inside binary systems, a higher proportion than had been thought, and the first precise determination of this number. More importantly, they found that the proportion of these pairs that are close enough to interact is far higher than anyone had imagined. They say this has profound implications for cosmologists’ understanding of galaxy evolution.
Lead author of the study Hugues Sana, of the University of Amsterdam, in the Netherlands, said: “These stars are absolute behemoths. They have 15 or more times the mass of our Sun and can be up to a million times brighter. These stars are so hot that they shine with a brilliant blue-white light and have surface temperatures over 30,000 degrees Celsius.”
O-type stars make up just a fraction of a per cent of the stars in the Universe, but the violent phenomena associated with them mean they have a disproportionate effect on their surroundings. The winds and shocks coming from these stars can both trigger and stop star formation, their radiation powers the glow of bright nebulae, their supernovae enrich galaxies with the heavy elements crucial for life, and they are associated with gamma-ray bursts, which are among the most energetic phenomena in the Universe. O-type stars are therefore implicated in many of the mechanisms that drive the evolution of galaxies.
How a hot, high-mass binary evolves. Both stars eventually explode as supernovae. Credit: ESO/L. Calçada/M. Kornmesser/S.E. de Mink
“The life of a star is greatly affected if it exists alongside another star,” says Selma de Mink, of the Space Telescope Science Institute, in the USA, a co-author of the study. “If two stars orbit very close to each other they may eventually merge. But even if they don’t, one star will often pull matter off the surface of its neighbour.”
Before this study, astronomers mostly thought that closely-orbiting massive binary stars were the exception rather than the rule, something that was only needed to explain exotic phenomena such as X-ray binaries, double pulsars and black hole binaries. The new study shows that to properly interpret the Universe, this simplification cannot be made – these heavyweight double stars are not just common, their lives are fundamentally different from those of single stars.
For example, in the case of vampire stars, the smaller, lower-mass star is rejuvenated as it sucks fresh hydrogen from its companion. Its mass will increase substantially and it will outlive its companion, surviving much longer than a single star of the same mass would normally. The victim star, meanwhile, is stripped of its envelope before it has a chance to become a luminous red super giant. Instead, its hot, blue core is exposed. As a result, the stellar population of a distant galaxy may appear to be much younger than it really is.
Hugues Sana added: “The only information astronomers have on distant galaxies is from the light that reaches our telescopes. Without making assumptions about what is responsible for this light we cannot draw conclusions about the galaxy, such as how massive or how young it is. This study shows that the frequent assumption that most stars are single can lead to the wrong conclusions.”