Like celebrities, the brightest stars have close companions
Like celebrities who never travel without a companion, the brightest stars in the universe generally have their own companion: a second star that orbits close by. At least three-quarters of these extremely bright, exceptionally hot stars have such companions, according to the first survey of so-called O-type stars.
An estimated 20% to 30% of the binary pairs will eventually merge, astronomers reported Thursday in the journal Science, and as many as another 50% of the O-type stars will have much of their hydrogen stripped away by their companions, which are commonly called vampire stars.
O-type stars account for less than 1% of the stars in the universe, but they are the brightest and heaviest. With a mass at least 15 times that of our own sun and surface temperatures exceeding 30,000 degrees Celsius, they shine a brilliant pale blue and are as much as a million times brighter than the sun. Although they are rare, they have a disproportionate effect on their surroundings. The winds and shocks coming from them 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. They are thus associated with many of the mechanisms that drive the evolution of galaxies,
An international team of astronomers headed by Hugues Sana of the University of Amsterdam studied 71 O-type stars using the European Southern Observatory’s Very Large Telescope (VLT) in Paranal, Chile. The VLT is composed of four main telescopes with 8.2-meter (323-inch) mirrors and four movable auxiliary telescopes with 1.8-meter (71-inch) mirrors. Even with the high resolution provided by the VLT, it is not possible to see the individual points of light emitted by the binary stars. But the VLT allows researchers to conduct interferometry that shows that three-quarters of the objects they studied comprised two light sources moving in orbits around each other.
More importantly, they found that the proportion of the pairs that were close enough to interact with each other was far higher than researchers had previously thought. Until now, astronomers had thought that close-orbiting binary stars were the exception, something that was needed only to explain exotic phenomena such as X-ray binaries, double pulsars and black hole binaries. The new study shows that they are much more common and their lives are fundamentally different from single stars. In particular, as the vampire star strips hydrogen from its brighter companion, both will become hotter and bluer in color, appearing younger than they actually are. If astronomers do not account for this, they may incorrectly calculate the age of distant galaxies.
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