APEX observations shed light on 17th-century star collision

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The rare collision is seen here in this composite image from the Gemini, SMA and APEX telescopes (Image: ESO/T. Kamiński)

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The European Southern Observatory's (ESO) Atacama Pathfinder Experiment telescope (APEX) has been used to solve a 340 year-old astronomical mystery. The findings reveal that an object that appeared in the sky in the 17th century was not a nova as astronomers at the time believed, but actually a rare stellar collision.

Accounts of the event, where a star suddenly appeared in the sky, have long been of interest to astronomers. Named in part for its year of discovery, Nova Vulpeculae 1670 was easily visible to the naked eye when it first appeared, but its remnants are now so faint that submillimeter observations were required to shed light on the mysterious event.

What has puzzled both modern and historical astronomers about Nova Vul 1670 is the manner of its apparent arrival and subsequent disappearance. According to historical accounts, the object was clearly visible in the night sky for two years after its first appearance, before disappearing and reappearing twice, and then finally vanishing.

While astronomers have studied the patch of sky for centuries, it wasn't until the 1980s that a faint nebula was detected, and even then, available technology wasn't equal to the task of deciphering what had actually been witnessed more than 300 years ago. The new observations finally answer the centuries-old riddle.

A chart showing the position of the assumed nova (red) that was first observed in 1670 and recorded by the astronomer Hevelius, published by the Royal Society in England (Image: ESO/Royal Society)

"We have now probed the area with submillimeter and radio wavelengths." said study lead author Tomasz Kamiński. "We have found that the surroundings of the remnant are bathed in a cool gas rich in molecules with a very unusual chemical composition."

The observations were carried out by the APEX telescope, the Submillimeter Array (SMA) and Effelsberg radio telescope. Together, the three installations delved into the chemical composition of the gas, measuring the ratios of different isotopes to reveal how the material came to be in its current state.

The findings reiterate that the event was not a nova, with isotope ratios not matching up to those expected following such an occurrence, in addition to too great a mass of cool material being present.

Instead, the data matched up perfectly with what would be expected following a spectacular stellar collision known as a red transient – a rare explosion caused by the merging of stars, resulting in the expulsion of stellar material into space. Repeated dimming and brightening is characteristic of such an event, neatly explaining the reports of the object's strange behavior upon its appearance in 1670.

Study co-author Karl Menten commented on the observations, stating, "This kind of discovery is the most fun: something that is completely unexpected."

Source: ESO

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