Astronomers discover 2,000-year-old nova remnant

M22

ARI researcher Sebastian Kamann helps confirm ancient Chinese sighting

For the first time, researchers have discovered the remains of a nova in a globular cluster. A nova is a transient astronomical event, observed when a star suddenly becomes almost 100,000 times brighter than the Sun and then slowly fades. This happens in binary systems – one star accretes gas from its companion, triggering an eruption on the surface of the accreting star. The material ejected in the eruption forms a slowly expanding nebula that can survive for centuries, long after the nova has faded. It was the glow from such a nebula that Sebastian Kamann helped to discover, in the Milky Way globular cluster Messier 22 located in the constellation Sagittarius.

"The position and brightness of the remains match an entry from 48 BC in an ancient collection of observations by Chinese astronomers," says Fabian Göttgens at the University of Göttingen, lead author of the study. This makes the ancient sighting one of the oldest observations of an event outside the solar system confirmed using modern instrumentation. The newly discovered nova remnant has a diameter of about 8,000 times the distance between the Earth and Sun. Despite its size, the nebula does not contain a large amount of material, with a mass around 30 times greater than Earth.

The observations were made with MUSE, an integral field spectrograph at the Very Large Telescope – one of the largest optical telescopes in the world – operated by the European Southern Observatory in the Atacama Desert in Chile. MUSE is currently being used to carry out a large survey of star clusters in the Galaxy and the local neighbourhood. “It is a bit funny,” says Sebastian Kamann, one of the initiators of the survey, “The centres of globular clusters like Messier 22 are some of the most frequently observed patches of the sky, but nobody ever saw the nebula.” Nova remnants only emit light in specific narrow wavelength ranges, and since most cameras integrate light over a much wider wavelength range, the remnants disappear in the noise. The power of MUSE is that instead of taking one image, it takes 3,500 monochromatic images simultaneously, each at a slightly different wavelength. “The nebula revealed itself in about a dozen of these images, while it remained invisible in the rest”, Kamann adds.

The MUSE observations not only enable the discovery of faint nebulae, but also the study of the stellar motions. In a previous publication based on the MUSE data, Kamann and his colleagues found that the motions of the stars in the clusters are not completely random; the clusters as a whole are slowly rotating. In addition, the data confirmed for the first time the presence of a stellar-mass black hole in a star cluster.

The results will be published in the journal Astronomy & Astrophysics.