Astronomers rediscover ancient ‘lost’ nova first spotted six centuries ago

A team of astronomers have pinpointed the location of a nova that was last seen almost 600 years ago.

In 1437, Korean astrologers spotted a bright new star in the tail of the constellation Scorpius and observed it for 14 days before it faded from view. Studying the ancient record made by the Royal Imperial Astrologers, modern astronomers determined that what they had seen was a nova explosion but, until now, they were unable to find the binary system that caused it.

“This is the first nova that’s ever been recovered with certainty based on the Chinese, Korean, and Japanese records of almost 2,500 years,” said the study’s lead author Michael Shara, a curator in the American Museum of Natural History’s Department of Astrophysics.

An image of the nova, which was first identified in 1437, taken with the Carnegie SWOPE 1-meter telescope in Chile. Image© K. Ilkiewicz and J. Mikolajewska. Above: a Korean star chart (rotated) first created in 1395, and reproduced in 1687. Image courtesy of Seoul National University / National Geographic Information Resource Map Museum

A nova is essentially a hydrogen bomb on a gigantic scale, produced in a binary system where a star is being devoured by a white dwarf (a dead star). Over roughly 100,000 years, the white dwarf builds up a critical layer of hydrogen that it then blows off, producing a burst of light that can make the star up to 300,000 times brighter than the sun for any period from a few days to a few months.

Shara has tried to locate the Korean nova for several years, teaming up with Durham University’s Richard Stephenson, a historian of ancient Asian astronomical records, and Liverpool John Moores University astrophysicist Mike Bode.  Their recent success came after expanding the search field and discovering the ejected shell of the classical nova. The finding was confirmed with a photographic plate from 1923 taken at the Harvard Observatory station in Peru.

“With this plate, we could figure out how much the star has moved in the century since the photo was taken,” Shara said. “Then we traced it back six centuries, and bingo, there it was, right at the centre of our shell. That’s the clock, that’s what convinced us that it had to be right.”

Photographic plates of the nova taken over six weeks in 1942. Image ©Harvard DASCH

The Peru plate is available online as part of the Digitizing a Sky Century at Harvard (DASCH) project and it was other such DASCH plates that helped reveal the system has now become a dwarf nova. The discovery supports the idea that novae go through an incredibly long-term life cycle, fading for thousands of years after eruption before slowly building back up to full-fledged nova once more.

It was previously believed that “cataclysmic binaries” – novae, novae-like variables and dwarf novae—were separate entities, but the rediscovery of this ancient nova as a dwarf suggests instead that they are one and the same, but at different stages in their lives. Following an eruption, a nova becomes nova-like, before taking the form of a dwarf nova.  Then there may be a period of hibernation, after which it becomes nova-like again and then a fully fledged nova. This cycle repeats, potentially up to 100,000 times over the course of billions of years.

The study, which is published in the journal Nature, was based on observations from the Southern African Large Telescope (SALT), and the Las Campanas Observatories’ Swope and Dupont telescopes.

Two potentially habitable planets discovered just 12 light years away

Astronomers have identified four Earth-sized planets orbiting a star 12 light years from our own Sun, two of which are in the habitable zone and so could be host to liquid water.

The planets, which are thought to be around 70% larger than Earth, orbit a star known as tau Ceti, which is in the Cetus constellation, and has a similar spectral range to our sun, although only 78% of the mass.

As a result, the astronomers, led by a team at the University of Hertfordshire, not only believe that the two within the habitable zone could be host to life, but could be targets for colonisation by our descendents.

The research, which is published today in the Astronomical Journal, represents a significant step in the ongoing hunt for life on other planets, providing one of the closest discoveries outside of our own solar system.

The Cetus constellation, which is traditionally depicted as a whale in mythology

The planets were detected not by direct imaging – that is, capturing an image of them using an instrument such as the Hubble space telescope – but by observing wobbles in the movement of the star, from which they can infer the existence, likely size and orbit distance of surrounding planets.

“We’re getting tantalisingly close to observing the correct limits required for detecting Earth-like planets,” said study lead author Dr Fabo Feng, a research fellow at the University of Hertfordshire.

“Our detection of such weak wobbles is a milestone in the search for Earth analogs and the understanding of the Earth’s habitability through comparison with these.”

This technique was first implemented by the astronomers in 2013, and has proved vital to the discovery of planetary bodies.

“We realised that we could see how the star’s activity differed at different wavelengths and use that information to separate this activity from signals of planets,” said Dr Mikko Tuomi, study co-author and pioneer of the technique.

“Since then we’ve painstakingly improved the sensitivity of our techniques and could rule out two of the signals our team identified in 2013 as planets. But no matter how we look at the star, there seems to be at least four rocky planets orbiting it.”

The planets orbiting tau Ceti, compared to those of Earth. The green band is the habitable zone, where planets are thought to have the potential to host life. Image courtesy of the University of Hertfordshire

However, while the discovery is undoubtedly exciting, the astronomers have identified another quality to the star that could make the survival of life, either in the form of biologically native organisms or occupying humans, more challenging.

Tau Ceti also have what the university describes as a “massive debris disc” surrounding it, which would likely mean that the planets would be frequently bombarded with asteroids and comets, playing havoc with any atmosphere are providing challenging environmental conditions.

Nevertheless, the planets could well prove to be a valuable stop for future humans, and may even provide a home for hitherto-undiscovered life.