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Light Echo: Hubble captures the last gasp of dying star

Posted 2 November 2022

The final flash of an exploding star has been captured by a team of international astronomers studying supernovae, preserving the rare cosmic phenomenon in a series of images taken by the Hubble Space Telescope.

Described in a new paper in The Astrophysical Journal Letters, this ‘light echo’ was observed by researchers from University College Dublin, as well as members based in Denmarks, Germany, Spain and the United States.

The team merged the Hubble images in a short video, showing first the supernova at the very centre, followed by rings of light spreading out from the cosmic blast and hitting various layers of dust in the surrounding area.

The resulting animation offers a trove of information about the evolution of dying stars, said lead scientist Professor Maximillian Stritzinger of Aarhus University, Denmark.

“The data set is remarkable and enabled us to produce very impressive coloured images and animations that exhibit the evolution of the light echoes over a five-year period. It is a rarely seen phenomenon previously only documented in a handful of other supernovae."

Co-author Dr Morgan Fraser, UCD School of Physics, said: “While the James Webb Space Telescope has drawn much attention, its predecessor Hubble continues to provide incredible images of the universe. HST has now been observing the sky for over three decades, so we can find things like this light echo that evolve slowly over many years."

Dr Lluis Galbany, Institute of Space Sciences, Barcelona, fellow co-author added: “The blast wave from this powerful supernova explosion is racing outwards at over 10,000 kilometers per second. Ahead of this blastwave is an intense flash of light emitted by the supernova, and this is what is causing the expanding rings we can see in the images. Supernovae are of interest as these cosmic explosions produce many of the heavy elements such as carbon, oxygen and iron, which make up our galaxy, stars and our planet."

The supernova captured by Hubble, named SN 2016adj, was first seen in 2016 and belongs to the well-known peculiar galaxy Centaurus A, situated between 10 and 16 million lightyears from Earth.

For five and a half years, the astronomers watched the area around the supernova after it slowly faded away.

Centaurus A is full of dust lanes and when the sideways spreading light from the supernova hit these dusty areas over time, they lit up further and further away from the original supernova position, creating a series of expanding rings of emission called light echoes.

"A good everyday analogy is to imagine the finale of a fireworks show - the bright burst of light from a shell at the end of the show will light up the smoke from earlier shells that is still lingering in the area,” said Co-author Dr Stephen Lawrence, Hofstra University, New York.

“By comparing a series of photographs taken over several minutes, you could measure all sorts of information that is not directly related to the most recent explosion that is lighting up the scene, things like how many shells had previously exploded, how opaque is the smoke from a given shell, or how fast and in what direction was the wind blowing."

Professor Stritzinger added: “Centaurus A is a huge elliptical galaxy. These are mostly quiet, dust free and without younger stars prone to go off as supernovae, but Centaurus A is obviously different. It is a strong radioastronomical source and it contains prominent dust lanes with new stars forming within.

"This is a sign that it has 'recently' gobbled up another smaller spiral galaxy, and matters have not yet settled down, as it might in a couple of hundreds of millions of years. Observing the development of these light echoes will help us gain more insight into these violent galaxy collisions."

The team, which includes Dr Ferdinando Patat, European Southern Observatory, Garching, Germany, plans to follow up on the observations with Hubble in the future, hoping that more light rings will emerge.

By: David Kearns, Digital Journalist / Media Officer, UCD University Relations (with materials from Caroline Byrne, UCD Research and Innovation)