Exploding star made the Hubble telescope turn

In the latest issue of the journal Nature, researchers from Stockholm University participate in a study of a supernova explosion where the astronomers made the Hubble telescope turn and look in a different direction than planned. The ultraviolet light from the explosion shows that the exploding red giant star was embedded in gas that puffed out in the years before the final bang.

Supernovae – exploding stars – suddenly appear in the night sky. The closest supernova in nearly 10 years, SN 2023ixf, was discovered by a Japanese amateur astronomer on Friday evening, May 19 last year (2023). Just 20 million light-years away in the Whirlpool Galaxy (M101) in the Big Bear constellation, it became a favorite among Earth's astrophotographers (see Image).

Two images of a spiral galaxy. In the right one a bright supernova is visible
Astrophotographers' favorite. The image was taken with a small amateur telescope from southern Gotland and shows the spiral galaxy M101 both before and after the explosion of supernova SN 2023ixf. Credit: Astrophotographer Lars Anmark

For the astronomers, it meant a unique opportunity to study a recently exploded star in ultraviolet light using the Hubble telescope.

This was the moment I had been waiting for throughout my research studies, so it was just a matter of dropping everything else

- This was the moment I had been waiting for throughout my research studies, so it was just a matter of dropping everything else and working all night to prepare the Hubble observations, says lead author Erez Zimmerman from the Weizmann Institute. A bit difficult just that it happened two days before my planned wedding.

Early observations are important because the supernova quickly removes all traces of the exploding star's surroundings. Less than an hour after the report of the new supernova, the astronomers had taken a spectrum showing that it was a massive star that had exploded (Type II supernova). Just such an explosion they wanted to observe with the Hubble telescope.

Since 1990, the space telescope has been hovering 600 kilometers above Earth's surface, outside the atmosphere that otherwise stops the ultraviolet radiation that dominates the early, hot supernova explosion. Observing with Hubble usually requires several months of careful planning. Only if something really exciting happens in space can NASA change its predetermined schedule, quickly upload new instructions and turn the telescope in a new direction.

- We have had just such a program at the Hubble telescope for many years, says Jesper Sollerman at the Department of Astronomy at Stockholm University, and one of the co-authors of the article. We have also been searching feverishly for just such a supernova all these years. Now we finally got the chance.

Erez Zimmerman and his colleagues activated their program on the space telescope, and early in the morning on Monday, May 22, the first Hubble observations were made, the earliest ultraviolet supernova spectra ever obtained. At the same time, observations were made from many observatories around the world, including with the Nordic Optical Telescope (NOT) on La Palma (see Image). SN 2023ixf became one of the best-studied supernova explosions of all times.

One of the interpretations in the Nature article is that the light from the supernova during the first days is affected by surrounding gas that the star lost during its last years of life. The Hubble observations can thus be used to deduce how the star developed just before the explosion. More than 30 scientific papers have been written about SN 2023ixf so far, and there will be more because the supernova is still clearly visible to astronomers' telescopes.

Schematic illustration of the combination of HST imaging and groungbased spectroscopy
Illustration of observations of supernova SN 2023ixf in M101 observed both by the Hubble telescope – in the ultraviolet region – and with the Nordic Optical Telescope at La Palma in visible light. The spectrum in the illustration is a combination of these and shows some emission lines from, for example, helium (He) and nitrogen (N) that come from gas very close to the exploded star. Credit and Copyright: Joel Johansson (Stockholm University) made both the data reductions of the NOT/FIES observations and the illustration.

The article "The complex circumstellar environment of supernova 2023ixf" is published in the journal Nature (online 27 March 2024) and is led by Erez Zimmerman and Ido Irani from the Weizmann Institute of Science in Israel. From Stockholm University, in addition to professor Jesper Sollerman at the Department of Astronomy, Joel Johansson and Claes Fransson from the Oskar Klein Center participated.

Contact: Professor Jesper Sollerman, jesper@astro.su.se


The paper on Arxiv