'Chameleon' supernova observed

Observations of a supernova that appears to have changed its type over the course of a year have thrown into question our understanding of how stars live out the end of their lives.

An image from NASA’s Chandra X-ray Observatory showing NGC 7331, the galactic home of a supernova that appeared to transform from a type I to a type II over the course of a year. The supernova, SN 2014C, is indicated by the box.
Credits: NASA/CXC/CIERA/R.Margutti et al

When massive stars die, they sometimes explode in violent events known as supernovae. By this process, new elements are created and thrown out into space, providing the ingredients by which new planets, stars and planetary systems can form.

But the study of a supernova located in a spiral galaxy about 36 million to 46 million lightyears away is challenging the current theory as to how this process occurs.

Observations of supernova SN 2014C revealed a change in appearance over the period of a year, apparently because it had emitted a lot of material late in its life.

These observations do not fit with current theories as to how supernovae occur.

The revelation is based on the detection of hydrogen being given off by the supernova. Hydrogen is a star’s fuel, enabling it to form and grow. Large stars nearing the end of their lives have naturally run out of hydrogen in their core, as it has been used up over the course of their existence.

Supernovae that contain a small amount of hydrogen are known as ‘type I’, while those with an abundance of hydrogen are known as ‘type II’.

But observations of SN 2014C revealed that it has transformed itself from a type I to a type II supernova. This is because initial observations did not detect any hydrogen in the supernova, but then after a year the explosion shockwaves were observed hitting a shell of mostly hydrogen material beyond the star.

The study found that SN 2014C had ejected hydrogen decades, even centuries, before it exploded. This is not normally the case, as far as current theories go.

"This 'chameleon supernova' may represent a new mechanism of how massive stars deliver elements created in their cores to the rest of the Universe," says study leader Raffaella Margutti, assistant professor of physics and astronomy at Northwestern University in Evanston, Illinois. "Expelling this material late in life is likely a way that stars give elements, which they produce during their lifetimes, back to their environment.”

The study has led to the conclusion that we are missing a vital piece in our understanding of the nuclear reactions that occur within massive stars that explode as a supernova.

Or, perhaps the star was part of a binary system and its partner had a role in the late expulsion of hydrogen.

“The notion that a star could expel such a huge amount of matter in a short interval is completely new,” says Fiona Harrison, principal investigator of the NuSTAR telescope used in the study. “It is challenging our fundamental ideas about how massive stars evolve, and eventually explode, distributing the chemical elements necessary for life.”


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