How Elements are Forged from Stars

Sometimes we take the stars in the sky for granted. When you look up at the night sky, you can see constellations that you know, and that seem like they will always be there - and many of them have been for thousands of years since humans started recording the patterns in the heavens. 

But, just like everything, the sky and stars change. In fact, depending on the type of star, the change can be quite sudden and violent with the lives of the largest stars end with a bang within an astronomically short timeframe - detonating in a massive supernova event. 

Now, in a new paper published in the preprint server arXiv.org, a collaborative group of international researchers have explored the roles that gamma-rays play in a subgroup of supernovae and their interaction with elements that are synthesised during these violent spectacles that can shine as bright as an entire galaxy from across the Universe. 

This research, led by Dr Fiona Panther from the University of Western Australia, specifically explores the aftermath of type Ia supernovae that come from very low-mass progenitors that produce a helium detonation in a thick helium shell. The team used simulations to look at the decay of the elements Chromium-48 and Vanadium-48 and their role in the explosive event. This is a little different from regular Type Ia supernovae who are known to exhibit optical lightcurves that present Nickel-56.

“[We] investigated how gamma-rays can be used to understand the types of nuclear fusion reactions that occur in a sort of exploding star system called a ‘sub-luminous type Ia supernova’. We used theoretical models of the supernova, created by my colleague Dr Stuart Sim, to calculate whether or not space telescopes can see the gamma-rays that would be emitted from this type of supernova,” said Dr Panther.  

Supernovae are explosive events that result in the materials and gases of a star being flung far and wide into the interstellar medium, later going on to become future generations of new stars, planets, moons, and even living beings. 

The process of creating the elements occurs in the heart of the star as it heads towards its last days - in a process known as stellar nucleosynthesis. This method is responsible for how many of the heavier elements that we see around us (including oxygen, carbon, calcium, nitrogen, phosphorus and more) are created, and is known as the slow-neutron capture process, or s-process.

Then when a supernova occurs, these newly created elements are dispersed out into the local region around the star, eventually coming together in a recycling process that forms the next generation of astrophysical objects. 

Typically, supernovae fall into two major categories. The first describes when massive stars have reached the end of their life, having burned through all their fuel, fusing heavier elements until they get to iron. 

By this point they can’t fuse these heavy isotopes, so they end up exploding into a supernova, leaving behind either a neutron star or a black hole. This type of supernovae is called Type II (or commonly referred to as the core-collapse model).

However, another type of supernova can occur, and this one is called a Type Ia, which is a little more complex. These supernovae are formed from a binary star system, where two stars orbit each other closely, with one of these stars being a white dwarf. 

The compact, yet massive white dwarf can draw material from the other star in the system, accumulating its own mass. By itself, a white dwarf is a fairly stable star, but as it steals material from its companion it becomes larger and more massive. 

Once the white dwarf has accumulated enough material to reach approximately 1.4 times the mass of the Sun (a mass known as the Chandrasekhar limit), it becomes hot enough and dense enough to trigger thermonuclear burning, causing the white dwarf to rapidly explode in a supernova event. 

Additionally, sometimes two white dwarfs merge, which also trigger a type Ia supernova event. 

“Different processes occur when stars end their lives. A Type Ia supernova occurs when a carbon-oxygen white dwarf - the remnant of a star with an initial mass less  than a few times that of the sun - interacts with its binary companion, which can be another white dwarf or even a main sequence or helium star.”

“If enough mass is taken from the companion star by the ‘primary’ carbon-oxygen white dwarf, the star becomes hot and dense enough to begin fusing together carbon and oxygen at an explosive rate. Type Ia supernovae produce most of the ‘iron peak’ elements in the universe: elements like iron, nickel and also elements like chromium and vanadium, the interest of this paper,” she said. 

Dr Panther emphasised the importance of looking beyond optical light to study nucleosynthesis. 

“When we look at optical light, we can use spectral lines as a kind of fingerprint to see what sort of elements were made in a supernova explosion. However, this only tells you the name of the element.”

“When we study nucleosynthesis, we are usually interested in the amounts of different isotopes of elements that are formed as this tells us about the temperatures, densities and abundances of raw materials that were involved in nucleosynthesis.” 

“The gamma-ray spectrum encodes information about the nuclear structure of an element, so it is a more accurate way of determining the ways in which nucleosynthesis occurs in a supernova,” she said. 

But the fusing power found in stellar cores, which churns out new elements via the s-process as the star is ageing only roughly half of the elements we see on the periodic table. So where do all the other elements that we see around us come from?

When supernovae occur, and material is being rapidly expelled from the star, atoms and nuclei can collide to form different elements. This is how some of the bigger elements in our universe have been created, with this the method is known as the rapid neutron capture process, or r-process. Heavy elements are also created via the r-process in the event of neutron star merger events, known as kilonovae. 

Astronomers are able to tell which elements are present during a supernova event (any category) by studying the spectra and light curve the transient event presents over a period of days, weeks and sometimes even years. Additionally, the light curve can be observed across the range of the electromagnetic spectrum - from gamma-rays to radio waves, with each band highlighting a different process occurring in-situ at the event. 

This method was first described by Titus Pankey Jr, during a PhD thesis he wrote whilst attending  Howard University in Washington in 1960, in which he derived the connection between the decay of Nickel-56 and observed Type Ia light curves. At the time, Pankey was one of only a handful of African-American men who held a PhD and yet his contributions to the field have been enormous. 

“Gamma-rays from the decay of this radioactive isotope are absorbed and re-emitted as optical light, which we can use to track how the brightness of a supernova varies with time (a ‘light curve’). The first theoretical explanation for this process was made by Dr Titus Pankey Jr, who completed his PhD at Howard University in the US in 1962,” said Dr Panther. 

Observations of these light curves can tell us lots about how supernovae events happen, about the localised environment surrounding the explosion, and what elements are being created in the process, thanks to people like Dr Pankey.

In this latest paper, Dr Panther and her team describe how the nucleosynthesis of the element Chromium occurs in this niche group of Type Ia supernovae, which rapidly decays into the element Vanadium, releasing gamma-rays in the process and affecting the intensity of the light of the supernova. The team studied a simulation of supernovae that produced such gamma-rays to determine how they can be best observed with telescopes, tuned into detecting high-energy frequencies from astrophysical sources.

The study of supernovae is important as they can hold a lot of information, such as the process of nucleosynthesis, the abundance of different elements, and the life cycles of stars. 

Type Ia supernova in particular, however, can also tell astronomers about distances. 

Because the standard Chandrasekhar limit dictates how big this type of supernova will be (thus producing a roughly standardised measure for each event), then these supernovae can be used as “cosmic candles” to measure distances across the universe by figuring out how far away the supernovae are.

“Some Type Ia supernovae can be ‘standardized’ - brighter supernovae of this type fade slower. When we standardize Type Ia supernovae they can be calibrated using  other  types of standard candles and so  be used  as distance markers.”

“The fact that some Type Ia supernovae can be standardized is due to the nucleosynthesis processes that produce Nickel-56,” said Dr Panther.

Dr Panther also commented on the future development of future telescopes and observatories that will help scientists use supernovae to constrain cosmological distances and other parameters.

“Many future telescopes and observatories are being proposed and developed. Dedicated optical surveys, like the Vera Rubin Observatory, will search for more supernovae and enable scientists to make more accurate measurements of things like the expansion of space.”

“In the gamma-ray part of the spectrum, several telescopes are being developed that will be more sensitive, mostly due to the development of technologies that reduce background noise. COSI is a detector being developed in the US. It will initially be flown on a high altitude balloon to be calibrated and tested.”

“Further into the future, the AMEGO telescope is a proposed gamma-ray telescope that would be launched  in the 2030s. AMEGO has the ability to survey the whole sky in gamma-rays, and is more sensitive than the current INTEGRAL telescope,” she said.