The universe’s biggest explosions made some of the elements we are made of. But there is another source of mystery there

After its “birth” in the Big Bang, the universe consisted mostly of hydrogen and a few helium atoms. These are the lightest elements on the periodic table. More or less all elements heavier than helium were produced in the 13.8 billion years between the Big Bang and the present day.

Stars have produced many of these heavier elements through the process of nuclear fusion. However, this makes the elements as heavy as iron. Creating any heavier element would consume energy instead of releasing it.

To explain the presence of these heavier elements today, it is necessary to find phenomena that can produce them. One type of event that fits the bill is a gamma-ray burst (GRB)—the most powerful class of burst in the universe. These can explode with a quintillion (10 followed by 18 zeros) times the luminosity of our Sun and are thought to be caused by several types of events.

GRBs can be divided into two categories: long bursts and short bursts. Long GRBs are associated with the death of massive, rapidly rotating stars. According to this theory, rapid rotation sends material ejected during the collapse of a massive star into narrow jets moving at extremely fast speeds.

Short bursts last only a few seconds. They are thought to be caused by the collision of two neutron stars – compact and dense “dead” stars. In August 2017, an important event helped support this theory. Ligo and Virgo, two gravitational wave detectors in the US, detected a signal that appeared to come from two neutron stars headed for a collision.

Seconds later, a brief gamma-ray burst, known as GRB 100817A, was detected from the same direction in the sky. For weeks, nearly every telescope on the planet was tracking the event in an unprecedented effort to study its aftermath.

The observations revealed a kilonova at the location of GRB 170817A. A kilonova is a weaker cousin of a supernova explosion. More interestingly, there was evidence that many heavy elements were produced during the explosion. Authors of a study in Nature analyzing the explosion showed that this kilonova appeared to produce two different categories of debris, or ejecta. One was composed mainly of light elements, while the other was composed of heavy elements.

We have already mentioned that nuclear fission can only produce elements as heavy as iron in the periodic table. But there is another process that could explain how the kilonova was able to produce even heavier ones.

The fast neutron capture process, or r-process, is where the nuclei (or nuclei) of heavier elements such as iron capture many neutron particles in a short time. They then rapidly increase in mass, yielding much heavier elements. However, for the r-process to work, you need the right conditions: high density, high temperature, and a large number of free neutrons available. Gamma ray bursts occur to provide these necessary conditions.

However, mergers of two neutron stars, such as the one that caused the GRB 170817A kilonova, are very rare events. In fact, they may be so rare as to make them an unlikely source for the abundant heavy elements we have in the universe. But what about long GRBs?

A recent study investigated one long gamma-ray burst in particular, GRB 221009. This is called BOAT – the brightest of all time. This GRB was captured as a pulse of intense radiation spanning the Solar System on October 9, 2022.

SHIP launched a similar astronomical observation campaign as the kilonova. This GRB was 10 times more energetic than the previous record holder and so close to us that its impact on the Earth’s atmosphere was measurable on the ground and comparable to a large solar storm.

Among the telescopes studying BOAT’s aftermath was the James Webb Space Telescope (JWST). He observed the GRB about six months after it exploded, so as not to be blinded by the afterglow of the initial explosion. Data collected by JWST showed that, despite the event’s extraordinary brightness, it was caused by a merely average supernova explosion.

In fact, previous observations of other long GRBs showed that there is no correlation between the luminosity of the GRB and the size of the supernova explosion associated with it. The boat seems no exception.

The JWST team also deduced the number of heavy elements produced during the BOAT explosion. They found no indication of elements produced by the r process. This is surprising since, theoretically, the luminosity of a long GRB is thought to be related to the conditions at its core, most likely a black hole. For very bright events – especially extreme ones like BOAT – conditions must be right for the r process to occur.

These findings suggest that gamma-ray bursts may not be the hoped-for main source of the universe’s heavy elements. Instead, there must be a source or sources still out there.