by After its “birth” in the Big Bang, the universe consisted mainly of hydrogen and a few helium atoms. These are the lightest elements in the periodic table. More or less, all the elements heavier than helium were produced in the 13.8 billion years between the Big Bang and the present day.
Many of these heavier elements are made by stars through the process of nuclear fusion. However, this only applies to elements as heavy as iron. If any heavier elements were created, energy would be consumed instead of released.
In order to explain the presence of these heavier elements today, it is necessary to find phenomena that can produce them. One event that fits the bill is a gamma-ray burst (GRB) – the most powerful class of explosion in the universe. These can erupt with five billion (10 followed by 18 zeros) times the brightness of our Sun, and are thought to be caused by a variety of events.
GRBs can be subdivided into two categories: long bursts and short bursts. Long GRBs are associated with the death of rapidly rotating massive stars. According to this theory, the fast-rotating beams of material were ejected when a massive star collapsed into narrow jets that move at very fast speeds.
The short bursts last only a few seconds. They are thought to be caused by the collision of two neutron stars – compact and compact “dead” stars. In August 2017, an important event helped to support this theory. Ligo and Virgo, two gravitational wave detectors in the US, detected a signal that appeared to be coming from two neutron stars moving in for a collision.
A few seconds later, a short gamma-ray burst, known as GRB 100817A, was detected coming from the same direction in the sky. For a few weeks, almost every telescope on the planet was focused on this event in an unprecedented effort to study its outcome.
The observations revealed a chilenova 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 who analyzed the explosion showed that this kilonova appeared to be two different categories of debris, or ejecta. One was composed mainly of light elements, and another 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.
A fast neutron capture process, or e-process, is where the nuclei (or cores) of heavier elements such as iron capture many neutron particles in a short time. They then grow rapidly in mass, creating much heavier elements. For an e-process to work, however, 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, like the one that caused the kilonova GRB 170817A, are extremely rare events. In fact, they may be so rare that they are an unlikely source for the abundant heavy elements in our universe. But what about long GRBs?
A recent study investigated one long gamma-ray burst in particular, GRB 221009. This was dubbed BOAT – the brightest ever. This GRB was selected as a pulse of intense radiation sweeping through the Solar System on 9 October 2022.
The BOAT inspired an astronomical observation campaign similar to 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 that studied the BOAT aftereffect 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 burst. The data collected by JWST showed that, despite the event’s extraordinary brightness, it was caused by an average supernova explosion.
In fact, previous observations of other long GRBs have shown that there is no correlation between the brightness of the GRB and the size of the associated supernova explosion. The BOAT seems to be no exception.
The JWST team also inferred the number of heavy elements produced during the BOAT explosion. They found no indication of the elements produced by the e-process. This is surprising because, in theory, the brightness of a long GRB is thought to be related to the conditions at its core, which is probably a black hole. For very bright events — especially one as extreme as the BOAT — the conditions should be right for the e-process to occur.
These results suggest that gamma-ray bursts may not be the critical source of the universe’s heavy elements. Instead, there must still be a source or sources out there.
This article from The Conversation is republished under a Creative Commons license. Read the original article.
Robert Brose does not work for, consult with, own shares in or receive funding from any company or organization that would benefit from this article, and does not disclosed any relevant affiliations beyond their academic appointment.
