During the first three minutes of the universe’s existence, a significant fraction of the hydrogen was fused into helium. For hundreds of millions of years, the normal matter in the universe consisted of 75% hydrogen, 25% helium, and trace amounts of some light elements. However, these elements do not permit the complex chemistry that life requires, nor do they fulfill the requirements for forming life-friendly planets. Something had to change.
Around 200–400 million years after the big bang, a critical change started. Stars formed. In the hearts of these first stars, the abundant hydrogen and helium were converted into increasingly heavier elements like carbon, oxygen, and iron. By studying the stars we see today, astronomers know that dust (formed from elements heavier than helium, which astronomers refer to as “metals”) facilitates the formation of stars similar in size to the Sun. Without these metals, the first stars formed were far more massive—hundreds to thousands of times more massive. Processes in the hearts of these massive stars produced copious quantities of metals that helped form future, smaller stars and also provided the material for rocky planets like Earth to grow.
However, these metals would have been useless without some mechanism to scatter them into the material from which newer stars formed. Fortunately, astronomers and physicists have calculated such a mechanism and refer to it as a “pair-instability” supernova. Observations of a metal-poor dwarf galaxy recently revealed just such a supernova. Unlike the more common core-collapse supernovae, pair-instability supernovae scatter a large fraction of the original star back out to form future stars.
Core-collapse supernovae result from a large iron core forming in a massive star (8–100 times the mass of the Sun). As the core grows, it eventually collapses because the gravitational forces overwhelm the thermal pressure. Consequently, core-collapse supernovae form massive, dense objects such as neutron stars and black holes.
Pair-instability supernovae occur from a different mechanism, but only in stars with masses greater than 100–150 times the mass of the Sun. Because of the great mass and lack of metals, the star’s core reaches high temperatures but at relatively low densities. Instead of having a dense iron core, the star will have a core made primarily of oxygen. However, as the temperatures rise, an increasing number of the photons in the core will produce electron-positron pairs. Just like converting a gas to a liquid dramatically reduces the pressure, this pair production also reduces the pressure in the core of the star. The star begins to collapse, heat up, and undergo catastrophic nuclear reactions. These runaway nuclear reactions blow the star apart, scattering the metals produced throughout the surrounding environment.
As described in a Nature article, the observed pair-production supernova blew off more than three solar masses of one nickel isotope. This much nickel production means that a large quantity of the star was converted into metals, which were then expelled into the surrounding environment. Because this particular nickel isotope is radioactive, it emitted lots of light such that the supernova was 10 times brighter than a typical Type Ia supernova (a more common kind of runaway-nuclear-explosion supernova) and lasted more than a year and a half. The extreme brightness and longevity implies that astronomers should be able to see pair-production supernovae from great distances.
As astronomers find more pair-production supernova, they will be able to better understand one of the first life-essential changes that occurred in the universe, namely the formation of metals. The initial results from this first detection indicate that the earliest stars rapidly increased the metal content of the universe. Without this foundation, no habitable Earth-like planet would have formed.