A Rare and Ancient Hypernova Detected in the Milky Way Galaxy

Just where did the chemical elements come from? With new observations of an ancient star orbiting within the halo of our Milky Way a team of international astronomers led by the Australian National University have provided a surprising answer. A previously unknown type of exploding star, a “magneto-rotational hypernova”, must have played an important role in seeding the Galaxy with heavy elements.

The Universe that emerged from the fiery Big Bang was a relatively simple place, being 75 percent hydrogen, 25 percent helium, and with a smattering of other light elements. In 1920, Arthur Eddington suggested that nuclear reactions within stars could be responsible for forging heavier elements, such as carbon and oxygen, and in the 1950s Fred Hoyle and collaborators calculated the detailed rates of nuclear reactions with the hearts of stars.

Most stellar nuclear reactions are relatively sedate, with elements building up over the lifetimes of the stars as lighter atoms are burnt into heavier atoms. However, astronomers realised that if the conditions were right, nuclear reactions could be extremely rapid, readily forging very heavy elements via what astronomers unimaginatively call the “r-process”. For this, you need an abundance of neutrons and energy, and these are only found in the most violent events in the Universe. 

Given that the Big Bang only produced the very lightest of elements,  the responsibility of producing the remaining ‘metals’ (a term used by astronomers to denote anything heavier than hydrogen and helium) was predominately placed on the churning cores of stars to pump out and seed into the Universe. 

Nucleosynthesis inside stars can follow a number of different processes, with the difference between the important two types of processes - the s-process and the r-process - having to do with the rate of neutron capture during synthesis.

The slow neutron-capture process (s-process), which is believed to occur mostly in the asymptomatic giant branch stars, where the stars are able to build elements and isotopes heavier than iron by capturing one neutron at a time. The newly formed element can then either stabilise or through radioactive decay, become stable prior to the next neutron being captured. Approximately half of the elements, up to 209Bismuth are expected to be formed through this process. 

In contrast, however, the rapid neutron-capture process (r-process) occurs when the nucleus of atoms absorb a neutron prior to it having a chance to decay. This process is very fast and usually takes place in under 10 seconds. This usually occurs when there is a higher flux of neutrons (such as during violent events like core-collapse supernovae or neutron star merger events) and can produce much more neutron-rich isotopes relative to the s-process, and also accounts for approximately half the elements on the periodic table. 

It is through both these mechanisms, and of course stellar fusion, that all the elements that we see around us in the Universe came to be - including the very elements that make us up. 

Historically, It was suspected that merging neutron stars, recently observed via their gravitational wave signatures (as well as an electromagnetic counterpart signal), were responsible for the rapid production of elements, but the observations of the star, SMSS J200322.54−114203.3, have forced a rethink.

Located about 7,500 light-years from Earth, this star is ancient and would have formed in the initial stages of the Milky Way, roughly eleven billion years ago. It is “extremely metal-poor”, astronomical jargon meaning that it is relatively unenriched chemically when compared to the Sun. This is not surprising as the gas from which this star formed would have had only limited enrichment from previous stellar generations. 

However, astronomers were surprised to see the signatures of zinc, europium and uranium in the starlight, the unmistakable signature of the rapid production of elements. Perhaps the initial gas in the Milky Way was polluted by merging neutron stars? But it was realised that something was not quite right.

Compared to other stars that are considered enhanced due to the r-process SMSS J200322.54−114203.3 exhibited the highest abundance ratios of these heavy elements measured - confirming how unusual it is. The high levels of nitrogen (relative to iron) also painting a picture that the progenitor star was rapidly rotating. 

Merging neutron stars produce a particular elemental fingerprint in the stellar light, but this was distinctly different to what was observed. This led to another suspect, a hypernova, a star that explodes with the power of more than ten individual supernovae. 

With a mass 25 times that of the Sun, this giant star would have lived a short life in the early Milky Way, with its explosive death flinging its rapidly formed elements into its surroundings and polluting the interstellar gas. And this kind of explosion has just the right elemental fingerprint to explain what was observed!

The discovery and identification of the red giant star SMSS J200322.54−114203.3 being extremely metal-poor was made using Australia’s SkyMapper telescope, located at the Siding Spring Observatory in NSW, and operated by the Australian National University’s (ANU) Research School of Astronomy and Astrophysics.

SkyMapper boasts a 1.3-metre primary mirror, and a powerful 268-million pixel camera that is able to capture a patch of the sky 40 times as large as the full Moon every 20 seconds - making it a powerful survey tool. Additionally, it features a unique series of filters to utilise in photometric observations, helping determine a star’s age, mass and temperature. 

Once SMSS J200322.54−114203.3 was identified to be of special interest, the 2.3-metre telescope at Siding Spring (also operated by ANU) was activated to scan the spectral detail of the star using the WiFeS integral field spectrograph (which has a spectral resolving power of 3,000). 

These initial studies of the chemical signature of the star provided preliminary confirmation of it to be very metal-poor, by determining its [Fe/H] metalicity ratio. This ratio is a number used by astronomers and based on the presence of metals over hydrogen detected in the spectrum of stars. Our Sun, a fairly younger and therefore much more metallic star, has a [Fe/H] ratio of 0, as it is considered the benchmark to measure other stars from. 

A review and analysis of historical spectral observations (from September 2017) for SMSS J200322.54−114203.3 were made, looking at data collected with the Magellan Inamori Kyicera Echelle (MIKE) spectrograph, located on the 6.5-metre Magellan Telescope in Chile. This instrument’s resolving power was much higher (at 22,000 - 28,000) and from this, astronomers determined the [Fe/H] ratio to be -3.5, providing evidence that the metallicity is many times lower than our Sun. Further, high-resolution spectra were obtained at the European Southern Observatory’s Very Large Telescope using UVES, the Ultraviolet and Visual Eschelle Spectrograph. Obtained in 2019, pushed the spectral resolution much higher, up to 110,000 and allowing the astronomers to identify the chemical fingerprint in detail.

These new observations reveal that the early Milky Way was an energetic place, shaped by these monstrous exploding hypernovae. Astronomers are continuing the hunt for other ancient and metal-poor stars (known to be very rare and special finds) to further shed light on this mysterious time in the life (and history) of the Galaxy.