The Youngest Supernova that Nobody Saw

The world was starting to bloom with science, construction, and reason in the latter half of the 19^th century.

If we were alive then, we’d have witnessed some of the great marvels in science such as the publication of On the Origin of Species by Darwin; Faraday, Maxwell and Ampère ignite a new branch of science known as electromagnetics and Edison inventing the phonograph or testing the long-lasting light bulb.

Construction of the Suez Canal, Big Ben and the first section of the London underground were completed. In America, slavery is abolished and in New Zealand, for the first time anywhere in the world, women are given the right to vote. The world was indeed a rapidly changing place.

Whilst our societies grew and flourished, light from a distant colossal detonation – one of the biggest explosions in our Galaxy known as a supernova – was only just reaching the Earth from the heavens. Its brightness would have rivaled the full moon during the night, outshining many stars that were slowly beginning to drown in the rising light pollution created by industrialisation. It’s possible, it would have been visible during the day, beaming through the blue sky like a distant Sun.

A star had died. Yet, nobody saw a thing. No one recorded it, nor spoke of it. No one even mentioned it. The last time humans had witnessed such an event was 330 years earlier, but the skies had fallen eerily silent since.

A new paper, published in the peer-reviewed astronomy and astrophysics science journal, the Monthly Notices of the Royal Astronomical Society (MNRAS) by scientists from several Australian institutions - has discussed new findings of the expanding shockwave from the youngest known supernova from within the Milky Way Galaxy.

“This project started out as a short-term summer research project, something we thought would be a simple project – looking at a standard object,” said Kieran Luken, from Western Sydney University, who is the lead author on the new paper.

“Three years later we find it is not as simple as we were hoping, it’s a lot weirder. We found that there is some evidence that might indicate it might not be a Type Ia supernova” he said.

Coming from a computer science background, Kieran’s work involves machine learning being applied to estimating redshifts in the universe. He works on the Evolutionary Map of the Universe (EMU) project, which utilises the Australian Square Kilometre Array Pathfinder (ASKAP) instrument.

The new paper outlines details about long-term radio wavelength study (spanning from 1984 to 2017) of the supernova remnant known as G1.9+0.3 with new developments detected during the 2016 – 2017 observational period.

G1.9+0.3 is the remnant expanding shell of a supernova event that occurred somewhere between the mid to late 19^th century. Supernova events have been observed from within our own galaxy – even with the naked eye since some of the first antiquity astronomers and even during the classical renaissance periods. Both Tycho Brahe (1572) and Nicolas Kepler (1604) observed and documented these transient events.

However, the event that caused G1.9+0.3 – even though it was located in a prominent part of the sky where most of the world could witness it – was not seen. This is because the dense clouds of gas and dust near our Galactic Centre obscured all of its visible light.

“If the supernova was visible to Earth when it erupted, there would have suddenly been a very bright star in the sky that wasn’t there before. Then after a short time, this star would have disappeared” said Mr. Luken.

In 2016, NASA – using the orbiting x-ray observatory Chandra and the Very Large Array (the VLA is located in the USA) confirmed that the trigger was a Type Ia supernova event and that it occurred in our very recent history.

Though we cannot observe G1.9+0.3 in visible wavelengths due to the obscuring dust between us and the remnant (it’s not even possible using the Hubble Space Telescope), we can still observe it with high-energy orbiting observatories like Chandra, or Earth-based radio telescopes.

The distance to the supernova remnant was is outlined in this most recent paper (Luken et al 2019) as 8.5 kiloparsecs (kpc) which equate to about 27,700 light-years away from Earth. Given that light travels at a fixed speed across the universe (300,000 km/s^-1) what we see when looking at G1.9+0.3 is as it was roughly 27,700 years ago.  It’s also located near the Galactic Centre in the constellation of Sagittarius – a winter constellation that rises high over Australian skies.

When scientists observed G1.9+0.3 using the VLA in 1985 (Green & Gull 1985), then again in 2008 (Green et al. 2008) – they found that the remnant shell – a shockwave of expanding material – had significantly expanded and emitting synchrotron radiation. By comparing the two observations, they were able to determine that G1.9+0.3 had expanded by roughly 15% over the 23 year period.

However, these latest findings – which have utilised multiple radio continuum images between 1984 and 2017 from multiple Earth-based telescope arrays – indicate the average rate of expansion of the supernova remnant is roughly 8,900 km/s^-1 (± 1,200 km/s^-1).

This equates to a staggering 32.04 million km/h and has allowed the scientists to calculate back the expansion time, to determine when the supernova event took place – which they state as 142 years (± 19 years), placing the event somewhere between 1858 and 1896.

Interestingly, the paper has also outlined that the surrounding material that the supernova remnant shell is expanding into – the interstellar medium (all the gas, dust, objects and radiation that exists between stars) is inhomogeneous. By observing the spectral index brightening by up to 195% in the remnants northern regions, scientists have derived that as the shockwave expands, collides and sweeps up these materials, it gives off brighter observed signals. Given that this increase does not occur everywhere around the expanding shell – it must mean that there are more materials to collide within this region relative to other regions.

Modeling shows that this brightening is far from over (from our humanistic timescales) and the supernova remnant G1.9+03 will continue to brighten (in radio) reaching a maximum approximately 500 years from now, somewhere around 2,500 C.E.

These latest data were captured using the Australian Telescope Compact Array (ATCA) which is an array of radio telescopes located in the NSW northern central town of Narrabri. The observations included three twelve-hour periods during January and March of 2016 and May of 2017.

Data taken from an existing 2013 survey (Galactic Plane CO survey by Burton et al. 2013) using another radio telescope called MOPRA (located in the Warrumbungle National Park in NSW) was also combined with these new observations. Both telescope facilities are operated by the CSIRO Australian Telescope National Facility (ATNF).

A number of Australian institutions were involved in this paper and joined a number of international collaborators, including:

• Western Sydney University
• CSIRO ATNF
• University of NSW
• University of Adelaide
• CSIRO Astronomy and Space Science
• International Centre for Radio Astronomy Research
• Pawsey Supercomputer Centre

In addition to the ATCA and MOPRA telescopes being used for these observations, previous studies of G1.9+0.3 have included work conducted with other Australian-based telescopes such as the Molonglo Observatory Synthesis Telescope (MOST) which through a Galactic survey in 1994 was able to resolve shell-like morphology of the supernova remnant, in addition to data from the GaLactic and Extragalactic All-Sky MWA survey (GLEAM) which helped calculate the revised spectral index for these latest findings.

Ever notice different colours in stars when you look up at them? Or when you see a photo of star trails, they appear to be flashes of blue, white, yellow and red? (next time you’re out in summer, look at the constellation of Orion. Its belt and upper right corner is made of bright blue stars, whilst its lower corner contains a shimmering red star).

The stars in our universe have a variety of colours, temperatures, and brightness –and to complicate matters a little more, stars evolve throughout their lives and change in both size, brightness and colour.

Here’s where our supernova story starts.

Stars like our Sun, burn their hydrogen fuel (through nuclear fusion) for about 10 billion years and eventually expand to be red giant stars. As they get a little older, they quietly puff off their outer layers to become beautiful planetary nebulae and leave behind a planet-sized hot cinder core known as a white dwarf.

But some stars are much bigger than our Sun – and after furiously burning through their fuel storages in their short lives (10 million years or so). They explode in violent events known as supernovae. These events blow nearly all of the star up and spread all the materials that once made up the star into the surrounding space. Lucky for us, this material is what formed our Sun, planets and eventually us!

Depending on how big the original star was, the remaining cores of these massive stars turn into extreme objects like neutron stars, pulsars or even black holes.

When a massive star is alone and it goes supernova, the blast and shockwave expand into space around the event. This is called a Type II supernova and occurs when very big stars collapse in on themselves after their fuel production energy output caves into the greater force of gravity.

However, many stars are locked together in binary systems and as one of them ages faster than the other – it ends its life first but leaves behind a compact object like a white dwarf (like our Sun will).

These massive compact objects then start to consume their sibling stars by ripping material from them and accumulating this material on the surface of the smaller, white dwarf. As material builds up on the surface – pressure also builds until the point where there is enough pressure to fuse the infalling surface material and trigger a secondary thermonuclear reaction. This is called a Type Ia supernova.

Once again, materials from the system are blasted into space and expand as a sweeping shockwave, collecting any material that exists in the surrounding region. In both scenarios, the expanding shockwave of material is moving at almost 10% the speed of light (about 30,000km/s) which heats up the upstream plasma to very high temperatures, outputting high energy (x-ray emissions).

But supernova remnants can also send charged particles spiraling around magnetic field lines at almost the speed of light, creating synchrotron emissions (these are non-thermal) which are received in radio wavelengths.

As such, we can observe supernova remnants in many different electromagnetic wavelengths – but where we have atmospheric blocking (Earth’s atmosphere is a great filter!) we need use our space-based observatories (such as the Chandra x-ray observatory) or where there are interstellar obstructions (like Galactic dust), we can use our Earth-based radio telescopes.

Three recent papers (Hurley-Walker et al. 2019a; 2019b & 2019c) from the GLEAM survey and utilising the Murchison Wide-Field Array (MWA) telescope located in the remote central desert of Western Australia reviewed the Milky Way’s Galactic plane in radio frequencies.

As part of these surveys, 101 existing supernova remnants were reviewed, confirming that 10 were indeed ancient relics of violent events from our Galaxy’s past, in addition to the discovery of 26 new remnants.

Earlier in 2019, Brose et al. (including the lead author of the latest paper K. J. Luken), released another paper with regards to supernova remnant G1.9+0.3, finding that there were indeed two shock waves flowing through the expanding object.

The first, detected as x-rays emissions resulted from the forward shock wave and were highly focused on the east and west region of the remnant shell. According to this earlier paper, these regions are expanding faster than the north/south structures, moving at staggering 15,000km/s^-1. No gamma-ray radiation has been thus far detected from the system.

The study also found that the radio emissions were a result of a reverse shock and coming from the brighter northern region and the dimmer southern region. The findings in this earlier paper represent the first supernova remnant detected to display non-thermal emissions (synchrotron radiation) which are linked to the reverse shock within the structure.

Astronomers and enthusiasts are continually seeking supernovae events, with an average of 1 event per century expected for our Milky Way Galaxy. Of course, other galaxies continue to have supernovae occurring – so we are continually seeing them go off in distant parts of the universe (and in fact have used them to help us measure distances between us and their host galaxies).

Two interesting, very big stars within our own Galaxy continue to spark intrigue and imagination for the next big one.

The red supergiant Betelgeuse, located in the constellation Orion is thought to be nearing the end of its life - especially given its recent rapid dimming. So large is the surface area of Orion that we can use baseline interferometry to ‘see’ its huge surface here from Earth – the only star outside our solar system that we can perform this task for. If you were to place Betelgeuse where the Sun is today, it’s surface would extend beyond the orbit of Mars, consuming the inner rocky planets within its interior.

Located roughly 640 light-years away from Earth, this massive star will undergo core-collapse and detonate in a spectacular event that would be as bright as the full moon during the evening. Some astronomers even believe it will be bright enough to see during the day for weeks on end.

The second star, which resides at the upper limits of mass a star can contain is the binary system of Eta Carinae. Its mass is roughly 100 – 120 times that of the Sun and it shines with a brightness of 5 million times greater. With such a high mass, when this star explodes, it is going to be a spectacular view in the night sky. The remnant compact object of such a heavyweight can only follow one evolutionary path – it is certain this star will become a black hole.

In the 1830s, Eta Carinae flared up with an eruption which increased its apparent brightness to outshine many of the existing brightest stars in Earth’s sky. With advancements in our technology, in particular, the development and deployment of the Hubble Space Telescope – we’ve been able to see the expanding shockwaves of the 1830s Eta Carinae eruption.

Even at such a great distance of about 7,500 light-years from Earth, some astronomers think that the Eta Carinae supernova could have impacts on our upper atmosphere and any astronauts in space, as the high levels of blast radiation reach us.

Given supernova remnant G1.9+0.3 is so young, an opportunity now exists for astronomers to test their theories and models into the future.

“In 500 years from now, it will be interesting to know if your models are right – but the other interesting thing is going to be how big supernova remnant G1.9+0.3 will grow by then? Will it continue to expand and push everything around it out of the way – or will it slow down?” said Mr. Luken, pondering what the space sciences, astronomy and radio observations will be with advancements in technology over the next half millennia.  

With the radio flux density increasing over the next 500 years, scientists like Mr. Luken hope future advancements in technology and infrastructure are able to further refine, analyse and better understand how his paper’s modelling compares to the actual results of the expanding supersonic shockwave near the centre of our Galaxy.

What’s more, the last supernova event witnessed by humans was in 1987 – when a giant star in a nearby galaxy exploded. This was before the age of fast-streaming internet, powerful mobile phone cameras and the ability to quickly trigger an array of space-based telescopes to point to any event in the sky within minutes.

When Betelgeuse and Eta Carinae are ready to meet their violent ends and ignite in a flash that will outshine the combined output of all the stars in the Milky Way galaxy – our world will be ready to witness the unimaginably huge detonation.

We’ll observe and collect data better than ever before in human history, learning about our place in the universe. G1.9+0.3 will no longer be the youngest supernova remnant in our Galaxy.

And the world will change once more.