Supernova secrets: How are black holes born?

Black holes usually cannot be observed directly, but can be observed by the effects of their enormous gravitational fields on nearby matter. Image / 123RF

When it comes to grand exits, little compares with the dazzling death of massive stars.

These gigantic explosions in space, which we know as supernovae, create a lightshow so bright they can be seen across the universe – sometimes even outshining entire galaxies.

That typically unfolded in two ways.

One was a loss of balance between the two factors essentially holding stars together: the gravity pushing onto them, and the heat and pressure streaming outward from their nuclear-fuelled cores.

As stars ran out of fuel and cooled, their internal pressure dropped and gravity won – causing the star to suddenly collapse and explode.

Exploded “massive” stars – or those about 10 times greater than our sun in mass – typically left the densest objects in the universe: black holes.

In other instances, these spectacular bangs occurred when two co-orbiting “binary” stars collided.

Although the existence of black holes were first theorised nearly two and half centuries ago, recent breakthroughs have transformed scientists’ ability to physically observe them.

In 2019, scientists revealed the first image ever made of a black hole after assembling data gathered by a network of radio telescopes around the world. Image / Event Horizon Telescope Collaboration

In 2015, the landmark detection of gravitational waves – cosmic ripples in space-time that were once generated by a pair of merging black holes – provided a new way to “weigh” their mass.

But University of Auckland astrophysicist Associate Professor Jan Eldridge said there were still many fundamental questions to answer.

One obvious problem was that gravitational waves, game-changing as they were, largely represented only the biggest black holes in space.

That made it harder to study the origins and dynamics of the lightest ones – those with masses a few times the mass of the sun, but in a sphere less than 18 kilometres across.

To answer these mind-boggling questions, Eldridge and colleagues have launched a new study that will draw on computer models able to simulate the evolution of stars, from birth to death.

“This means we can predict the masses of black holes different stars will make,” she said.

“Importantly, because some aspects of how stars evolve are uncertain, we’re going to try out many different sets of stellar models, where we vary the input physics, to see how they change the predicted black hole mass distribution.

“Then, when we compare them with observations, we’ll be able to see which descriptions of physics in stars match observations and which don’t.”

Their new study, supported with a $913,000 Marsden Fund grant, will also create millions of new detailed models focused on what are called “X-ray binaries”.

In these systems, one of the two stars in the binary has already died, leaving the surviving “normal” star orbiting with either a neutron star – the collapsed core of a massive supergiant star – or a black hole.

Eldridge said the new model would simulate how normal stars essentially “fed” on their companion black hole – creating observable X-rays in the process – and also how black holes in turn affected the star.

“This is a really critical phase to get right, as it’s how black holes can grow really big,” she said.

“As there is so much about the physics that we’re not sure about, it’s going to be interesting trying to understand these exotic binary stars.”

University of Auckland astrophysicist Associate Professor Jan Eldridge and colleagues are investigating how black holes form when massive stars die. Photo / Supplied

She added that although most other research groups used approximate stellar models, each taking a fraction of a second to compute, the more complex ones her team were developing would take minutes to run their calculations.

“This means we need more computers to do that same amount of work – but we’re more accurate,” she said.

“We’re also using all of the information about black holes that exists – and will exist in a few years’ time – to compare to our model predictions. Using so many different observations will really give us strong constraints on our results.”

In the end, they hoped to come away with a far deeper understanding of how black holes were formed during the deaths of stars – and also how they could grow by feeding on other stars.

“And hopefully, when we observe black holes either from gravitational wave events or in X-ray binaries, we’ll be able to understand the full history of the stars that had to die to create them.”

More widely, there was the potential to better understand the universe itself.

“For example, we’re already finding good evidence that the stars that produce most of the gravitational wave events we see today are also the stars that helped re-ionise the universe early in its history,” she said.

“This is the process by which the universe became transparent, so we are able to observe everything we can today.”

NZ scientists aid the hunt for life on Titan

This near-infrared, colour view taken from the spacecraft Cassini shows the sun glinting off Titan’s north polar seas. Image / Nasa/JPL-Caltech

Meanwhile, another Kiwi-led effort is helping lay the geochemical groundwork for an ambitious mission to explore Saturn’s largest moon.

When Nasa’s Dragonfly spacecraft touches down on Titan in the mid-2030s, scientists hope to discover chemical species central to astrobiology, and the very origin of life.

That’s because Dragonfly’s predecessor, Cassini, has already revealed the moon is hiding an underground ocean beneath its icy surface – putting it among a handful of worlds in our solar system potentially containing habitable environments.

Although there’s so far no evidence of life on Titan, its unique environment and complex chemistry made it an exciting destination for exploration.

Alongside the programme, a team of researchers led by Otago University’s Dr Courtney Ennis is studying complex molecular minerals called co-crystals.

“Over the past decade, a number of new Titan relevant minerals have been discovered by our team that are likely to be found around Titan’s dune regions and hydrocarbon lake shores,” Ennis explained.

Frozen solid, co-crystal grains contain a mixed composition of cyanide and hydrocarbon compounds; originally formed as aerosols that lingered in Titan’s atmosphere before falling to the surface.

This illustration shows Nasa’s Dragonfly rotorcraft-lander approaching a site on Saturn’s exotic moon, Titan. Dragonfly is scheduled to land there in the mid-2030s. Image / NASA/JHU-APL

After being exposed to the harsh UV light and cosmic rays that continually bombard the Titan sky, the co-crystals are thought capable of generating a suite of biological building-blocks on the ground.

In a new study, just awarded a Marsden Fund grant, Ennis’ team sought to answer just which co-crystals were formed in Titan’s atmosphere, and whether they follow the same radiation-driven chemical reactions as predicted using computer models.

More intriguingly, they wanted to know whether such chemistry could help create life.

To push their hypothesis forward, the team will carry out crystallographic studies of co-crystal minerals thought to be relevant to Titan’s, telling them much more about how they’re formed.

Using methods such as neutron and x-ray diffraction, and Raman spectroscopy, the researchers will also effectively replicate Titan conditions at Otago’s astrochemistry laboratory for further examination.

“The techniques and instrumentation are quite well understood, with our team having over 10 years’ experience on co-crystal synthesis and irradiation experiments,” Ennis said.

If they succeed in synthesising target compounds from co-crystal minerals, an entirely new field of efficient ice-phase chemistry will be born.

“This leads to the possibility of life-bearing molecules residing on icy outer-planets and their moons, as well as comets and interstellar dust,” he said.

“Therefore, the catalogue of life-initiating molecules – which could have been delivered to an early Earth by impact to seed the first biologically relevant reactions in Earth’s ocean – will be expanded by our work.”

Ultimately, the hypothesis would only be verified by Dragonfly’s upcoming voyage – although Ennis said his team’s project could offer Nasa new chemical targets for the spacecraft to locate and analyse.