4 mins read 20 Jan 2022

Too Much Heavy Metal Changes Future Stellar Production

Astronomers indicate that the elements that stars produce will change over time, with many metals not being spread across galaxies from future generations of stars.

M82 is a very active star-forming Galaxy. Credit: Hubble Space Telescope/APOD.

Stars produce most of the elements in the Universe, including those that make up our Earth and us. However, new research has indicated that the production of these elements will vary as time goes on - as older stars give way to younger generations of stars, forged with materials from their generational predecessors.  

The production of all the elements in the Universe - including what makes up humans, moons, the Earth, and even other stars - comes from the powerful internal engines of stars. This wonderful synthesis occurs through the process of nuclear fusion inside the cores of stars - leading to the creation of heavier elements, and the release of energy and radiation.

It starts with hydrogen, which is fused into helium, and eventually trickles into much heavier elements like carbon, oxygen, neon, magnesium and so on. This continues all the way up through the periodic table until you get to iron before no more fusion can occur (iron absorbs more heat than it generates, so puts a halt to fusion processes). 

Eventually, the star either blows these elements out into its local environment through strong stellar winds, puffs away its outer layers gently dispersing these new materials into space, or violently explodes in a supernova, spraying them outwards at high velocities. This newly forged material then goes on to form the next generation of stars. 

Astronomers tend to give funny names to things, and one perfect example is how they name the elements found across the Universe. One term you will often hear them discussing is ‘metals’ - which practically means everything on the periodic table that is not hydrogen and helium. 

And that’s because both hydrogen and helium were the two major elements formed in the primordial fires of the Big Bang. Everything else was churned through a stellar core - like an engine. 

Now, two new papers published in Monthly Notices of the Royal Astronomical Society elucidate how the youngest generation of stars will eventually stop contributing metals back to the Universe.

“We know the first two elements of the periodic table – hydrogen and helium – were created in the Big Bang,” said Amanda Karakas, first author of a paper studying metal-rich stars.

“Over time, the stars that came after the Big Bang produce heavier elements.”

Stars like our Sun are considered to be “metal-rich”, and spew out the elements they produce into space enriching the composition of the galaxy over time. However, the ability to produce more metals is highly dependant on the composition of the star (and thus, the nebulous material from which it forms) at its birth. That nebulous material, contained within giant molecular clouds of gas, ice, dust and particles dictates the composition of the new star prior to it even forming. And that in itself is driven by the former generation of stars polluting their environments with more metals. 

“Introducing just a tiny bit more metal into the stars’ gas has really large implications on their evolution,” said Giulia Cinquegrana. Her paper uses modelling from the study mentioned earlier to study the chemical output of metal-rich stars.

“We discovered that, at a certain threshold of initial metal content in the gas, stars will stop sending more metals into the Universe over their lifetime,” said Cinquegrana.

At approximately 4.5 billion years old, our Sun is a typical “middle-aged” star. Compared to the first stellar generations, our Sun is “metal-rich” and has a heavy metal content. This is similar to many other stars in the centre of the Milky Way.

“Our papers predict the evolution of younger stars (most-recent generations) which are up to seven times more metal-rich than the Sun,” says Karakas.

“My simulations show that this really high level of chemical enrichment causes these stars to act quite weirdly, compared to what we believe is happening in the Sun,” says Cinquegrana.

“Our models of super metal-rich stars show that they still expand to become red giants and go on to end their lives as white dwarfs, but by that time they are not expelling any heavy elements. The metals get locked up in the white dwarf remnant,” she says.

“But the process of stars constantly adding elements to the Universe means that the make-up of the Universe is always changing. In the far distant future, the distribution of elements will look very different to what we see now in our Solar System,” says Karakas.

The first paper is available on the pre-print server

The second paper is available on the pre-print server