This Month’s Media is from PhD student Giulia Cinquegrana from Monash University, who brings us research on totally metal stars! Giulia and her supervisor, A/Prof Amanda Karakas, have had some media attention about two papers they recently published. They find that stars with greater amounts of heavy elements in their gas at birth are not contributing the fresh metals they make back to the universe over their lifetime. Instead, they’re keeping the heavy elements they make all to themselves, to be locked inside their dead remnant for all eternity!

Before we can get into the weeds of this research, we’re going to need to brush up on our stellar evolution knowledge – that is, how each generation of stars formed, died and “returned” to the Universe. First, as with all things in the Universe, we start with the Big Bang (BB), the single point in time and space that everything we know about came from. Shortly after the BB, the two most basic and lightest elements formed: hydrogen (H) and helium (He). With the tiniest amount of our first “metal”: lithium (Li).

You’ll have to excuse astronomers, our definition of “metal” is different to most. There’s hydrogen and helium, and then literally everything else on the periodic table is a metal. Keeps things simple.

From this first Universe-wide cloud of H, He and Li, gravity eventually pulled bits of it together so much that they were crushed under their own weight. The pressure and temperatures soared, and around 1 billion years after the BB, the first stars “turned on”.

In general, stars create new metals out of the building blocks they began with – this process is called nucleosynthesis. When the core of a star is primarily turning H into He, it is said to be a “main sequence” (MS) star, which is the phase our Sun is in. Stars will spend about 90% of their lifetime in this MS phase. After the star has fused enough H into He in its core, it moves off the MS and depending on how massive it is, it could do a number of different things.

For the most massive stars (massive meaning it’s got lots of stuff crammed in), the end of its evolution will see it explode as a supernova. Here the star can either rip itself apart completely or leave behind a small remnant object left over from its core: a neutron star or a black hole. The material blasted off during the explosion is returned to the Universe.

Left: M87*, the first image of (the shadow of) a black hole – though this one didn’t form through the collapse of a star, it’s much too big! Credit: Event Horizon Telescope Collaboration. Centre: Artists impression of a neutron star. Credit: Casey Reed, Penn State University. Right: Cassiopeia A Supernova Remnant. Credit: NASA/CXC/SAO.

Less massive stars, like the ones Giulia is studying will not end so violently. Instead, these stars will merely swell their outer layers to hundreds of times their original radius and stellar winds will erode the outer layers. All that is left is just the very dense core, while those outer layers are returned to the Universe. This is how our Sun will end in billions of years time.

Left: Artist’s impression of a red giant expelling material into space. Credit: JAXA. Right: The planetary nebula NGC 2440, surrounding a white dwarf remnant. Credit: NASA, ESA, and K. Noll (STScI).

The first stars only had H, He and Li to work with, but when they died they enriched the Universe around them with new elements. From these remains new stars formed, and they had a different mixture of elements to play with. This happened over and over again as the remains of previous stars are constantly recycled. Our Sun is made up of about 2% metals because of all the stars that “lived” before it.

Scientists have observed stars in our own galaxy that contain more than twice the metal content of the sun (~4%). But it’s likely that there are stars out there in the universe with even higher percentages of metals in their gas. However, for these high metallicity stars, it seems like they are keeping their metals to themselves. Instead of producing more metals than they started with at “birth”, these stars are returning gas back to the Universe with the same metal content that they started with.

Why are they not producing more metals?

To answer that we need to consider where in the star these metals are made. For nuclear reactions to occur, we need really high temperatures and densities. This means that most of the reactions are happening in the central region of the star (not in the outer layers called the “envelope”). But, for a star to recycle metals into space the metals need to be in the puffy envelope that gets eroded by the stellar winds.

The trick here is a process called a “dredge up,” where the star is able to mix the products from the central region up into the big puffy envelope. When that envelope is then eroded, the metals will be recycled back into space!

Giulia and her supervisor Amanda found that stars with a higher initial metal content at birth struggle with this mixing process. More metals seem to cause a less efficient mixing process. So, at a certain percentage of initial metal content (~8-9%), that mixing process stops altogether. Which means all the good metals that the star has produced over its lifetime are locked inside the core remnant and we’ll never see them again!

Mixing efficiency versus number of star pulses. The purple dots show a star with the same beginning metallicity to our Sun (2%) that undergoes an efficient drudge-up process and will return new metals to the Universe at the end of its “life”. The blue line shows the mixing efficiency of a star with twice the starting metals as our Sun (4%), with the yellow squares showing that with only a more starting metals again (5%). As the beginning metallicity increases, the mixing efficiency for each reduces, meaning less and less of the new metals will make it into the Universe when the star “dies”.

Why are these stars so bad at mixing their metals?

In the last phase of evolution for stars like our sun, they undergo pulses. We think that the strength of the pulse drives the mixing, which happens after the pulse. The higher the initial metal content, the weaker the pulse seems to be. So at 8-9% metals the stars are undergoing much weaker pulses and the envelope isn’t able to reach far enough into the centre to mix the metals up.

Chances are, these weaker pulses are somehow stopping the stars from dredging-up their metals, but the exact mechanics are yet to be unravelled. These are mysteries to be solved another day, and for another Monthly Media!