Today, 13.8 billion years after the Big Bang, stars have converted about 2 per cent of the universe’s hydrogen and helium into other elements.
They now exist in varying amounts, depending on the frequency and productivity of the processes that create them. Platinum (atomic number 78), for instance, is a million times more rare than iron because neutron star mergers don’t happen very often. That’s one reason why precious metals are precious, John Cowan, an astrophysicist at the University of Oklahoma said.
The presence of elements like carbon and oxygen helped cool corners of the galaxy so that smaller stars like the sun could form. And the appearance of metals allowed solar systems to emerge from the discs of gas and dust that swirled around these new stars.
“There’s now enough junk in the disc that you can form planets,” said Jennifer Johnson, an astronomer at Ohio State University who wrote a review of elemental origins in Friday’s issue of Science to celebrate the sesquicentennial of the periodic table.
“The more iron compared to hydrogen, the more likely we are to find a Jupiter.”
The increasing ratio of iron to elements like oxygen also increased the chances of forming rocky planets with large cores, like Earth. (Large cores can serve many functions, including generating a magnetic field that protects life.)
As the universe ages, the elements within it will get heavier. And in about 10 trillion years, when star formation has fizzled, its composition will stop changing.
There’s debate about how much hydrogen will be left at that point. Johnson thinks a fair amount will remain in the intergalactic medium, while Anna Frebel, an astronomer at Massachusetts Institute of Technology suspects most of it will have been transformed.
But it will still exist in another sense, she said, since all elements are really just rearrangements of the hydrogen atoms that formed in the first minutes after the Big Bang. They have been kicking around the cosmos ever since, in one element or another. Some wound up here on Earth, where they make up everything. Including us.
The beloved astronomer Carl Sagan was fond of saying that “we’re made of star stuff.”
That’s not all, Frebel said: “We are also Big Bang stuff.”