Gold on Earth Owes Its Existence to Dwarf Galaxy Mergers

 Our Milky Way galaxy, like all large galaxies, grew by accreting many smaller galaxies billions of years ago. And, you or your spouse’s, wedding ring owe those galaxies a debt of gratitude.

When our universe first came into existence nearly 14 billion years ago, it contained only hydrogen, helium, and a trace amount of lithium. These are the three lightest and simplest elements. A glance at a periodic table shows our universe contains 92 naturally occurring elements with uranium being the heaviest. We have created another dozen or so elements in laboratories around the world. Of those 92, all of them up to iron are created inside normal stars, like our sun.

Stars form these heavier elements through a process called nuclear fusion. The incredible heat in a star’s core slams lighter elements together making heavier and heavier elements and releasing more energy into the star’s core. But making iron requires more energy than it creates. Fusion reactions almost instantly halt.

Stars like our sun die rather quietly after that point. But, stars more than ten times larger than the sun die in a spectacular supernova explosion. These events release so much energy that elements heavier than iron can form. However, supernova explosions cannot account for the amount of “jewelry store elements,” like gold and platinum, in the universe.

Recent studies revealed that collisions between two neutron stars release enough energy to create the amount of gold in the universe. Astronomers have discovered hundreds of gold-rich stars in the Milky Way. But they wanted to know when and where these stars formed. Scientists from Notre Dame and Tohoku University in Japan used a powerful supercomputer to run simulations. Their work showed these stars formed in dwarf galaxies that merged with the Milky Way some ten billion years ago. In their study, they said “The gold-rich stars (we see) today tell us the history of the Milky Way. We found most gold-rich stars are formed in dwarf galaxies over 10 billion years ago. These ancient galaxies are the building blocks of the Milky Way.”

Colliding neutron stars. Credit NSF-LIGO-Sonoma State Unioversity-A. Simonnet

Not only did these many dwarf galaxies grow our Milky Way, they also created most of the gold that exists on Earth. The universe does work in mysterious ways.

Each month, I write an astronomy-related column piece for the Oklahoman newspaper. After it is published there, I post that same column to my blog page.

This is reprinted with permission from the Oklahoman and www.Oklahoman.com.