The Fate of the First Stars

Soon after the Big Bang, the first generation of monstrously large stars ignited, lit up the universe, and then died.

The resulting swarms of supernova explosions enriched the universe with the first heavy elements and lots of black holes.

They shaped everything that came after.

These were the stars of population three, and they are one of the most enduring mysteries in astrophysics.

The sun is a latecomer to our universe.

In its light, we see telltale signs of the generations of stars that came before it.

See, the sun and all stars are made of the raw material forged in the heat of the Big Bang itself, hydrogen and helium.

Mostly.

When the sun's light is broken into a spectrum, it reveals traces of many of the heavier elements of the periodic table.

These elements were forged in the cores of earlier generations of stars, stars that exploded as supernovae and spread their element-enriched guts through the galaxy long before the sun was even a twinkle in the eye of a giant molecular cloud.

Astronomers categorize stars according to the relative quantity of heavy elements that they possess.

By the way, astronomers call any element heavier than helium a metal.

And the relative quantity of metals versus hydrogen and helium is a star's metallicity.

Stars that formed more recently tend to have the highest metallicities because they contain the dust of more stellar generations past.

We divide stars up into three populations.

The sun is a population of one star, meaning 2% to 3% of its mass is metals.

And that's a lot.

Pop I stars formed the most recently and are still forming today, typically in the disks of spiral galaxies.

Population II stars are metal-poor, with metallicities around 0.1% or even lower.

These are the oldest stars that we see in the Milky Way.

They were born long ago, when galaxies like the Milky Way were still forming in the early universe.

Today, they're found in the galactic bulge or in globular clusters, which are ancient dense islands of stars that orbit far out in the galactic halo.

Population three stars have no heavier elements whatsoever.

They were the first ever stars shining in the first ever proto-galaxies, born of the pristine hydrogen-helium gas that filled the universe soon after the Big Bang.

I'd like to tell you where they are today, but it's not clear that we've ever seen one.