The Fate of the First Stars

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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 3

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 one star, meaning 2% to 3% of its mass is metals. And that's a lot. Pop-1 stars formed the most recently and are still forming today, typically in the disks of spiral galaxies. Population 2 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 3 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. And that's not for lack of trying. Astronomers have been searching for the mythical Pop 3 generation for decades. Yet they must have once existed. We're starting to think they may be all long dead. Okay, so these things formed at the beginning of the Universe make sense they'd all be gone now, right?

Except that the longest-lived stars, red dwarfs, have lifespans of trillions of years. No red dwarf has ever burned out. Even stars a little smaller than our sun, the orangish K-type stars, live for longer than the current age of the Universe. Still, our lifespan gets shorter the more mass of the star, and I'll get back to why. But stars of the sun's mass and higher that formed over 13 billion years ago near the beginning of the Universe would now be long gone. And this brings us to the leading theory as to the mysterious disappearance of Population 3

They were gigantic. All of them. And every single one has long since burned out. Before we get to why Pop 3 stars were so large, let's unravel this whole lifespan thing. Massive stars live fast, die young, and leave beautiful space-time warping corpses.

One might think that having more mass, more hydrogen to fuse in their cores, would allow a star to burn longer.

However, the light that burns twice as bright burns half as long, and these stars burn so very, very brightly. OK, physics time. The cores of stars are under extreme pressure due to the gravitational crush of their great mass. The more mass, the greater the pressure, and by the ideal gas law, temperature increases with pressure. So the cores of very massive stars are much hotter than our sun's. Up to a couple hundred million Kelvin versus the sun's 15 million K.