The Mystery of Population III
By: Ella Alderson
Tags: Technology, Science, Physics, Universe, Space

Galaxy CR7 lies in the Sextans constellation of the sky. It’s a rare galaxy — among the oldest and most promising locations to find objects that have up until now been merely hypothetical. These objects assert in our best theories of cosmology that they must exist and yet avoid any direct observation. The galaxy, which stands for Cosmos Redshift 7, is jeweled by a collection of blue and red stars. The color differences help to distinguish between star types with the red stars being of the familiar Population II family. But it’s the others, the stars gowned in their mysterious sapphire glow, that hold something of the new and exciting for us. Both the age and the composition of CR7 make it a perfect place to find Population III stars, embedded there in its ancient bright bed of matter. In studying these stars we’d learn about the formation of galactic structures, of planets, and of life itself.

Population III stars are so named because they were the last to be identified by scientists, following the Population I and Population II families. The most crucial difference between the population types is their metal content. Young stars like our sun have a mass that’s 2–3% metal, giving them a high metallicity and denoting them as Population I. Living in crowded islands of old stars — known as globular clusters — or at the central bulges of galaxies are the Population II stars with .1% or lower metallicity. But no matter how low their metallicity, the key is that they do have a metal content and it’s much higher than the gas after the Big Bang. This gas would have contained only the lightest elements of hydrogen and helium, with some trace amounts of lithium and beryllium. It was a pristine, unadulterated gas swelling in the expanse of space. 800 million years after the Big Bang, the first stars would have formed from the collapse of this primordial gas. Unlike Pop I or II stars, Pop III stars contain no heavy elements at all but instead hang huge and momentary, growing to masses several times that of the sun and burning thousands of times brighter. Their hotter cores give way to higher rates of fusion, in some cases reaching temperatures of a couple hundred million Kelvin compared to the cool 15 million Kelvin at the heart of our sun.

Above is a visualization of a Pop III star during formation. Earlier estimates put the first stars at up to an immense 1,000 solar masses with later simulations giving us more modest figures of 60 to 300 solar masses in size. This would classify them as high-mass stars. Captivating, but short-lived. Visualization by Ralf Kaehler. Simulation by Tom Abel.

Any elements heavier than hydrogen and helium are made either at the core of large stars (lithium to iron) where they eventually leech out into the interstellar medium, or during the star’s grand finale as a supernova. This explosion fizzes and bestows the surrounding space with chemical elements, creating anything heavier than iron within its first few seconds. A new generation of stars is born from this gas and each generation of stars after gives way to a gas that’s increasingly metal rich, thus producing young stars with the highest metallicity and older stars with the lowest metallicity. That is, interstellar gas is recycled and enriched, leading to each fresh set of stars having a higher metal content than the last.

The cores of these stars would then have seeded the supermassive black holes at the center of galaxies. Each monstrous black hole left behind by a Pop III star would merge with other equally monstrous black holes, eventually reaching billions of solar masses (this year a black hole of 40 billion masses was announced). They pull in matter and dark matter in a sweeping swirl. A swirl which eventually becomes a galaxy laden with planets, moons, and the curious beginnings of life. And more than just living in the galactic homes created by Pop III stars, we’re dependent on the elements they’ve created. Skin, blood, and bone from carbon, oxygen, and nitrogen.