A few hundred million years after the big bang, the first stars formed. We aren’t exactly sure how, but we do know that they contained Hydrogen, Helium, and a little bit of Lithium. These were the only elements in the entire universe at the time. Within these first stars, the fusion of heavier elements began. Oxygen, Nitrogen, Carbon, Iron, and all the other elements that make up everything we know formed Billions of years ago in these first stars and in their progenitors. It was a slow process to produce these elements and seed them throughout the cosmos, but over time, the universe did evolve. Even the second generation of stars had only trace amounts of these heavier elements compared to stars that we see today.
The amount of the heavier elements in a star is what astronomers refer to as ‘metallicity.’ When we talk about metals in a star, we mean everything heavier than Helium. To an astronomer, Oxygen, Carbon, and Nitrogen are metals. It acts as a great tracer of the age and evolutionary history of a star, because the metallicity of the universe has always increased over time. Stars with high metallicity are younger, while low metallicity stars are older, since there just wasn’t an abundance of heavy elements in the universe when they formed.
Generally, the most massive stars, being hotter and larger, will burn through their fuel quickly, rapidly fusing Hydrogen and Helium into heavier elements. This also means that they live much shorter lives, some lasting only a few hundred million years (An astronomical blip on the cosmic radar).
Smaller stars can live much longer, and red dwarf stars, being the smallest and coolest, can last for many billions of years, even longer than the universe has been around. In recent years, with the advent of better instrumentation for astronomers, we can precisely measure the metal content in distant stars, giving us an estimate of their age.
There is a growing group of astronomers who are looking to metal-poor stars as a way of mapping the early universe and the behaviour of the first stars. Getting an idea of the number of extremely metal poor stars and their distribution can help astronomers map the evolutionary history of the cosmos, in the same way looking at fossil characteristics can help piece together the history of life’s evolution.
Some of the stars that are being found have a metallicity as low as one-ten thousandth as the Sun, making them nearly as old as the universe itself. These stars contain within them the initial conditions of the early universe from before galaxies formed, like an ancient time capsule buried underground. They are hard to spot, as they are rare and lost in the field of incredible stellar density that we see at the center of the Milky Way. Even though the stars are metal-poor, they can still be relatively young, but only if we can prove that they formed in a lower density region where the heavier elements hadn’t been distributed yet. We can find clues about this by looking at the star’s trajectory. If its moving quickly, it may have traveled a great distance, but if it’s slow and regular, chances are it hasn’t ventured far from its birth cloud.
The field of astroarchaeology is a new and burgeoning field, as we are just reaching the basic technology needed to utilize it. Over the next decades, the distribution and characteristics of these ancient stars will reveal some of the oldest secrets of the universe. It’s like doing cosmology in our own backyard.