The First Stars: Jewels in the Night
After a slumber, the Universe rewakes;
ready to warm and shine once again.
The Universe is now cold and lumpy. The thin gas of the dark ages. leftover from the Big Bang has clumped into small regions which have started to form some structure. Over time these will become the stars in our night sky. A Universe we’d recognize is finally starting to form. Let’s dive in, to the early Universe.
The First Stars and Galaxies: How and when?
Stars all form the same way. They start with enormous clouds of gas spread out across billions of kilometers. Gravity then works on these clouds by pulling the bits of gas in the cloud together into clumps, the same way it pulls a leaf off a tree towards the ground. The difference is that the clouds in space are much less dense than the Earth, so it takes much longer for them to clump together, often millions of years.
Once gravity has pulled enough gas together, it starts to burn and turn into a star. As more dust comes together, the center of the cloud starts getting denser as gravity pulls it together more tightly. Over time, millions of years, the center of the star starts to get really hot – similar to the center of the Earth. Eventually, gets much much hotter – hot enough to essentially start a nuclear reactor. Except, the nuclear reactor is a bit different to the ones we’ve built on Earth – instead of splitting atoms, it combines them under extreme temperature and pressure. At their most simple, stars are just big nuclear explosions formed from gravity squeezing gas together until the star uses up its fuel and burns out.
It’s easy for this to sound a bit weird – how can gas suddenly just start burning? Do things really get that hot when you just push them together really hard? The answer is, yes, they do. On earth, you can grab two bits of wood and rub them together and start a small fire. And that’s with a tiny little stick. Then think about how hot a massive raging bushfire can get. The center of the Earth gets to around 5,000 degrees celsius, due to all the pressure of the rock above it, pressing down. The smallest stars are millions of times heavier than the earth. The difference between a planet and a star is not like a mouse and an elephant, it’s the difference between an ant and a continent. When the weight of a star starts pushing down, it will split atoms in half.
The first galaxies and stars formed about 500 million years after the Big Bang (see GN-z11 the oldest known galaxy). However, there wouldn’t have been many. And for a lot of the early Universe, there would not have been galaxies of stars, big sweeps of them like we see in our night sky. Instead, they would have just been randomly scattered throughout the Universe, occasional pinpricks through a dark blanket. However, these stars created the first metals and heavier elements (see below). These heavier elements then had more weight and allowed the next generation of stars to form more quickly and burn for longer, with more substantial fuel. The process of building some variety and complexity in our Universe had begun.
The Early Stars: Our Universes element factories
By the end of the cosmological dark ages (see this post), about a billion years after the Big Bang, the Universe would have started to look recognizable. There would have been a few galaxies with a few stars in them burning brightly. And even more sweeping nebula waiting their turn to become stars. There would not have been anywhere near as many, so the night sky would have been far darker. However, over time within a few billion or so years, there would have been plenty of them.
However, our solar system only begins to form about 7 billion years later. So what happened in between?
The main difference between our Universe and the early Universe with a few stars is the amount of metal. Even 500 million years after the Big Bang, there was basically still only hydrogen and helium in the Universe. Because the matter of the universe formed directly out of rogue particles created by the big bang, every electron just grabbed the nearest proton they could find (like enthusiastic kids at a school dance) and turned into hydrogen or maybe a helium atom. There was no waiting around to form larger more complex atoms like metals.
Because the Universe was only made out of hydrogen and helium, basically nothing chemically complex or interesting can happen. No rocks can exist, no water, let alone life. Instead, you just have sweeping clouds of gas and the occasional giant burning gas ball.
So how did things other than hydrogen and helium come into being? Stars made them.
A star burns very differently to the way things burn on Earth. A woodfire on earth operates by breaking down chemical structures, for example, breaking wood down into charcoal. Similarly, a nuclear bomb works by splitting an atom. However, at very high temperatures, like in the middle of stars, you can also create energy by putting things together. For instance, if you take two hydrogen atoms and push them together hard enough, you can create what is called ‘nuclear fusion’ where you fused their nucleus together and create a helium atom. Similarly, you can then take two helium atoms, force them together and create a beryllium atom.
Once a star has finished doing this, and run out of fuel it will generally explode and distribute its newly made elements into the Universe. Then the next star can come along, and gobble up its remnants and start burning those more advanced elements and created even more complex elements. For something like gold, it needs to go through this process many times – every gold atom has burned in the heart of many stars. All the complex elements in our Universe were produced by chains of stars, gathering up elements, burning them, exploding them out into the rest of the Universe and then repeating the process.
So, are we actually stardust?
It’s a bit of a cliche, but it is true. Every single atom of oxygen, nitrogen, carbon, iron, gold, silver – has been created in the center of the star. And not just one star, often many. The laptop or phone you are reading this on has almost one of every single element the Universe can create. Each of them was probably created by a chain of different stars billions of years ago.
Every part of you is a product of the Universe. And when you die and eventually, our planet dies and is absorbed into the sun in its final blaze of glory, we will be re-absorbed into the Universe.
We are the Universe trying to understand itself. We just happen to be, at the moment, pieces of the Universe that can smile.
