# primary

## Making the World Around Us

Atoms are very tiny objects that make up the world around us. They are about a hundred thousand times smaller than a human hair and way too small to be able to see! Atoms come in differnet types, which we call Elements: Oxygen, Gold and Carbon are all examples of different Elements.

For a long time, scientists believed that Atoms were the smallest, simplest objects that existed, but now we know that atoms are actually made up of three even smaller objects - Protons, Neutrons and Electrons.

These three tiny objects, or particles, each have a different job in the Atom. The Protons and Neutrons are found right in the middle of the Atom, which we call the nucleus. Protons have a positive charge: this is a bit like one half of a bar-magnet. It is difficult to get Protons too close to one another, just like two North-poles of magnets. To help the Protons stick together, Neutrons are packed in between the protons. Neutrons are neutral and have no charge.

The number of Protons in an Atom tells us what type of Element it is - Oxygen, Gold and Carbon will all have different numbers of Protons. If you add or remove a Proton from an Atom, then it will become a different Element. Neon has 10 Protons and is a gas; add one more Proton and you will form Sodium which is a metal.

In the picture below, you can see Protons and Neutrons in the middle of the Atom, and Electrons around the outside. Electrons have a negative charge, opposite to the Protons. Normally an Atom will have the same number of protons and electrons, so that its total charge is 0.

The Element below is an Atom of Lithium, which has 3 Protons, 4 Neutrons and 3 Electrons.

## Periodic Table

The Periodic Table of the Elements shows all the different discovered Elements from the smallest to the biggest. Hydrogen has one Proton and is the first Element; Helium has two Protons and is next; and so on. All the Elements are placed in the table according to how many Protons they have.

It is called the Periodic Table because of the order and patterns that different Elements have. For example, all the elements in the very first column (called a “Period”) are metals that will explode when dropped into water; and the elements in the last period are all gases that will not have any chemical reactions. The Periodic Table has a very unique shape, which is based upon how the electrons are arranged around the outside of the atom.

We will see that all of the elements in the Periodic Table are formed in stars, in many different processes. The smallest elements are made in small stars, but to make bigger and bigger elements we need bigger stars, higher temperatures and the Biggest Bangs that the Universe can offer.

## Nuclear Fusion

In a star Protons and Neutrons are added together to form larger and larger Elements. Sometimes a Proton will change to a Neutron, or the other way round, if there are too many of one type.

Stars are incredibly heavy - the sun is 300,000 times heavier than the earth. They are so heavy that all the atoms in the star are being pushed into the middle of the star by Gravity.

Atoms in a star have no Electrons, and therefore have a positive charge, like a bar magnet. It takes lots of energy to make two Atoms touch one another, because they will try to push each other away. In a star the Gravity pushes all of the atoms together so strongly, that when they are pushed together, two small atoms can join together to make a bigger one.

In this way stars make new Elements by joining smaller ones together. This process is called Nuclear Fusion.

When this happens, lots of heat energy is released, which is what makes the Sun and all the other stars in the sky hot (the Sun is over 1 million degrees)!

## Fusion in our Sun

The Sun is a simple star and there are only two Elements found in the sun. They are the simplest and smallest Elements - Hydrogen and Helium. A Hydrogen atom contains one Proton and zero Neutrons. When two of these join together, one of them turns into a Neutron and makes a different kind of Hydrogen (called 2H, because it has 2 particles in the atom).

[Beta decay for secondary]

As more simple Hydrogen (1H) atoms are added to this new Hydrogen (2H), we form a new element called Helium. Helium has 2 protons and 2 neutrons, and has the symbol 4He.

It is this Nuclear Fusion that makes the Sun so hot. All the heat and energy in our Sun come from the creation of Helium.

The Sun is not the same all the way through. In the middle, where it is hottest, Nuclear Fusion takes place, making Helium. Around the outside it is not hot enough to form Helium (although still hundreds of thousands of degrees), and there is a layer of Hydrogen waiting to be used.

The sun will take 8 billion years to turn all of its Hydrogen into Helium, and it is only about halfway through! Once it has used up all of its Hydrogen, the Sun will turn into a different type of star called a Red Giant.

[Periodic Table]

## Heavier Stars, Heavier Elements

The Sun is only a little star compared to some of the other stars in the galaxy, even though compared to us on earth it seems enormous.

In much heavier stars, once all of its Hydrogen has been turned into Helium, they will start joining Helium atoms together to make bigger elements. These are the elements which make up all of life on earth i.e. Carbon, Oxygen and Nitrogen.

Making these elements releases lots and lots of energy, making these bigger stars even hotter than the Sun.

These new elements will be made in the middle of the star, causing layers of different elements to be formed. You can think of this a little bit like an onion, where each different layer represents another element that has been made.

[Star Structure - layers]

In the heaviest stars more Helium atoms can be added to Carbon to make bigger and bigger atoms. The biggest atom formed like this is Iron, which has 26 protons and 30 neutrons (56Fe). The new elements are made by joining the elements we already have to one another.

[Binding Energy per Nucleon]

[Periodic Table]

## Supernova

In stars there are two opposite forces happening, that balance each other out while Nuclear Fusion is happening: Gravity and Pressure.

Gravity pushes all the atoms into the centre of the star, making the star as small as possible. Stars are super heavy and therefore the Gravity is very strong.

Pressure is made by the temperature in the centre of the star. When atoms get hot, they move around faster and faster, and push outwards to take up more space (a thermometer works because the Mercury inside gets slightly bigger as it gets hotter). Atoms in a star are moving around very fast, pushing outwards as they heat up, trying to make the star bigger.

Normally these two opposite forces balance each other out and nothing changes. But when a star runs out of atoms and cannot do any more Nuclear Fusion, it stops creating heat. This means that there is not enough Pressure, and the Gravity makes the star collapse into the middle.

All of the atoms rushing into the middle cannot fit and bounce back from the centre. This makes the star explode, throwing most of its atoms back out into space. This is a Supernova!

During a supernova even bigger elements can be created.

The huge temperature of a supernova explosion creates new elements in a new way. Instead of adding two small elements together, Neutrons are added to the atoms from the Supernova.

When lots of Neutrons are added to an atom, one of the Neutrons will turn itself into a Proton! Having a new Proton in an atom means that we have created a new element. In Supernova 80 Neutrons might be added to an atom, creating elements more than twice as big.

In this way, Supernova can create over 20 new elements in the explosion of the star, including Copper, Silver and Bromine. The largest element made in a Supernova is Barium, which has 56 Protons and 82 Neutrons.

[Periodic Table]

### Supernova Remnants

When a Supernova explodes, we can look at the elements thrown out into space to see what was being made in the core of the star. Each element will have its own unique colour, which can be detected by telescopes looking at the leftovers from a Supernova – a Supernova Remnant.

Looking at the colours of the elements might seem strange initially, after all, aren’t some gases colourless and most metals the same silvery-grey? Each element will burn a different colour if it is hot enough. The colour is based on the number of Protons, Neutrons and Electrons, which will be different for each different element.

We can look at the colours in a Supernova Remnant to see what elements were made inside the star before it exploded. Above is a tiny part of a Supernova with all of the elements shown. On the left are the elements separated out into their specific colours. This part of the Supernova Remnant contains three elements: Hydrogen (red), Oxygen (blue) and Sulphur (green).

Every element will have its own unique colour. For example, if you take different metals and burn them in a hot enough flame, they will give off a flame in their unique and sometimes surprising colour.

Five different metals can all be seen with very different flame colours. The metals used above are Lithium (Li), Strontium (Sr), Sodium (Na), Copper (Cu) and Potassium (K); and they burn very bright, distinct colours! The colours seen here would be exactly the same as seen is a supernova remnant. Sodium burns a very bright orange colour, which is the same colour as street lamps, because street lamps use Sodium metal in them to produce the light.

### Neutron Star Mergers

So far we have seen how we make most of the elements that we know about on earth, including the most important and common elements. Carbon, Oxygen and Hydrogen make up most of our bodies; Iron makes up the core of the earth; and other elements like Calcium and Aluminium which we use everyday.

The very biggest elements like Gold and Uranium are very rare and made when two super heavy stars collide into one another. These super heavy stars are what is left after a supernova, and they are called Neutron Stars.

A Neutron Star is the densest object known to exist in the universe. Imagine the Sun, which is 300,000 times heavier than the earth, being squeezed down until it is only 10 miles across!

A Neutron Star is made almost entirely from Neutrons, one of our three building blocks that make up atoms. Around the outside of a Neutron Star there is a thin layer of heavy elements from before the supernova - Copper, Silver and Barium.

Two nearby Neutron Stars will gradually move closer and closer together until they eventually collide. When they collide, there is a very short moment (less than half a second) when the temperature is hot enough for neutrons to be added to the elements on the outside of the stars. Although this happens really quickly, over 100 neutrons can be added to an atom in this time!

Some of these neutrons will turn into protons, creating brand new, massive elements. Radioactive elements like Uranium and Plutonium are made in Neutron Star Mergers as well as rare metals like Gold, Platinum and Lead.

A Neutron Star Mergers are the biggest and brightest explosions in the entire universe, and make the Biggest Bangs that can ever be made.