Stars have long fascinated humankind – we look for patterns and pictures among them, or try to predict the future by their apparent movement; we give them names and attach mystical significance to them. People have used the stars to navigate across the oceans and deserts, to work out the right time to plant their crops and to note the passing of the seasons. Changes and the night sky have been considered so significant that they are thought to herald world-changing events – such as the birth of a Messiah.

The Universe

Several lines of evidence suggest that the Universe began in an event about 13.6 billion years ago (13,600 million years).

Firstly, every star in the universe appears to be travelling away from Earth. This is tricky to understand, but the key idea behind it is that this is only an illusion: what is really happening is that space itself is expanding. We can tell this because the further away stars appear to be travelling faster – if they really were moving, they should all be moving at the same speed or the further ones should be moving slower.

Secondly, we can still see the ‘glow’ of the early, hot universe in the sky. This forms something called the Cosmic Microwave Background (CMB). Although the radiation in the early universe was made of extremely short waves – what we would call gamma rays – space itself has expanded since then. The electromagnetic waves that made up the radiation in this early, hot universe have been ‘stretched out’ by the expansion of space in the universe. This causes the original short waves are to be stretched out to about 4 cm in length, which we call microwaves.

Everywhere we look in the sky we see these microwaves. The short wavelength radiation that once filled the whole universe has nowhere to go, so it is still there 'in the sky'. Over time it has shifted into the microwave spectrum. A ‘map’ of this is shown below:

The picture above shows the microwaves in the whole sky, converted to false colour. More information here.

We call the event that started it all off ‘the Big Bang’. This name is misleading: it wasn't an explosion. The term represents the idea that if you ‘run time backwards’ and see what happens to the universe in the past, you get to a point where the universe is very small and very hot. Try and run further than this and you can’t go back any more because our present ideas about physics don’t make sense , particularly our ideas about time and space. This is why we can't say what was 'before' the Big Bang, because the way we think about time doesn't allow us to run the 'clock' back past this time (I can't explain this simply, because it involves explaining about Einstein's theories about the relationship between time and space. If you are interested, read further here).

Distances in Space

Space is big. You've probably heard that before, Even scientists have trouble getting their heads around just how big. In this section we are going to look at some of the cosmic distances and briefly explain how we know how far away things are.

Distance and size in our Solar System

The Earth is a bit more than 12,000 km across and is about 150,000,000 (150 million) km from the Sun. The Sun is about 1,000,000 (one million) km across. The distance to the Sun was worked out by looking very carefully at the position of Venus on the rare occasions when it passes across the face of the Sun as seen from Earth (this is what Captain Cook went to Tahiti to do in 1769).

Here's another way to think of it: some of you might have flown to Australia. This takes about 3 hours, travelling at about 900 km per hour.

If you were able to travel at that speed from the Earth to the Sun it would take you a bit over twenty years to get there, flying continuously. That's 20 years to travel 1 astronomical unit travelling at the speed of a passenger jet.

Neptune, the furthest true planet, is 30 astronomical units from the Sun. On our model above that distance would be 4.5 km - that's about the distance from Sacred Heart to Panmure Bridge.

Astronomical units sound big, and they are on a human scale. But the closest star outside our Solar system is over 250,000 astronomical units away. We use astronomical units for Solar System distances (our own and other Solar Systems), but we need bigger units for stars and galaxies.

For distance between stars, we often use light years. A light year is what it says: the distance that a ray of light would travel in space in one year. Light travels at a speed of 300,000 km per second. It takes light only eight minutes to travel the 150 million km from the Sun to the Earth.

Formation of stars

Stars begin as giant clouds of hydrogen molecules known as Giant Molecular Clouds or GMCs. These get shocked into swirls and eddies by factors such as supernova explosions and gravitational disturbances from black holes.

Areas with higher density have more gravity, and draw in more gas.

As the gas is drawn in by gravity, it starts to spin and gets flattened into a spiral disk. The centre of the disk starts to heat up, as all gases do when they are compressed.

When the centre part reaches a hot enough temperature, the hydrogen starts the process of nuclear fusion. This turns the hydrogen into helium and gives off a lot of energy . This is when it becomes a star.

Types of star

Not all stars are the same. The two main factors in different sorts of star is how big they are and how old they are.

Our Sun has a mass of 2 x 10³⁰ kg - that is 2 million million million million million kilograms. This is known as a solar mass. Other stars range from 2% of the Sun's mass up to about 50 solar masses.

The mass of a star has a huge influence on its life story; the larger a star, the brighter it is and the shorter its life

Looking at stars in the sky

A shape of stars in the sky is called an asterism. Two of the easiest asterisms to find in the night sky in Aotearoa are the Pointers and the Southern Cross:

The two stars in the Pointers are called Alpha Centauri (Rigel Kent) and Beta Centauri (Hadar). They appear to be a similar brightness.

Alpha Centauri is the closest visible star to Earth at about 4.5 light years away. Beta Centauri is much further away: 390 light years.

You can work out that Beta Centauri must be much brighter than Alpha Centauri to appear the same brightness despite being so far away. The amount of light a star gives off is called its luminosity. If we use our Sun as a standard with a luminosity of 1, stars vary from luminosities of about 0.0001 to over 100,000.

Stars are also different colours. The difference is more obvious through telescopes than it is with the unaided eye. The colour of a star depends on its temperature. The least hot stars are red, and they go through orange, yellow, white and blue as they get hotter.

If you plot a graph of temperature against luminosity for a whole lot of stars, an interesting pattern emerges:

Stars don't just plot randomly anywhere on the graph. Instead, they fall into certain areas as shown above. This is caused by the life cycle of the star.

Some points about the diagram: Stars on the Main Sequence are smallest (least heavy) on the right and largest on the left, so this goes from biggest top left to smallest top right. Supergiants at the top are large stars which have cooled because they have left the main sequence when they ran out of hydrogen. Giants are Main sequence stars from the middle when they reach old age, and white dwarfs are the same stars after they've died.

Star life cycles

When a star first starts glowing, it is turning hydrogen into helium by nuclear fusion, We say for shorthand it is 'burning' hydrogen.

The bigger a star, the hotter its core and the faster it burns its hydrogen. This makes larger stars brighter and shorter lived. The period when a star is burning hydrogen is called the Main Sequence, and it can last as long as a trillion years for a Red Dwarf star such as Wolf 359. Very large stars, such as those in Matariki, may be on the Main Sequence for as little as 10 million years.

The next smallest stars are orange and yellow dwarfs like our Sun. These stars form layers because the helium they make builds up in the core. When their hydrogen runs low, they expand into Red Giant stars and blow off a lot of matter as a 'planetary nebula' . After this they collapse into a White Dwarf.

A White Dwarf the corpse of a dead star. It is made of helium, carbon or oxygen usually. They are hot but very faint. They are not producing any energy, only radiating away the leftover energy from their earlier life. Over billions of years they cool down into a black dwarf.

Aldebaran and Arcturus are examples of stars similar to our Sun but in its Red Giant stage. Sirius is a Main Sequence star that will expand into a Red Giant much sooner than the Sun because it is bigger. Sirius has a White Dwarf companion that is the remains of what was once a larger star in the same system.

Large Stars

Stars which are more than 8 times the mass of our Sun have short violent lives. Their cores are so hot that other fusion reactions start early in their lives: helium into carbon, carbon into neon and oxygen, oxygen into silicon and so on. These stars are the element factories that made many of the atoms in your body and around you.

After a supernova explosion there is a remnant left over. A smaller supernova forms a type of dense, tiny object called a neutron star. Larger supernova remnants form black holes.