The universe and the life of stars

Lovely day, huh. Yeah. But hey! I was awake thinking last night. About the Big Bang and all the atoms created.

What happened then? How did the stars come to be? Like the sun? They are born in giant clouds. What?

In clouds? Yeah, but not those ordinary white clouds. A different kind of cloud. -- When the universe was about 380 000 years old, atoms had been created, but still there were no stars or planets. The atoms were floating around in gigantic clouds of gas. They consisted of Hydrogen and Helium, and are called nebulae.

Nebulae are around today as well, for instance the Orion Nebula, that can be seen with the naked eye on a clear night. It's in the nebulae that stars are born. For instance our sun, which also is a star. They form when matter in the gas cloud is concentrated into one place. The more matter that accumulates, the denser and hotter the pressured gas gets.

The gravity increases, and more matter is drawn into the accumulation, which keeps getting hotter and denser. When the temperature becomes high enough, a reaction starts, where hydrogen nuclei are fused into helium nuclei. This releases enormous amounts of energy, and this is what causes the amassed matter to emit light and heat. A gathering of gas transforms into a star. Gravity also makes huge numbers of stars attract each other and gather in vast groups: galaxies.

It's estimated that there are several hundred billions of stars in the galaxy where we live, and over a thousand billion galaxies in the universe. All stars and galaxies in the universe are created this way, in nebulae. But are they around forever once they have come to be? No, just like a bonfire goes out when the wood is burned up, a star goes out when the fuel is gone. The life span of a star is limited. -- Stars 'die' in different ways, depending on how big they are.

The sun goes out when there are no more hydrogen nuclei that can fuse into helium nuclei. But before it dies, it grows and devours the planets nearby. And if there is still life on earth, it ceases to be. The sun changes colour and becomes a red giant. Large amounts of gas are hurled away from the sun, and what's left is a small white dwarf.

Slowly, all nuclear reactions halt, and the sun goes out. So when the sun dies: when it goes from its current phase, to red giant, to white dwarf, some of the gas will be hurled into space. New gas clouds appear, new nebulae. In these nebulae, a new generation of stars can be born. It has been 13.8 billion years since the universe was formed, and 4.5 billion years since our sun was born.

What does the future look like for the universe? We don't know. Some scientists believe that the universe will contract again into one single point, a singularity. Just like at the moment of the Big Bang. Others hold that the universe will exist for all future time, but that it will become sparser and sparser and sparser.

So far, all the scientists can see is that the universe expands, at an accelerating pace. There is a force that pulls the universe apart, but no one knows for certain what causes that force and what will happen with the universe. -- What do you think, Philip? I think we need to hurry up, if we want to catch some fish before the sun dies out. Plenty of time. We have 5 billion years.