Living with a Sun (Choose your own Star Part 2)*

The Sun-like star looked like the perfect choice. But, quite suddenly, things began to change...

*This series of posts are an homage to the Choose your own adventure book series, which my Earthling alter-ego so much enjoyed in her youth.

Evolution of your Sun-like star. Credit: NASA Goddard Space Flight Center

After studying the stars in the shop, you decided to take home the Sun-like star. And you are quite happy with it. It's been shining with a yellowish glow for nearly ten billion years, without any important disturbances. Ok, one storm from time to time, but who is perfect?

You have had time to complete your kit with some planets and, after some experimentation, you have managed to place one at the right distance for it to maintain liquid water on its surface (an atmosphere turned out to be very useful for that) and develop some forms of life. All in all, you are proud of your work.

But now, you have started to observe some changes in your star: It seems to be getting bigger and redder. What is happening?

A red giant

You call a star doctor (a.k.a. an astronomer), who, after careful examination, concludes that something is going on in the star's core: It is running out of hydrogen! This means that your star is no longer able to produce enough energy to balance the force of gravity. Its core is shrinking!

A consequence of this is that the core is also heating up. This heat is transferred to the outer layers of your star, which are expanding, getting further and further away from the core, and cooling down again because of the large distance. So the star's surface is now actually much cooler than before, 3,300 degrees Celsius or less. This is the reason for its red colour.

The star is also much bigger. So big that is threatening to devour your little life-harbouring planet!

So it works: Ignition of helium fusion in the core of a solar-mass star. Credit: T. Kallinger (University of British Columbia & Universität Wien)

A brief solace

When you are already desperate about the fate of your star, there is a flash of hope. The process seems to have stopped! The star, in fact, seems to have decreased in size a little bit, and its colour has turned a bit more yellowish. According to the star doctor, the core is now so hot that helium fusion has begun, producing a lot of energy again. The balance has been re-established! And this seems to have happened in a matter of minutes.

Alas, this was a very short relief. After only hundred thousand years, here we go again: Once more, the star is expanding. Now it is clear to you what is happening: The core must be running out of helium. True, the star still has a shell of burning hydrogen and a shell of burning helium, but the core itself is mostly carbon (maybe some oxygen too) and is not releasing any more energy.

So the star grows and grows, getting cooler and cooler... The outer layers are so far away from the core, and the stellar winds are so strong, that it seems impossible to hold them anymore... Off they go, getting spread in space.

NGC 6751, a planetary nebula: The last breath of a dying solar-mass star. Credit: ESA, NASA and the Hubble Heritage Project (STScI/AURA)

A white dwarf

Yet your star is not done with surprising you. Suddenly, the cloud of gas it has turned into begins to glow. To somebody that had not witnessed the whole process, it may look as though this were a planetary system in the making. It is not, of course; rather the opposite. But nevertheless, you are tempted to call this cloud "a planetary nebula".

However, this is just the swan song of your dying star... Too soon, in a few thousand years, all that remains is the naked carbon core... A hot, bluish ball the size of a rocky planet, surrounded by a ring of dust which is all that remains from the former planetary system. A white dwarf that you can only watch cool down and dim for billions of years...

A dust disk around the white dwarf is all that remains of a planet that has been ripped apart during the star’s cataclysmic death. Credit: University of Warwick/Mark A. Garlick.

The End

If you don't like the end of the story, go back to the initial post and choose again.