D-7: The Bride

The following was originally posted on the Stellaris forums.


A big thank you to Murmeldjuret or another fantastically informative weekend! Coming up this week we have all sorts of interesting stuff for you in the astroknowledge series. We’re close to the end now, very close. I’ve been holding back some of the best stuff in space until now, so as to close this series with a bang. As a result, I hope that this week will be as good for you as it is for me.

We’ll also have another guest article from the pen of the delightfully learned Admiral Howe.

Let’s talk about Pollux. This is a star that humans have known about for a very long time; together with Castor it makes up the two bright stars that the Gemini constellation is named after.

(Image courtesy astropixels.com)

Once upon a time, back before humanity existed, Pollux was a white A-class star on the main sequence. Sadly stars of this sort, while very beautiful, don’t live very long. Let’s look at what happened next, to understand what happens to stars when they reach the end of their lives.

Gas falls in
Light comes out
That’s what stars
Are all about

A star is a balancing act. Gravity pulls gas inwards towards the core, and light (and other radiation) that streams out of the core pushes it back. This might seem weird to you if you aren’t used to thinking of light as being something powerful enough to hold up a heavy weight, but this gives you an idea of just how bright the cores of stars are. We call this “thermal pressure.” *

When Pollux exhausted the hydrogen in her core, this light began to fail. Gravity, which had been kept at bay for hundreds of millions of years, is a patient force: the core contracted and enormous amounts of fresh hydrogen poured into it.

What happens when you pour enormous amounts of fresh fuel into a dying fire? It doesn’t just restart gradually: it explodes.

This sudden reignition of the core pushes back against gravity hard. The outer layers of the star are blasted outwards, as is a lot of the still-burning hydrogen that just began to flood into the outer core. We call this the “first dredge-up.”

At this point Pollux became vastly larger, which is where she is today: she’s only twice the Sun’s mass but is almost nine times her radius and thanks to her enormous surface area is a full forty-three times as luminous.

(Image courtesy NASA. That small yellow thing next to Pollux is the Sun, included for size comparison.)

Pollux, which had been a bright white star for a few hundred million years, became a larger, cooler orange giant. Her outer layers are the only part we can see, which create her orange colour. Deep inside, her core has now started to fuse helium instead of hydrogen. This gives her only another few hundred million years of life. When this fuel source ends then it will be like before: her core will contract, bringing in more helium, which will cause a second dredge-up. She’ll get even larger as her outer layers inflate even further, and her surface will be even cooler and redder.

At this point her core will be full of oxygen and carbon, which she isn’t heavy enough to fuse. She’ll continue to glow but this will be from the outer layers rather than the core. Eventually the outer layers will fade and drift away, creating a nebula that slowly cools to black; and a core of oxygen and carbon, still glowing white from residual heat.

Don’t weep for Pollux. Space is a fantastically good insulator, and so she’ll cool extremely slowly. While her life up until this point will have been less than a billion years, her lifespan as a white dwarf is projected to be considerably longer than the current age of the universe. Everything up until this point has really just been her childhood: a brief hydrogen- and helium-burning phase she went through before she settled down to be what makes her truly happy: being small, and beautiful, and white as snow.

Tomorrow we’ll talk about her planet.


* Thermal pressure is a combination of radiation pressure (that is, light) and convection pressure (that is, hot gas rising.) For small cool stars convection pressure dominates; for larger hotter stars radiation pressure dominates.


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