…..And the rocks melt wi’ the sun

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Happy Burns Day everybody. While I contemplate my failure to acquire a haggis and some Irn Bru in Honolulu, I thought I may as well find a tenuous link between Burns and astronomy.

Robert Burns was not only a poet in his own right, he was also an important librarian of Scots traditional songs. Sometimes he would add his own input to the lyrics, sometimes he wouldn’t. One of the songs he catalogued was My Love is Like a Red, Red Rose,

O my Luve’s like a red, red rose
That’s newly sprung in June:
O my Luve’s like the melodie
That’s sweetly play’d in tune.

As fair art thou, my bonnie lass,
So deep in luve am I:
And I will luve thee still, my dear,
Till a’ the seas gang dry:

Till a’ the seas gang dry, my dear,
And the rocks melt wi’ the sun;
I will luve thee still, my dear,
While the sands o’ life shall run.

And fare thee weel, my only Luve
And fare thee weel, a while!
And I will come again, my Luve,
Tho’ it were ten thousand mile.

Here’s Eddi (It’s got to be-ee-ee-ee-ee-ee-ee perfect) Reader singing it while some people wander by and bend down in the background.

So what part of this song made me think of astronomy? This part,

Till a’ the seas gang dry, my dear,
And the rocks melt wi’ the sun

That line has always made me think of the eventual fate of the Earth.

When the Sun stops burning Hydrogen in its core, it will expand to over a hundred times its current size. This period in its evolution is known as the red giant phase. It’s possible that during this phase the Earth will be swallowed up by the Sun. Whether this will happen is a matter of debate but this paper from a few years back came up with some possible answers.

So the first part,

Till a’ the seas gang dry, my dear,

Well it is possible that the seas will dry up if the Earth becomes so hot that it can’t support liquid water. The region around a star where a planet can exist with liquid water is called the Habitable Zone. The paper tries to estimate at which point the Earth will cease to be in the Habitable Zone and hence cease to have seas and oceans. It’s sooner than you’d think. Not only will the Sun become a red giant once it stops burning Hydrogen, it also evolves during its Hydrogen burning lifetime, getting brighter as time goes on. The paper uses the latest models of stellar evolution to estimate the point when the Earth will get too hot to have liquid water on its surface. This isn’t an easy calculation as greenhouse gases in the atmosphere will have some effect, but the authors reckon the Earth’s oceans could possibly disappear in about a billions years. Long before the Sun becomes a red giant.

And so to the final part,

And the rocks melt wi’ the sun

Well again it isn’t just a case of working out when the Sun will get to the size when it will engulf the Earth. When the Sun evolves it will lose mass meaning the orbits of planets in the solar system will get larger. It’s probable due to this that the Earth’s orbit will lie outside the maximum size of the Sun. However, another effect comes into play. When the Sun gets to the red giant phase its rotation rate will slow and the surface will get close to the Earth’s orbit. The Earth’s gravity will deform the Sun slightly, leading to a bump on its surface. As the Sun will be rotating more slowly than the Earth will orbit it, the bump will lag behind the Earth and produce a drag on it. This will pull the Earth towards the Sun. The authors calculate that, combined with the drag of the Earth moving through the Sun’s atmosphere, the Earth will be sucked into the Sun in about 7.6 billion years.

So,

Till a’ the seas gang dry, my dear,
And the rocks melt wi’ the sun

Or in one billion or 7.6 billion years.

Schröder, K., & Connon Smith, R. (2008). Distant future of the Sun and Earth revisited Monthly Notices of the Royal Astronomical Society, 386 (1), 155-163 DOI: 10.1111/j.1365-2966.2008.13022.x



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