Lightweight Light

In a galaxy of smaller stars,
With few that ever get to boom –
They only get to fuse to silicon,
By steady burn.
Besides the odd Type 1,
Then none will face a sudden doom –
And just ten elements (bar traces)
In the churn.
Though ‘smaller’ stars are relative –
We still get whites and blues –
But nothing that can cross
The cataclysmic iron line.
In truth, the silicon is rare,
Without a few Type 2s,
But the largest lose their mass to stop
Their super-shine.
So there’s enough to build some silicates
That build a rocky world,
Though lacking radioactivity
To heat its core.
But it has a liquid ocean,
In which chemicals are swirled,
As the ultraviolet starlight warms
Its barren shore.
It may miss plate tectonics,
But it holds an atmosphere,
And it has no need to hurry
When its stars are here to stay.
Organic molecules will still
Eventu’ly appear –
However long it takes for life
To find a way.

The 10 elements mentioned are Hydrogen, Helium, Carbon, Nitrogen, Oxygen, Neon, Sodium, Magnesium, Aluminium, & Silicon.  And although needing fewer protons, the missing ones (Lithium, Beryllium, Boron, & Fluorine) are very hard to acquire without the by-products of a supernova.

In truth, the oxygen-burning needed to produce silicon (and small amounts of phosphorus & sulphur) usually only happens in the final months before a Type 2 supernova, which in turn will produce iron from burning that silicon unless the candidate star is only just over the 8-solar-mass threshold – though it is possible to get some ‘localised’ oxygen-burning in stars just below the limit when they’re on the asymptotic-giant branch of their evolution.

In terms of life, it is fascinating to think if it would be possible for life to arise – but it would be greatly increased if our rocky planet of silicates could avoid having its early atmosphere stripped away.  Now, a lack of a magnetic core prevents an Earth-like magnetosphere, but an equally powerful dynamo can be generated from metallic hydrogen inside a gas giant of Jupiter-or-greater mass.

And having our terestrial world be a large moon of such a planet will also give it plenty of tidal heating to compensate for its lack of radioactive decay to provide internal heating.  It may even be able to have some form of plate tectonics and volcanism to prevent the carbon dioxide from getting locked away in the crust and losing all of our liquid water to ice.

Of course, there’s absolutely no reason to think that gravity could only form stars upto a maximum of 8-solar-masses but no greater.  This is simply a thought-experiment into how to generate life using the least-possible number of elements.

And as an aside, I have always found it hard to hear talk of ‘carbon burning’ and mean ‘carbon-fusing’ instead of ‘carbon-oxidising’.  Of course, ‘oxygen-burning’ means the same either way…