In 2004, astronomers discovered a collapsed white-dwarf star, BPM 37093, in the constellation Centaurus whose carbon core had crystallised into a 10 billion trillion trillion carat gem just 4000 km across*.
Twinkle, twinkle, little star!
Now we know just what you are:
A crystallised white dwarf, so high.
You are a diamond in the sky!
Astronomers have sussed you out,
Their instruments have left no doubt:
Your seismic oscillations showed
Your hot, dense core’s a priceless load.
Your carat count is astronomic;
But mining you’s not economic –
Fifty light-years is too far!
So keep on twinkling, little star. . .
*See this abstract from the SAO/NASA Astrophysics Data System