A star that’s almost as old as the universe. The article is shy on the details, but they’re analyzing the decay of uranium and thorium, or more specifically, uraniums 235 & 238 and thorium-232.
The half lives of uranium 235 and 238 are approxiamtely 7 x 10^8 (70 million years) and 4.5 x 10^8 (40 million years). By the ratio of isotopes in each sample examined, you can determine how many billions of years old it is. For our 13 billion year old star, amount of half-lives of U-235 is 18.5 and 238 it’s 28.9 (all approximated to make the math a little easier). So for a 13 billion year old star, your isotopic ratio of U-235/U-238 should be 18.5/28.9, or 0.64.
Thorium-232 has a half life of 1.4 x 10^19 years (14 billion years!), so if just over half a sample has degraded, then you know the age of that sample.
It’s pretty cool to think that about half of all the thorium made in the big bang is still around in the universe.
Did you read about the planet that’s hot? Really hot? It’s a gas giant that’s close to its star and it absorbs so much energy from the star that it actually glows.