Isotopes in the cosmos

April 4, 2018

 

I mentioned the basics of isotopes and why they are useful in our lives in a previous post. Here I wanted to talk about why isotopes matter in the Universe.

 

We usually ask questions like “How are the elements created in stars?” or “What is the origin of heavy elements in the Universe?” or “How do stars make gold?". These are all great questions, but in reality things are a bit more complicated than they appear. The complication is that each of the elements can have many stable isotopes (and even more rare ones) and each of these isotopes can be formed in a completely different stellar environment.  As a result elements are not formed in a single stellar event but typically have contributions from a variety of sources. 

 

Let’s take, for example, the well-known element tin.  Tin has the largest number of stable isotopes out of all elements: 10. There are reasons for this (that have to do with “magic”) but I’ll have to come back to it later.

 

Out of the 10 tin isotopes that we can find in our solar system and other stars, the 3 lightest ones were created in some type of supernova, the 2 heaviest were probably produced in the rare merging of two neutron stars, and the 5 middle ones were synthesized in dying low-mass stars.

 

There are different processes, different nuclear reactions and different conditions involved in creating each of these isotopes. The final result, what we see around us, is the sum of the endless cycle of isotope production in stellar ovens.

 

Because each isotope, or perhaps group of isotopes, is produced in a different stellar environment, it is conceivable that different stars might have different amounts of each isotope, “isotopic abundances” as we call them. It’s extremely challenging to get isotopic information from the light we observe from stars. Even the best telescopes cannot distinguish between different isotopes of the same element because to a large extend their signatures look identical. However, we can get very detailed information from studying meteorites in the lab.

 

Meteorites are literally rocks that fall from the sky. They typically come from a particular region of our solar system called “asteroid belt” between Mars and Jupiter. They also typically have isotope abundances that are identical to what we find on Earth. Hidden inside these meteorites, however, are pieces of stardust that come from a time before the solar system was formed, hence called “presolar grains”.  It’s amazing if you think about it: a piece of stardust originating from before the solar system was even created, managed to hide itself in a piece of rock, and somehow, a few billion years later, it made it to Earth. Meteorite experts isolate these grains from the rest of the meteorite, study them, and find that they carry the signs of stellar events that are not found in the solar system. This is because the distribution of isotopes for each element is very different, which brings us back to the significance of isotopes in the cosmos. 

 

From presolar grains, solar system samples, and also various astronomical observations, we know that there is a large diversity in stars. We are always looking for ways to understand how the Universe works, and isotopes are one of the ways that help us reveal its secrets. 

 

 

 

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