Scientific Method / Science & Exploration

Which came last—The supernova or the red giant?

Understanding how an element is made rewrites the pre-history of our Solar System.

Enlarge / A red giant star really is quite gigantic compared to our Sun.

Radioactive decay is a powerful tool. The predictable decay of radioactive isotopes can be used for far more than just dating old rocks. Scientists have used radioactive isotopes to determine the age of the Earth and the age of the Solar System itself. Now, a team of scientists has used radioactive dating to study the pre-history of the Solar System more accurately than before, in the process reconciling data that had seemed to be contradictory.

The contradiction came in the form of data from two different isotopes. The radioactive elements iodine-129 and hafnium-182 are found throughout meteoroids in the Solar System. The abundance of those elements, in relation to the abundance of their non-radioactive counterparts, should give estimates of the time when those elements were produced. The problem is that the date calculated from the iodine (~72 million years prior to the Sun’s formation) does not match the date from the hafnium (~15 million years). Since the two elements should have been produced in the same event (typically a supernova), this was quite a problem.

Both these isotopes are produced via a neutron-capture process. Under certain conditions, an atomic nucleus can pick up a loose neutron. While it remains the same element, it ends up being a different isotope with a different atomic weight. There are two known types of neutron-capture processes: the s-process and the r-process.

The r-process

Until the new study, it was thought that radioactive hafnium and iodine were both created in the r-process. The r-process is so-named because it is extremely rapid, taking place in incredibly high-energy events.

For example, a supernova explosion can trigger the r-process. But this doesn’t occur in every supernova, and it's not the norm. Another possible source of the r-process is the collision of two neutron stars. Neutron stars have the masses of large stars compacted to roughly the size of Manhattan. A collision between two of these rapidly spinning monsters could result in the transformation of isotopes through the r-process.

In either case, the two elements should provide the same date if both were produced in the same r-process event, whether it was a neutron star collision or supernova.

The s-process

As its name implies, the s-process (for slow) can take quite a while. It usually occurs in the cores of red giant stars, specifically stars on the “asymptotic giant branch” (AGB). Red giant stars are the final phase in the evolution of stars with up to 10 times the mass of the Sun.

Until the new study, it was thought that the s-process could not produce significant amounts of radioactive hafnium-182, because a form of radioactive beta decay (in which a neutron releases an electron, becoming a proton in the process) happens faster than the s-process, canceling out its effects. This is also why scientists concluded that both elements must have been produced in the r-process.

But in the new study, the scientists were able to demonstrate that hafnium-182 can be produced by the s-process after all. Once produced, the element can be expelled by the star’s winds until some of it mixes into the material that would form the Solar System.

A brief history of the Solar System

The Sun is not a first-generation star. That is, it contains elements that could not have formed in the Sun, but must have formed in its predecessors—earlier stars that created the elements we observe throughout the Solar System today.

At some point before the formation of the Sun, the material that would become the Solar System became isolated from the interstellar medium. That would have happened sometime after whatever event caused neutron capture to occur (whether via an s- or r- process) made its contribution to the proto-Solar System material.

After that, the material formed into a molecular nebula, which then condensed into the Sun and the planets roughly 4.5 billion years ago. According to the conflicting dates given by the iodine and hafnium found in meteoroids, this would have been either 72 or 15 million years after the last event that triggered neutron capture.

The new study

There were reasons scientists thought that the s-process was incapable of producing enough radioactive hafnium to account for the abundance found in the meteoroids. A key one was that previous models held that hafnium-181, the isotope that becomes hafnium-182 upon neutron capture, had energy states that would increase the rate of beta decay. If this was the case, it would have dramatically slowed the rate of hafnium-182 production that could take place inside AGB stars.

But recent experiments have not found any evidence for the existence of those energy states. The experimenters, therefore, removed those states from their calculations. As a result, the amount of hafnium-182 predicted to be produced in AGB stars increased by a factor of four to six.

That was enough of an increase to make the s-process a plausible mechanism for the production of the hafnium-182 in our Solar System after all. That means these two different isotopes could have come from two different sources—an r-process and an s-process event.

A new brief history of the Solar System

Using this information, the team was able to calculate new times for the last events of each type that contributed material to the Solar System, before it was separated from the interstellar medium. The last r-process, whether it was a supernova or neutron star collision, occurred ~80 to 109 million years before the Sun’s formation (which itself occurred 4.5 billion years ago), based on the iodine-129 abundance. This agrees with (within the error bars) earlier calculations based on curium/uranium ratios and with the aforementioned date (~72 million years, which comes from a different isotope of iodine).

The hafnium—along with another radioactive element, palladium—were not produced in the same event. The dates derived from these elements agree with each other, giving a date for the separate event that produced them: ~10 to 30 million years before the formation of the Sun. The intervening years represent the time it took for the proto-Solar System to condense and form the Sun and planets.

This time fits well with the typical lifetime, from formation to dispersal, of giant molecular clouds observed elsewhere in the Universe.


Some have questioned whether discoveries like this really matter. After all, to those not scientifically inclined, piecing together the Solar System’s early history can seem like a pointless intellectual pursuit. Scientifically interesting, sure, but does it really matter what kind of event the radioactive material came from?

No matter how remote or trivial it may seem, this research does address one of the biggest questions: why are we here?

Because of our understanding of physics, because of radioactive dating, scientists were able to piece together a story. A supernova (or neutron star collision) ejected material into space, which would ultimately encounter and mix with material from an AGB star tens of millions of years later, to form a 'soup' of gas and dust, a molecular nebula. That nebula would spin for further millions of years until it became our Sun and planets. We are the product of those events.

So why does a discovery like this matter? Simply put, this is our history.

Science, 2014. DOI: 10.1126/science.1253338  (About DOIs).

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