Stellar smashup creates elements

This exercise is a part of Educator Guide: Strontium Found in Neutron Star Crash / View Guide

Directions for teachers: After your students read “Strontium found in neutron star crash,” ask them to answer the following questions.

1. What recent discovery related to a neutron star merger does the article describe?

Astronomers found that a merger of two neutron stars formed the heavy element strontium.

2. Why is this discovery important?

Strontium is the first heavy element clearly detected from a neutron star merger. The discovery provides the most direct evidence yet that these collisions can trigger the r-process, a chain of nuclear reactions thought to have formed about half of the universe’s elements heavier than iron.

3. How did astronomers make this discovery? What data did astronomers use and what instrument collected the data?
The Very Large Telescope in Chile collected data on wavelengths of light, or spectra, from the merger in the days after the event. Researchers looked at the spectra and spotted marks specific to strontium.

4. The neutron star merger was first observed in 2017, so why hadn’t the discovery been made sooner?

At the time, the spectra suggested that the neutron star merger created many heavy elements. Scientists had been focusing on the signals from elements heavier than strontium. It wasn’t until they expanded their analysis to consider other elements that they discovered strontium’s unique, spectral fingerprint.

5. Name and briefly describe the series of reactions that produced strontium and other heavy elements in the neutron star merger.

The series of reactions is called the r-process. In this process, atomic nuclei steal subatomic particles called neutrons from their surroundings and radioactively decay to become heavier elements.

6. How is strontium similar to other elements created in the neutron star merger? How does its atomic structure differ from the other elements? How did this difference contribute to the discovery?

Strontium and other heavy elements were made during the neutron star merger. But unlike heavier elements, strontium has a simple atomic structure. That structure means it generates a few spectral marks that have been measured in the lab, which made strontium easy to identify. Heavier elements with more complex atomic structures can generate millions of spectral marks. Many of those spectral marks haven’t been associated with specific elements.

7. What does the discovery reveal about the material that was released during the neutron star merger? Explain.

Strontium’s presence indicates that the material had relatively few neutrons. In the r-process, atomic nuclei snap up neutrons from their surroundings to become heavier elements. If the material had a lot of neutrons, the r-process would not have yielded strontium, which has relatively few neutrons in its nucleus. Atomic nuclei would have snapped up more neutrons to become elements heavier than strontium.   

8. What does astrophysicist Brian Metzger think contributed to the material’s unusual composition?

Brian Metzger suggests that the material has a low neutron density because some of the neutrons were destroyed, perhaps when they were hit with subatomic particles called neutrinos that were released during the merger.