“Historic Papers” is a series of posts, in which we explore famous scientific papers that were the stepping stones for the advent of nuclear astrophysics, and we also learn about the life of the scientists involved.

This week we are exploring a milestone work in observational astronomy that was extremely influential for nuclear astrophysics, titled “Spectroscopic observations of stars of class S” by Paul Merrill, which was published at the Astrophysical Journal in 1952.

The abstract of the paper published in the Astrophysical Journal (1952).

More than two decades after the seminal work of Cecilia Payne, which helped scientists understand what stars are made of, astronomers were trying to detect all elements that exist on Earth in stars.

Technetium is a very interesting element. It has 43 protons in his nucleus (atomic number) and was discovered by Carlo Perrier and Emilio Segrè in molybdenum that had been irradiated with deuterium (heavier version of hydrogen). The origin of this element’s name is Greek, meaning artificial or man-made. That is because technetium is an element which has no stable isotopes. Its longest-lived isotope is \(\mathrm{^{97}Tc}\) with a half-life of around 4 million years, meaning that if you have 1g of this material in 4 million years you’ll have half, in 8 million years a quarter and in 1 billion years almost nothing left. The material of course is not lost, it is transformed by converting one of its protons into a neutron, becoming stable \(\mathrm{^{97}Mo}\). Some other technetium isotopes (same number of protons, different number of neutrons) are widely used in as a radioactive tracers for medicine. For example, \(\mathrm{^{99m}Tc}\) has a half-life of only 6 hours and as it decays to its ground state it emits characteristic \(\gamma\) rays which doctors use to find fractures or tumors in bones via a bone scan.

As in all other elements, scientists were very curious in learning more about the spectrum, or the light fingerprint, of technetium. William Meggers and Bourdon Scribner, working at the National Bureau of Standards (now called NIST) characterized technetium’s spectrum in late 1950.

Stellar spectrum containing technetium lines from Burbidge et al., Rev. Mod. Phys. (1957).

Enter Paul Merrill. He was a former colleague of Meggers at the Bureau of Standards. Merrill was approaching the end of his career, when in 1952 he published “Spectroscopic observations of stars of class S”. The title sounds rather uninteresting, but the results were remarkable. Merrill reported that he identified technetium in R Andromedae and other S-type stars. He detected four spectral lines: 403.1, 423.8, 426.2, 429.7 nm, and this was sufficient to identify the element \(\mathrm{^{98}Tc}\). What is really amazing is that the fact that he doesn’t even mention it in the paper’s abstract!

As we discussed in the above, all technetium isotopes decay and thus such detection could mean that either there must be a process that actively creates it in the star, otherwise all of it would have decayed, or there is a peculiar event that enriches the star with technetium. Scientists now agree that nuclear reactions take place in stars and create the technetium observed by Merrill. This was the first evidence that such a process is possible.

To be more specific, technetium is created via a slow neutron capture process, dubbed the s-process, where neutrons are slowly captured by nuclei which then decay back to a stable nucleus before they have the chance to capture another one. This s-process occurs in cool giant red stars, like R Andromedae, which are heading towards the end of their lives. There is also another neutron capture process, called the r-process (rapid) in which case there is a huge number of neutrons that are captured by nuclei (more on that another time!).

Meet the scientist

Paul Merrill was born in Minneapolis and received his Ph.D. from University of California. In 1919 he joined Mount Wilson observatory in California, where he stayed for the rest of his career. Merrill was an expert in stellar spectroscopy and interstellar matter. He was the one that defined the S-type class of stars which we now know that is a site for the astrophysical s-process.

Paul Merrill seated beside a spectrograph mounted on the 60-inch telescope, Mount Wilson Observatory. Image courtesy of the Observatories of the Carnegie Institution for Science Collection at the Huntington Library, San Marino, California.
Scientists playing pool in the Hooker cottage, Mount Wilson Observatory. Merrill is at the left. Image courtesy of the Observatories of the Carnegie Institution for Science Collection at the Huntington Library, San Marino, California.

P. Merrill, Astrophys. J 116, 21 (1952)

Paul W. Merrill: Bibliographical Memoir by Olin C. Wilson

The synthesis of elements by Giora Shaviv