# Paul W. Merrill
**Paul Willard Merrill** was a specialist in the spectra of unusual classes of stars: he defined spectral class S in 1922,[1](https://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1954ApJ...120..484K&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf) and in 1952 he reported the detection of technetium in stellar spectra, the observation that opened observational tests of how stars form heavy elements.[2](https://theconversation.com/elements-from-the-stars-the-unexpected-discovery-that-upended-astrophysics-66-years-ago-93916)
| Key fact | Detail |
|---|---|
| Born | August 15, 1887, Minneapolis, Minnesota |
| Died | July 19, 1961 |
| Doctorate | Ph.D., University of California, 1913; thesis on class B stars with bright hydrogen lines[3](https://astrogen.aas.org/front/searchdetails.php?agnumber=636) |
| Signature work | Defined spectral class S (ApJ, 1922); detected technetium in stars (Science, 1952)[4](https://doi.org/10.1086/142716) • [5](https://doi.org/10.1126/science.115.2992.479) |
| NAS membership | Elected April 1929 |
| Henry Draper Medal | 1945, National Academy of Sciences[6](https://iopscience.iop.org/article/10.1086/125820/pdf) |
| Bruce Gold Medal | 1946, Astronomical Society of the Pacific[7](https://iopscience.iop.org/article/10.1086/125789/pdf) |
## Life and career
The [University of California](https://www.edgepedia.org/university-of-california) awarded him the Ph.D. in 1913 for the thesis "Class B stars whose spectra contain bright hydrogen lines."[3](https://astrogen.aas.org/front/searchdetails.php?agnumber=636)
From 1913 to 1916 he was a member of the staff of the Michigan Observatory at Ann Arbor.[7](https://iopscience.iop.org/article/10.1086/125789/pdf) In 1919 he joined the [Mount Wilson Observatory](https://www.edgepedia.org/mount-wilson-observatory) staff of the Carnegie Institution.
## Representative work
**Spectral class S.** Merrill introduced class S in 1922 to designate stars intermediate between ordinary giants and the carbon stars of classes R and N.[1](https://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1954ApJ...120..484K&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf) The defining paper, "Stellar spectra of class S," appeared in *The Astrophysical Journal* in 1922 (volume 56, page 457).[4](https://doi.org/10.1086/142716) • [8](https://doi.org/10.1007/978-1-4419-9917-7_940) S stars are late-type giants whose spectra show zirconium oxide (ZrO) bands in addition to the titanium oxide bands typical of M stars, with a carbon-to-oxygen ratio between that of M stars (about 0.5) and carbon stars (above 1), and overabundances of s-process elements.[9](https://ar5iv.labs.arxiv.org/html/1904.04039)
In 1920 he also published the first paper by a professional research astronomer discussing observations from airplanes, an early vision of airborne astronomy.[10](https://ntrs.nasa.gov/search.jsp?R=19960003704)
## Technetium in the stars
On May 2, 1952, Merrill reported in *Science* the discovery of technetium in stars.[2](https://theconversation.com/elements-from-the-stars-the-unexpected-discovery-that-upended-astrophysics-66-years-ago-93916) He identified the resonance lines of neutral technetium (Tc I) at 4297 Å, 4262 Å, and 4238 Å in several S, MS, and Mira-variable M stars, an observation later described as marking the beginning of observational tests of nucleosynthesis schemes.[11](https://doi.org/10.1017/s025292110006276x) [Technetium](https://www.edgepedia.org/technetium) has no stable isotopes; the isotope 99Tc, with a half-life of about 2×10⁵ years, is the only one along the path of the slow neutron-capture (s-process) nucleosynthesis occurring inside asymptotic giant branch (AGB) stars.[12](https://ar5iv.labs.arxiv.org/html/2201.02851) Any technetium visible at a stellar surface must therefore have been made there recently, and Merrill offered that interpretation among three: stars create heavy elements.[2](https://theconversation.com/elements-from-the-stars-the-unexpected-discovery-that-upended-astrophysics-66-years-ago-93916) He also showed that the Tc I lines tended to be stronger in stars with more pronounced S-type characteristics, and since no technetium isotopes are stable he suspected that S and MS stars represent a transient stage of stellar evolution.[11](https://doi.org/10.1017/s025292110006276x) In 1956 he detected technetium in the carbon star TX Psc.[11](https://doi.org/10.1017/s025292110006276x)
## Honors and recognition
The National Academy of Sciences elected Merrill in April 1929. According to a notice in the *Publications of the Astronomical Society of the Pacific*, the Academy's Henry Draper Medal for 1945 was formally presented to him on April 23 in Washington, D.C., at the Academy's annual dinner.[6](https://iopscience.iop.org/article/10.1086/125820/pdf) In 1946 the Astronomical Society of the Pacific unanimously voted him the Catherine Wolfe Bruce Gold Medal, on nominations from at least five observatories including Harvard, Yerkes, Lick, Greenwich, Stockholm, and Córdoba.[7](https://iopscience.iop.org/article/10.1086/125789/pdf) The Royal Astronomical Society elected him an Associate on April 9, 1948.[14](https://www.ras.ac.uk/obituaries/Paul_Willard/Merrill)
## Legacy and later research
Merrill's technetium detection prompted the 1957 paper "Synthesis of the Elements in Stars," which set out the theory of stellar nucleosynthesis, with A. G. W. Cameron independently arriving at the same theory that year.[2](https://theconversation.com/elements-from-the-stars-the-unexpected-discovery-that-upended-astrophysics-66-years-ago-93916) A 2026 review of s-process spectroscopy cites Merrill's 1922 class S paper and his 1952 *Science* paper as the foundational observations of the field.[13](https://link.springer.com/article/10.1140/epja/s10050-026-01854-z)
**The technetium diagnostic today.** The presence of technetium divides evolved stars into Tc-rich "intrinsic" S stars, genuine thermally-pulsing AGB stars, and Tc-poor "extrinsic" S stars enriched by mass transfer from a former AGB companion, making technetium a sensitive probe of third dredge-up and AGB nucleosynthesis.[12](https://ar5iv.labs.arxiv.org/html/2201.02851) A 2019 study located six low-mass (1–1.2 solar masses), Tc-rich intrinsic S stars just above the predicted onset of third dredge-up, direct evidence that dredge-up occurs in AGB stars with initial masses as low as about one solar mass.[9](https://ar5iv.labs.arxiv.org/html/1904.04039) By the late 1980s, about 70 technetium stars were known, with 15+ possible additional cases among 240+ stars searched, spanning 27+ M, 13+ MS, 16+ S, 2+ SC, and 9+ C stars.[11](https://doi.org/10.1017/s025292110006276x) A 2025 study treats technetium-rich M stars as diagnostics of recent third dredge-up events on the AGB; the simultaneous enrichment of zirconium and technetium is well understood within s-process nucleosynthesis, while Tc-rich M stars remain an enigma.[15](https://www.aanda.org/articles/aa/abs/2025/10/aa55581-25/aa55581-25.html)
## References
1. P. C. Keenan, ApJ 120, 484 (1954). https://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1954ApJ...120..484K&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf
2. "Elements from the stars: The unexpected discovery that upended astrophysics 66 years ago," The Conversation. https://theconversation.com/elements-from-the-stars-the-unexpected-discovery-that-upended-astrophysics-66-years-ago-93916
3. AstroGen, The Astronomy Genealogy Project: Paul W. Merrill. https://astrogen.aas.org/front/searchdetails.php?agnumber=636
4. P. W. Merrill, "Stellar spectra of class S," ApJ 56, 457 (1922). https://doi.org/10.1086/142716
5. P. W. Merrill, "Technetium in the stars," Science 115(2992), 484 (1952). https://doi.org/10.1126/science.115.2992.479
6. "Presentation of the Henry Draper Medal to Paul W. Merrill," PASP. https://iopscience.iop.org/article/10.1086/125820/pdf
7. F. J. Neubauer, "Address of the Retiring President of the Society in Awarding the Bruce Gold Medal to Dr. Paul W. Merrill," PASP (1946). https://iopscience.iop.org/article/10.1086/125789/pdf
8. "Merrill, Paul Willard," Springer encyclopedia entry. https://doi.org/10.1007/978-1-4419-9917-7_940
9. "Observational evidence of third dredge-up occurrence in S-type stars with initial masses around 1 M⊙," A&A (2019). https://ar5iv.labs.arxiv.org/html/1904.04039
10. "An infrared astronomer's early vision of airborne astronomy: Paul Merrill 1920," NASA NTRS. https://ntrs.nasa.gov/search.jsp?R=19960003704
11. "The Role of Technetium in the Evolution of Red Giants." https://doi.org/10.1017/s025292110006276x
12. "Tc-rich M stars: platypuses of low-mass star evolution," A&A (2022). https://ar5iv.labs.arxiv.org/html/2201.02851
13. "Tracing the s-process: spectroscopic insights into chemical abundances in O- and C-rich evolved stars," EPJ A (2026). https://link.springer.com/article/10.1140/epja/s10050-026-01854-z
14. RAS Obituaries: Paul Willard Merrill. https://www.ras.ac.uk/obituaries/Paul_Willard/Merrill
15. "Technetium-rich M stars: Prime diagnostics of recent third dredge-up events on the asymptotic giant branch," A&A (2025). https://www.aanda.org/articles/aa/abs/2025/10/aa55581-25/aa55581-25.html
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