Personal Story
Published in the RSAA Lunations
Vol1 Issue27 1–30 April 2022
I attended primary and high school in Hobart before enrolling for BSc at the University of Tasmania, where I studied maths, chemistry and physics. My honours project was in radio astronomy and I helped construct and operate 1.5 and 1.0 Mhz dipole antennas and receivers to study Galactic Low Frequency Radio Emission. On many occasions during solar minimum the ionosphere opens up over Tasmania and low frequency cosmic radio waves can be received. Toward the end of this project Fred Hoyle happened to visit the Department and after seeing the observations, published a paper in Nature explaining the results and including me as a co-author. My publication record is thus almost neatly bookended by my also being a co-author on David Yong’s Nature paper last year!
I was lucky enough to be awarded a Vacation Scholarship to Mount Stromlo in 1962, a great experience, that led me to apply to return to Stromlo for a PhD in optical astronomy.
My PhD on Delta Scuti and Dwarf Cepheid variables was supervised by Alex Rodgers and Leonard Searle and encompassed spectroscopy, spectrophotometry and model atmospheres, topics that I have followed throughout my career.
Midway though my PhD, Bart Bok moved to Steward Observatory in Tucson and Olin Eggen became director. My first meeting with him was when he came to the office that I shared with Harry Hyland and directed us to lay out a croquet pitch on the lawn outside the Director’s residence. That done, croquet competitions were held most lunch hours for staff and students for many years. (However, I don’t remember Olin ever playing!)
My first post-doc was at Yerkes Observatory of the University of Chicago, on Lake Geneva in Wisconsin, with Bob O’Dell ,where I helped commission a new 40 inch reflective telescope and spectrograph, and had several observing runs at McDonald Observatory in Texas. (As an aside: a court-case brought by the children of Mr McDonald established that you aren’t necessarily of unsound mind to give money to astronomy rather than your children.)
Hiltner and Morgan were two of the older astronomers at Yerkes when I was there and I benefited greatly from discussions with them and their students. Dimitri Mihalas also joined the staff and he was another important influence on me. Chandrasekhar and Parker were staff members at the University of Chicago but only occasionally visited Yerkes. I became interested in young OB associations that some of Morgan’s students were working on, especially Sco OB1, and when I returned to Stromlo as a Research Fellow in 1969 became involved with Dayal Wickramasinghe in studying Xray binaries that had just been discovered.
My prior spectroscopy of the supergiants in Sco OB1 enabled me to identify two of these objects as supergiants and not low-mass post-AGB stars that had been proposed, and the wonderful coude spectrograph at the 74 inch enabled us to get excellent high resolution photographic spectra of the primaries.
We also worked on the study of white dwarfs, in particular, cool white dwarfs that were touted as a good way to measure the age of the old disk. However, although we found a few interesting cool white dwarfs, for me the more interesting discoveries were that of extreme metal deficient K dwarfs and metal deficient M dwarfs and the first [Fe/H]=-4.0 K giant, CD -38 245. All these objects were discovered whilst searching for cool white dwarfs amongst the ultraviolet deficient red stars.
My main interests concern how stars of different mass evolve and how this ties in with the build-up of elements in stars. This involves working at the cutting edge between theory (model atmospheres and interiors) and observations (photometric and spectroscopic in wavelength regions from the satellite UV to satellite IR), and comparing stars in various parts of our galaxy and in the Magellanic Clouds. Closely associated with this work has been the determination of the temperature scale of stars and the providing of empirical and theoretical relations between colors and temperatures. I have long been identifying the most metal-poor stars in the Galaxy using medium dispersion spectra with the DBS and WIFES and have also continued finding and observing low mass M stars, most recently, nearby young M dwarfs that are candidates for searches for protoplanetary disks and protoplanets and brown dwarfs. I have worked closely with engineers and technicians to design, procure and use spectrophotometric instrumentation.
The great advantage of working at Mt Stromlo, ANU, has been access to excellent instrumentation of our own and also access to the front-line AAT photon counting high-resolution spectroscopy. Of equal importance has been the wonderful ANU astronomers and students that I have had the privilege to collaborate with. The combination of first-rate theory, cutting edge instrumentation, and access to workshops and skilled and knowledgeable technicians has been a huge advantage. I have also greatly benefited from being able to visit and collaborate with a wide range of overseas astronomers having complementary interests and expertise.
My involvement with photometry and spectrophotometry has been driven by the need to derive accurate temperatures and gravities for stars so their position in the HR diagram is assured and their metallicities are well defined. I have done much work on standard photometric systems, pushing their use and establishing passbands so that synthetic photometry can be derived from theoretical spectra. In more recent times standard star photometry has become less essential as the incredible data from all-sky-surveys, such as the ground based SkyMapper, PanStarrs and 2MASS, and the space based Gaia DR2/3/4 and future planned surveys, becomes ubiquitous.
Looking back, I have seen first-hand the amazing advances that have taken place in astronomical instrumentation. My first spectroscopic observations were with photographic plates on the 74 inch and the AAT. And I have vivid memories of my first digital observations with the IDS alongside Joe Wampler and Lloyd Robinson and later the IPCS with Alex Boksenberg. And more recently the introduction of CCDs with FORS on the AAT and CCDs on the DBS and later WIFES.
The echelle spectrograph UCLES with the IPCS on the AAT was the only UV sensitive high resolution spectrograph in the world for many years. Ralph Sutherland and I had just derived a linelist for OH lines and generated a synthetic spectrum for CD -38 245. I had a run on the AAT in 1991 in an attempt to measure oxygen and OH in such an extremely metal deficient star. The night assistant and I watched as the UV spectrum built up and after about 30 min I pulled out the synthetic spectrum to compare with what was on the screen. The match was perfect. The night assistant was less impressed and said why had I observed for 30 min when I already had a good spectrum of the star!
Observing with CCDs, such as those in WiFeS and SkyMapper, is wonderful, but it can’t replace the excitement of watching lines slowly appear, be they red-shifted quasar emission lines or hard sought for absorption lines and molecular bands, as the spectrum builds up and the S/N increases. However, I get a great buzz reducing the accurate spectrophotometry from WIFES observations on the 2.3m telescope and then fitting them to Thomas Nordlander’s beautiful synthetic spectra to establish the astrophysical parameters of another possibly very interesting star for David Yong to follow up. Long may that continue!
Mike Bessell
Picture: 1962 vacation scholars with Arthur Hogg and Bart Bok. You may recognise some others.
