Postdoc Spotlight: Astronomer Sergio Dieterich

Hubble Space Telescope
Wednesday, November 16, 2016 

On April 24, 1990, the Hubble Space Telescope launched from the Kennedy Space Center. It was the first major optical telescope to be placed in space, soaring far above the distortion of the atmosphere or any light pollution from Earth, showing scientists the universe in unprecedented detail. It was that day, while witnessing the launch live on television, that DTM NSF Postdoctoral Fellow Sergio Dieterich decided he wanted to explore the universe for the rest of his life. For his undergrad, he attended Johns Hopkins University, where he interned on campus at the Space Telescope Science Institute, the institute that manages the Hubble mission for NASA. A few years later, he was first awarded time on Hubble, the very telescope that ignited his fascination for astronomy all those years ago. Today, his research at DTM focuses on understanding low mass stars, both as individuals and as a population, which he examines using telescopes all over the world. 

We asked Dieterich why astronomy fascinates him, and what research he hopes to do in the future in his endless pursuit to explore the universe. 

NSF Postdoctoral Fellow Sergio Dieterich stands with his amateur telescope that he uses to stargaze on campus. Photo by Robin Dienel, DTM.

DTM: When did you first become interested in your field of research? Why? 

Sergio Dieterich: April 24, 1990. I was an 11-year-old boy watching TV when regular programing was interrupted and the image switched to the space shuttle launch pad at Kennedy Space Center. The announcer explained that the Hubble Space Telescope was about to be launched. I had never heard of the Hubble Space Telescope before, but for days after the launch I could not stop thinking and looking up as much information as I could about it, this being before the age of the internet. The fascination that started that day eventually led me to choose Johns Hopkins University for my undergraduate studies so that I could be an intern on campus at the Space Telescope Science Institute, the institute that manages the Hubble mission for NASA. It was there that I began my work in understanding very low mass stars and brown dwarfs, which continues to this day. I feel that much of the fascination with exploring the universe that glued my eyes to the TV screen during the Hubble launch when I was 11 continues to drive me today. A particularly sweet moment for me happened a few years ago, when I was first awarded time on the Hubble Space Telescope to study very low mass stars.

Space shuttle Columbia on Pad 39A "watches" the picture-perfect ascent of sister ship Discovery, carrying the Hubble Space Telescope, during its liftoff on April 24, 1990. Photo by NASA.

DTM: How did you first hear about DTM? What brought you here?

Dieterich: It is hard not to hear about the reputation and ground breaking work of DTM astronomers in astronomy graduate school, no matter where you go. In my graduate level planetary sciences class, we read several of Alan Boss's papers on giant planet formation theory. More specifically, the work I do requires measuring precise distances to nearby stars via the trigonometric parallax technique. The DTM astrometry group is one of the few groups in the world that has a dedicated program to do that. It was that observational resource that drew me here more than anything else.

In this April 25, 1990, photograph taken by the crew of the STS-31 space shuttle mission, the Hubble Space Telescope is suspended above shuttle Discovery's cargo bay some 332 nautical miles above Earth. Photo by NASA.

DTM: What excites you about your work? 

Dieterich: What excites me the most is the unknown. It is the fact that if I actually knew how to do my work, it wouldn't be science. Particularly as a postdoc, my collaborators and I have to figure out how to accomplish each step, every step of the way.  There are many instances in which we think of an idea of how to carry out an observation, or fit a particular mathematical model, and we say to ourselves or to each other, "yeah, sure this can be done!" But the truth is at that moment we might have no idea how to do it. And then over the course of a day or so, the ideas solidify and a plan of attack emerges. You are far from being always right, but you always end up learning something along the way.

The other part that really excites me is to think of what actually happens at the observatory and at the telescope, specifically Carnegie’s Las Campanas Observatory in Chile. You are in a self-contained mountaintop where people come to work, away from the rest of the world, for seven days at a time. Your nearest neighbors are goat herders who don't have electricity. Wild animals roam free. And yet amidst dirt roads, in this very rustic environment, humans have built incredibly complex technology to connect us to the universe. And in that environment, the dislodging of electrons by photons falling in your camera's CCD detector allows you to develop a fairly sophisticated picture of what the stellar atmosphere where that photon originated dozens of year ago is like. You need to care as much about the quantum efficiency of your light detector as you do about the way the wind is blowing, where the clouds are, and whether or not the airplanes flying out of Santiago on their way to the U.S. are leaving behind con trails in that particular night. The last time I was at Las Campanas I had the idea that if I left the outside door open the wind would generate less turbulence and my images would be sharper. That worked great until a cute little mouse came into the observing room through the open door. What do I do then? Do I concentrate on what the telescope images are telling me about stars that are light years away? Do I get the mouse out before it starts chewing on the computer cables? Do I look at the cloud camera to see if there are con trails in the very last bit of the path of those photons? It is this synthesis of what happens very near and very far away, of the rustic and the technology, of Earth and Galaxy, that fascinates me about ground based observational astronomy. 

Dieterich at Las Campanas Observatory in Chile. Photo by Cindy Hunt Benson, Carnegie Observatories.

DTM: What research projects are you working on now at DTM?

Dieterich: My research focuses on understanding low mass stars, both as individuals and as a population. It has as much to do with what the inner working of these objects are as it does with how the stellar formation process produced the diversity of objects we see today in our solar neighborhood. Some specific questions I am trying to answer are: Why is it that lower mass stars become more and more common at lower and lower masses, but then at masses below about 10% the mass of our Sun the number of stars suddenly plummets? As a graduate student, I advocated that the smallest possible stars are actually significantly brighter and hotter than what theoretical models of stellar structure predict. Was I right, and if so, what accounts for the discrepancy? And as a broader question, how can we understand what is going on inside a star based on clues from its surface, which is after all the only part of the star we can directly observe? Practically answering these questions means a lot of observing at Las Campanas. Most of my time there is spent observing and analyzing observations to figure out the distance to these stars and studying their colors to see if any of them are new candidate nearby stars. We then use the large Magellan telescopes for detailed spectroscopic observations for a much smaller subset of these stars.

Luminosity - Radius Diagram for a sample of nearby stellar and substellar objects nearing the stellar/substellar boundary. Diagram by Sergio Dieterich, DTM. 

DTM: What research do you hope to do in the future?

Dieterich: I hope to be fortunate enough to continue in stellar astronomy for the rest of my life. Every 10 years the U.S. astronomical community comes together to plan its priorities for the next decade in a so called "Decadal Review." One of the things we have learned from this exercise is that it makes a lot of sense to have detailed plans for the next 10 or 15 years, but after that we are just not smart enough to even know what the relevant questions will be then. I am sure the astronomers reading this are giggling at their own favorite example of a project that seemed like a good idea at the time 30 years ago and today seems silly. Again, if we knew how to do our jobs ahead of time, it would not be real science.

In the long run, we will have to wait and see what nature holds for us. In the short term, there are a few ideas I am currently developing. One of them aims at understanding the nature and shape of magnetic fields in low mass stars, and whether or not these fields exist internally and are actually strong enough to sustain some of the star's weight, thereby inflating its radius. I believe that if we combine Magellan high resolution spectroscopy with simultaneous observations by radio telescopes we have a good chance of cracking this problem.  Another idea has to do with expanding my current studies, all focused on our immediate vicinity in the Galaxy, to more distant regions and more extreme environments in our Galaxy. A new generation of space and ground based telescopes are currently under construction, or in the planning phase. These new tools will allow us to observe what we now observe on our cosmic door steps at much greater distances, and I am excited to figure out how we can get the most out of these new telescopes in the context of understanding low mass stars. Which one of these ideas will develop, if any, depends mostly on which lines of research I can get funded. 

Hubble spies a charming spiral galaxy bursting with stars. Photo by ESA/Hubble, NASA and S. Smartt (Queen's University Belfast) Acknowledgement: Renaud Houdinet.

DTM: What do you hope to be doing at your next job?  

Dieterich: As with all other Carnegie scientists, I am incredibly fortunate to get paid to do what I like. Any sort of job that allows me to continue to explore the universe in some capacity will make me very happy. Two particular aspects that I would like to expand on would be to increase student involvement in my research and also to branch out to other areas of astronomy. I believe I will be equally as happy being a professor or working at an observational research center where some of my time is dedicated to my own research, some is dedicated to helping other astronomers with their observations, and some is dedicated to mentoring students.  

DTM: Where do you see yourself in 10 years? 20?

Dieterich: Both Bonny, my wife, and I really like the Washington area, and it is one of the few metropolitan areas in the world where an astronomer has access to several potential employers without having to move to another city. So don't be surprised if 20 years from now I am still crashing BBR beer hour on Friday afternoons.

Sergio Dieterich

Interview by Robin Dienel
November 16, 2016

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