John Lester's Homepage

	John Lester Professor Department of Astronomy and Astrophysics
	Address:	University of Toronto Mississauga Mississauga, Ontario L5L 1C6, Canada
	Telephone:	905 828-3818
	Fax:	905 828-5425
	E-mail:	`lester` AT departmental mail server

Academic History

B.A. - Northwestern University (1967)
M.S. - University of Chicago (1969)
Ph.D. - University of Chicago (1972)
Postdoctoral Research - Harvard-Smithsonian Center for Astrophysics (1972-76)
University of Toronto at Mississauga (1976 -)

Undergraduate Teaching in 2007-2008

Research Interests

I am interested in the physical processes that occur in the atmospheres of individual stars. While stellar atmospheres are often used as tools for the study of "macro" topics, such as the evolution of the Galaxy or the extragalactic distance scale, and these applications interest me too, I am more intrigued by the "micro" astrophysics that takes place in the stellar atmosphere, which is an extremely complex boundary layer between the stellar interior and space. There are fundamental physical processes occurring in this environment that are still poorly understood because the scale, temperatures, pressures, velocities and magnetic fields cannot be recreated in terrestrial laboratories. My approach to these problems is primarily observational, although I also employ modeling.

Current Projects

With my current graduate student, Hilding Neilson, I am working on a modeling project. Because classical Cepheid variable stars undergo periodic radial pulsation, their atmospheres should incorporate hydrodynamics instead of the traditional assumption of hydrostatic equilibrium. The usual approach has been to compute the pulsation in the context of the stellar interior structure, and then somehow to attach an atmosphere to the interior to "process" the radiation for observational purposes. Our approach is to work from the outside in, incorporating the pulsation mechanism into the structure of the atmosphere. This will create a self-consistent structure of the atmosphere that includes the pulsationally modified structure into the observable layers of the star. A major test of the models will be to predict the surface brightness distribution (limb darkening) of Cepheids as a function of both phase and wavelength, which will be testable by optical interferometers.
My major observational effort for many years has been to measure stellar radiation in fundamental units of W/(m² nm), spectroradiometry, which is the purest way to test the predictions of model atmospheres. The current system of stellar spectroradiometry was developed in the 1960s and 1970s, but it has been stagnant since then, while there have been great advances in stellar photometry, in high resolution, high signal-to-noise stellar spectroscopy and in modeling of stellar atmospheres.
New Instrument - I have developed a small Fourier Transform Spectrometer (FTS) specifically designed to measure spectral irradiance while avoiding the systematic errors that have plagued spectroradiometry. Although I did not realize it when I began this project, this is a resumption of the approach pioneered by H. L. Johnson in the 1970s. With funding from grants provided by the Research Corporation and by the Natural Sciences and Engineering Research Council of Canada, the FTS has been designed and built by ABB Bomem in Quebec City. The spectrometer is now located at the Lowell Observatory, Flagstaff, Arizona, which is hosting this project.
How Bright is the Sun? Although we know more about the Sun than any other star, its absolute spectral energy distribution is, surprisingly, more uncertain than the bright nighttime stars. Because the Sun is so bright and its angular size is so large, the measurement of its spectral irradiance is dominated by systematic effects, not the number of photons as is usually the case in astronomy. Because of the fundamental importance of the Sun, and because the FTS is able to measure large, bright sources with great accuracy, the Sun is the first star in this program.
Very Tiny Telescope - Because of the Sun's great brightness, it can be observed with a very small telescope. My approach is to mount a small heliostat on top of the stationary spectrometer, as shown in this image of the heliostat. This image also shows Ralph Ney and Jim Darwin of the Lowell Observatory, who created this arrangement. The heliostat tracks the Sun and reflects the light to stationary folding mirrors that direct the light into the "telescope", which is a small lens just above the entrance aperture of the spectrometer. The amount of light is still too much for the detectors of the FTS, so a neutral density filter must be placed in front of the lens, reducing the effective diameter of the telescope to only 50 microns, surely one of the smallest telescopes in the world.
Spectroscopy - To illustrate the spectroscopic performance of the instrument, this observation of H alpha, is compared to the same feature in the Kurucz et al. Solar Flux Atlas, convolved to my resolution.
Photometry - The tests from 2004 June measured the solar spectral irradiance every 30 seconds for several hours, and these data were used to derive the atmospheric extinction. For example, the extinction at 600 nm agrees with the value expected for Lowell observatory in June. Extinction such as this is measured at every wavelength with a spectral resolution of 2 cm^-1. For example, the violet extinction shows the rising extinction due to Rayleigh scattering.
Calibration - The next stage of the project is to develop a calibrated light source to remove the instrument's reflections and transmissions.

Selected Publications

Former Graduate Students

Last updated: 24 August 2007