Gustavo Bruzual &
Stéphane Charlot talk with ScienceWatch.com
and answer a few questions about this month's Front in the
field of Space Science. The authors have also sent along
images of their work.
Article: Stellar population synthesis at the
resolution of 2003
Journal: 2003 MON NOTIC ROY ASTRON SOC 3, 344
(4): 1000-1028 OCT 1 2003
Addresses: Ctr Invest Astron, AP 264, Merida 5101A,
Ctr Invest Astron, Merida 5101A, Venezuela.
Max Planck Inst Astrophys, D-85748 Garching, Germany.
CNRS, Inst Astrophys Paris, F-75014 Paris, France.
Why do you think your paper is highly
Our paper provides interested users with a tool to interpret the spectral
energy distributions of stellar populations in galaxies, from the Milky Way
all the way to the most distant galaxies discovered in the universe.
The evolution in time of the light emitted by stars in a galaxy or a star
cluster is a valuable clue to understanding the basic properties of stellar
populations, which in turn inform us about the history of galaxy assembly.
Age, chemical composition, and mass in stars are among the quantities that
can be readily inferred from comparisons of our models with galaxy
The paper appeared at the moment when observations from the Sloan Digital
Sky Survey (SDSS) were becoming available in large quantities. The spectral
resolution of the SDSS spectra matched quite well that provided by our
models, and the models became almost "de facto," a convenient and
ready-to-use tool to interpret these data.
Additionally, we provided the users with a series of computational tools,
which made applications of the models easy and convenient. Finally, the
wide spectral coverage of these models makes them a useful tool to
interpret data collected at all major ground-based and space-based
ultraviolet, optical, and infrared telescopes.
Does it describe a new discovery, methodology, or
synthesis of knowledge?
The paper assembles a large amount of astrophysical information available
in the literature and on the Internet and makes specific predictions about
the behavior in time of most of the observational properties of stellar
populations in galaxies—independently of the effects of interstellar
gas and dust.
By design, in fact, spectral evolution models constitute a synthesis of our
knowledge of how stars evolve and shine, from which we can compute the
amount of light emitted at any time and at any wavelength by an evolving
stellar population. In a way, our models also defined a new methodology,
which has become "a standard" in the field of spectral studies of galaxies.
Would you summarize the significance of your paper
in layman's terms?
Our work allows observational astronomers to establish with relative
certainty the age of a galaxy and the chemical composition and the total
mass of stars shining in it. As we observe more and more distant galaxies,
we also look further back in time because light travels at a constant
Hence, the stars we see in distant galaxies are typically younger than
those in the nearby universe, and the light they emit is bluer and more
intense. Interpreting observations of distant galaxies requires a careful
modeling of these effects. This is precisely what we provide in our paper.
How did you become involved in this research and
were any particular problems encountered along the way?
This research is the fruit of a long-term project we both got involved in
at different epochs during our Ph.D. studies—GB about 10 years before
SC. Perhaps the most major problem we kept encountering along the way has
been the lack of reliable prescriptions to describe the light emitted by
stars in some complicated phases of stellar evolution.
This can weaken considerably the applicability of the models in specific
cases where the missing ingredients play a dominant role. Major progress in
our research therefore relies on the progress accomplished in related
fields of astrophysics, such as stellar evolution theory and stellar
Where do you see your research leading in the
This kind of research will undoubtedly lead to a better understanding of
the universe. The rate of progress in our theoretical understanding of the
physics of stellar evolution and stellar atmospheres makes us foresee that,
in the near future, the use of spectral synthesis models in studies of
galaxy evolution will become more sophisticated.
The answer to problems like how galaxies are built up, how their mass is
assembled, how copiously they form stars, and how their gas becomes
enriched in different chemical elements, which at present are only hinted
at, will be part of textbook astronomy.
Do you foresee any social or political implications
for your research?
From the social point of view, research in extragalactic astronomy, and
astronomy in general, has contributed to our understanding of the universe
and has increased the cultural heritage of mankind to levels unthinkable
only a few years ago.
Gustavo Bruzual, Ph.D.
Centro de Investigaciones de Astronomía (CIDA)
Stéphane Charlot, Ph.D.
CNRS and UPMC
Institut d’Astrophysique de Paris
KEYWORDS: ASYMPTOTIC GIANT BRANCH; COLOR-MAGNITUDE
DIAGRAM; EARLY-TYPE GALAXIES; DIGITAL SKY SURVEY; SURFACE BRIGHTNESS
FLUCTUATIONS; INTERMEDIATE-MASS STARS; OPEN CLUSTER M67; M-CIRCLE-DOT;
EVOLUTIONARY SYNTHESIS MODELS; CIRCUMSTELLAR DUST SHELLS.