|
The Chemistry of Stars - Current Research at UW Oshkosh
The astronomical research taking place today at UW Oshkosh
is centered around studies of the evolution of low mass stars. It is important
that we understand such stars because the more massive a star is, the faster
it evolves. If we look around at the oldest stars in the Universe today, they
are all about 80% the mass of the Sun or less. All the more massive stars
from those early days have since evolved into white dwarfs, neutron stars,
or maybe even black holes.
We can use this to our advantage. By observing these old stars, and knowing how they work, how they live and how they die, we can figure out how old they are.
The globular cluster M5 |
This is the procedure which is used to date the Galactic Globular clusters - large clusters of stars (hundreds of thousands!) found scattered throughout
the halo of our galaxy (see the pictire of M5 to the left). Astronomers typically derive ages for these objects of
12 - 14 Billion years, implying this is about how long ago the Milky Way started to
form. Of course it also means that the Universe must also be at least 12 - 14
Billion years old.
Another reason to look at these old stars is that their chemical composition
should reflect the distribution of elements during the time at which
they formed. In a sense these stars are fossil remains of when our
galaxy was just forming. By studying them, we can get some idea of the processes
which were operating and the conditions during that early epoch.
But the picture that is evolving may not be simple. One of the most puzzling
observations (at least to me) is the distribution of light elements among member
stars of globular clusters. The traditional model of these systems is that they
are "a whole bunch of stars which all formed at the same time in the same place
out of the same cloud of gas." Thus all of the stars within a given cluster should
have the same chemical composition. Yet in the case of elements like Carbon,
Nitrogen, Oxygen, and even Sodium and Aluminum, this isn't necessarily
the case, especially N, which has been found to vary by more than a factor of
ten from star-to-star in some clusters! The question then is where did these
abundance differences come from?
There are two possible answers to this:
- The stars started out this way (the primordial scenario).
Possibly some process operating early in
the cluster's history polluted the material from which the present day stars
formed. This would then raise many more questions. How long was the
epoch of star formation in the clusters? Some stars must have formed and
evolved before star formation was complete in the clusters. What was the
source of the chemically modified material? Almost all present day clusters
do not have enough mass to retain the ejecta of supernovae. What effect
do the differing compositions have on evolution? This is particularly
important in the case of O, as significant supplier of electrons in the stellar
interior (electron scattering is an important opacity source in these stars,
which in turn effects their rate of evolution).
- The stars have modified their own surface composition by mixing the
products of nucleosynthesis to their surface (the mixing scenario).
The elements C, N, O (and
possibly Na, Mg, and Al) are all associated with hydrogen burning.
Could material exposed to nucleosynthesis somehow be
brought to the surface during the later stages of evolution? If this is indeed
taking place, then there must be something missing from our standard
theories of the structure and evolution of low-mass stars, as they do not
predict this mixing to take place. Moreover, the abundances of some
elements (namely O) which are important for determining the ages of
the clusters are measured from spectra of the brightest and most evolved stars
in the cluster. If these stars have somehow modified their surface abundances
of these elements, they no longer reflect the composition of the less evolved
stars which are used to compute the ages.
Most of my research here at UW Oshkosh is geared toward performing observations
which should help us figure out this puzzle. I am taking two independent
approaches: 1) exploring the chemical makeup of the least evolved cluster
stars to compare them with their evolved counterparts. Any differences between
the two groups must be due to internal (mixing) mechanisms. 2) looking at
abundance patterns among evolved cluster stars to try and pin down the underlying
trends. Most recently this has centered on studies of isotopic carbon ratios
to look for correlations which might be explained as the presence of material
exposed to either the CN or CNO cycle of H-burning.
You can read about these observations and their implications in the
publications below. Also, if you would like to learn more about this astronomical
problem, I might suggest the 1994
Review by Dr. Robert Kraft or the 1994 Review by Briley et al. (Canadian
Journal of Physics, 72:772 - no online version available). Or,
you can just send me some
mail.
If you are interested in this type of work, some possible
projects are listed on my Student
Research Opportunities Page.
|
