Yosemite Falls and the Celestial Pole.
One day I hope to also include a site for New York - completing the trio of great cities of the United States.
Here are the two main themes (to date) of my astronomical work:
Globular clusters are spherical groupings of hundreds of thousands of stars found in our galaxy and others. They are interesting objects because they are among the oldest objects in our galaxy (usually over 10 billion years old), and their stars seem to have been born at very nearly the same time and with nearly identical mixes of chemical elements. This makes a star cluster a laboratory where we can observe how the properties of stars of different stellar mass change over time. By measuring how much light the stars emit in different wavelength ranges, we can infer their surface temperatures and the total rate at which they release energy (their "luminosities").
Below are images (from the Digital Sky Survey) of globular clusters I have worked on so far: Messier 5 (M5) and Messier 30 (M30). On the right is Messier 12 (M12), a cluster that my graduate student Jonathan Hargis has worked on. They are roughly the same distance away (M5: 24400 light-years; M30: 26300 light-years; M12: 15800 light-years), but their appearances are different - globular clusters have their own personalities in many ways. If you would like to learn the details of the studies, click on the highlighted papers in the reference sections.
M5 is one of the most massive clusters in our galaxy, and is also not terribly dense in its center. Look here for a color photograph by David Malin of the Anglo-Australian Observatory. These facts allow us to make observations over a wide range of cluster radii for a large number of stars. The image below was taken at Cerro Tololo Interamerican Observatory's 4 meter telescope in Chile - the frame shown was taken in I band with approximately 1.5 arcsecond seeing.
From this data set, I have a sample of approximately 43,000 stars. Here is the calibrated Color-Magnitude Diagram using B (visible blue) and I (infrared) filters. This data was reduced using the DAOPHOT suite of programs (developed by Peter B. Stetson at the Dominion Astrophysical Observatory) for photometry of crowded star fields. The horizontal axis involves the difference between the B and I magnitudes measured, and can be thought of as a temperature sequence (with temperature perversely increasing to the left). The vertical axis gives the I band magnitude (a measure of the star's brightness over a particular range of infrared wavelengths). Increasing magnitude (perversely) indicates a fainter star.
It is possible to see some striking features in the diagram: very well-defined evolutionary branches (especially notice the horizontal branch living up to its name, and the red giant branch (RGB) in the upper right), a large sample of asymptotic giant branch (AGB) stars (very upper right, to the left of the RGB), and a nicely defined RGB "bump".
M30 has one of the densest concentrations of stars of any of the known globular clusters. This fact means that stellar collisions may be important within this cluster (see below for some potential effects of stellar collisions). In fact, there seems to be a notable deficit of giant stars in the cluster - larger stars are somewhat more likely to be involved stellar accidents than the more numerous main sequence stars. Our data for this globular cluster were taken at Cerro Tololo Interamerican Observatory's 4 meter telescope in Chile. I measured the brightnesses of over 25,000 stars in this cluster.
With all of the planets being discovered around other stars, it is natural to wonder what might have happened differently there than here. In fact, it is somewhat strange to find that many of the planets that have been discovered in past years are gas giants (like Jupiter, Saturn, Uranus, and Neptune) that orbit their star closer than Mercury orbits our Sun. We have taken our curiosity one more step - we have tried to see what would happen if one of these planets were to be dragged into the star.
The obvious answer is: it will melt! But there is interest in how long it takes the planet to be stripped away. Jupiter in fact seems to have more than its fair share of heavy elements (anything other than hydrogen and helium: for example, carbon, nitrogen, oxygen, iron, ...). If planets around other stars are like Jupiter in that respect, and a planet that falls into a star is dissolved fairly close to the star's surface, these heavy elements can leave their signatures on the light coming from the star. So in principle we might be able to determine if a star has eaten a planet!
The images above show how Jupiter gets distorted by its passage through the envelope of a star at an orbital speed of about 400 kilometers per second. (That is pretty close to one million miles an hour!) The planet has been sliced in half to show you some detail of the interior: "hotter" colors indicate gas of higher density. The first image occurs about 2 hours after the start of the simulation, and the last is after almost 5 hours. Quite quick... (Thanks to Alan Kendall for producing these images.)
Binary stars are systems in which two stars circle each other as a result of their gravitational attraction for each other. There are binary stars that appear to be peculiar though - they orbit very closely together (moving around each other in less than a day!) even though one of the "stars" is a burnt-out hulk, which must have been much larger during its life.
Such binary stars may look peaceful now, but we can infer that their pasts were violent. As stars grow old, they tend to expand to enormous size - hundreds of times the size of our own Sun. If one of the stars in a binary system were to expand in this way, the surfaces of the two stars would get close enough together that the smaller size star will pull much of the larger star's gas away, and even fling it out into space. In the process of doing this though, the drag from the gas causes the smaller star to spiral into the other star, causing more mass to be thrown outwards. Eventually, the smaller star can even strip the majority of the gas away from the larger star, leaving it a much smaller, burnt-out "white dwarf".
With my collaborators, I have carried out simulations of the process by which the small star strips gas off of the larger one, and spirals closer to it. A movie can be found below:
Blue stragglers are peculiar stars that are found in many places in the galaxy. On the surface they appear to be aging gracefully, but are too bright and too blue relative to most of their neighbors. This seems to imply they are younger or more massive. It is known that some of the blue stragglers are actually "contact binaries" - two stars that are orbiting so closely together that their surfaces merge together. This hint seems to tell us that blue stragglers may be more massive than their neighbors in general.
Number of accesses: