Neutron stars (NS) are one of the potential stellar endpoints for stars greater than eight times the mass of our Sun.  These stars are extremely compact and can exert immense gravitational forces on nearby objects.  By pulling in material from a neighboring star, a NS can "spin up" and begin rotating very rapidly.  This so-called "accretion" of material releases jets of energy (shown as the green cone in the NASA image above) from the NS that, due to the rapid rotation, can sometimes be observed from Earth as periodic signals.  We call these kinds of stars pulsars, and some of these objects can be spun up to the point that we observe their "pulses" once every few milliseconds!

Part of my research is dedicated to observing and timing these so-called "millisecond pulsars" (or MSPs).  I am particularly interested in observations of MSPs that exist in binary star systems with different kinds of low-mass companions.  We call these systems "spiders," due to the parallels between the pulsar slowly wearing away its companion and the behavior of some species of spiders.  Spider systems with very-low-mass objects (on the order of a few percent of the mass of our sun) are called "black widows", and systems with low-mass, main sequence stars (e.g., red dwarfs) are called "redbacks."  We only know of a few dozen spiders, yet they play an important role in our current, and certainly future, understanding of the MSP "recycling" process (the process by which the NS gets spun up).  Finding more of and studying the population of these intriguing systems can therefore help further our understanding of this complex process.

White dwarf (WD) stars are the final stage of evolution for stars like our Sun, and over 97% of stars in our Galaxy will eventually become WDs.  These stars are compact, and can exhibit different kinds of "variable" behavior.  Also, WDs can exist in binary systems with various companions such as main sequence stars, giant stars, and even other WDs.  For some systems, the orbital period — the time it takes one star to complete one orbit around the other — can be as fast as only a few minutes!  The population of binaries with orbital period of less than 80 minutes is referred to as "ultracompact binaries" (or UCBs) since they have uniquely compact orbits.  A majority of these systems contain two, low-mass (a few tenths the mass of the Sun) WDs; however, some of them are composed of a WD and other kinds of evolved stars such as a hot subdwarf.

Part of my research involves active monitoring of known UCBs in an effort to track any changes to their orbital period (much like timing MSPs).  Since UCBs have such tight orbits and have objects with strong gravitational fields, they produce gravitational waves that will cause the orbital period to get faster and the stars to get closer over time.  Measuring this shrinkage is important for missions such as the upcoming Laser Interferometer Space Antenna (LISA).  Currently we only know of about a dozen UCBs that could exhibit a change in orbital period due to the gravitational waves, so I also perform observations to find and characterize new UCBs that can be added to the list of timeable systems.