David Bersier - Research

Ths Sun is seen to move from East to West every day. The appearance and position of the Moon change from night to night. The planets wander across the sky. Comets or even new stars sometimes appear. In short, some of the most familiar astronomical phenomena involve variability, either in brightness, shape, or position. Although it is nothing new, the study of phenomena that involve a change over time are nowadays called time-domain astronomy. Most of my research focuses on variable objects, near or far.

Stars going boom: Gamma-ray bursts and supernovae

Gamma-ray bursts are just that: brief flashes of gamma-rays. Gamma rays are like ordinary light but it carries a lot more "punch". We now know that GRBs mark the birth of a black hole. GRBs involve extreme physical conditions and are pushing almost every limit in terms of matter density, magnetic field, speed of matter.
We now think that GRBs lasting longer than about two seconds are caused by the collapse of a massive star (somewhere between 20 and 40 times the mass of the Sun) and is accompanied by a supernova (see below for supernovae). In almost every case where we can find evidence of a SN accompanying a GRB, we have found such evidence.
As for short GRBs, those lasting less than about two seconds, they are thought to arise from the merger of two neutron stars or a neutron star and a black hole.

The supernova (SN) phenomenon is a spectacular display, a kind of cosmic fireworks that marks the end of the evolution of two type of stars.

Core-collapse supernovae: When a star with a mass larger than about eight times the mass of our Sun reaches the end of its life (i.e. when it has exhausted its nuclear fuel), its core will collapse suddenly. More often than not (or so we think), the star will explode and will shine very brightly for weeks and months.
Type Ia supernovae Another type of supernova come from white dwarf stars. When a star with a mass less than about 8 times the mass of the Sun reaches the end of its life (i.e. when there is no more nuclear fuel), the stellar core that is left behind will cool: that's a white dwarf.
A white dwarf in a binary system can gain some mass from this companion if the two stars are close enough. It may gain so much mass that it can collapse under its own weight. Nuclear reaction will start again and the star will explode.

In August 2010, we started to observe SNe nearly every night with the Liverpool Telescope. These SNe are found by the Palomar Transient Factory.

Stars going Up and Down: Cepheids and variable stars

Stars are very much like a simple pendulum. A pendulum is stable when it is perfectly vertical. When it is moved away from its equilibrium position, it tries to get back to it, "overshoots (goes too far), then moves back, etc. If there is no friction, the pendulum can oscillate forever. Stars do the same. They can shine at a constant luminosity but they can also be a little away from their equilibrium, in which case they will pulsate. They will appear periodically bright and faint. Go to the web site of the American Association of Variable Star Observers (AAVSO); it has a lot of background information on variable stars.

One particular type of variable stars I am interested in are Cepheids. They vary in size and luminosity extremely regularly. As time-keepers, they are much more accurate than most wrist-watches one can buy but this is not their main use. It turns out that the pulsation period of a Cepheid — the time it takes for a whole cycle of getting-faint-and-getting-bright-again, typically between a few days and a few weeks — is related to its average luminosity. This means that if we measure the period of a Cepheid, we can measure its luminosity, hence its distance. This Period_Luminosity relation (PL) is what makes Cepheids very useful to measure distance in the local Universe. Most of my work on Cepheids is connected to their use as distance indicators.

One particular issue I have been working on is the metallicity dependence of the PL relation. To build a large sample of Cepheids all at the same distance, we have surveyed the nearby galaxy M33. Complete results are not yet out but you can already check out the M33 variability survey page

Stars coming up or fading away: Star formation in nearby galaxies

Understanding the evolution of galaxies is one of the major challenges of modern astronomy. There are two ways to attck this problem. One is to look at different galaxies over cosmic time. By comparing the properties of nearby galaxies with those of very distant (hence old) ones, we can paint a broad picture of galaxy evolution. The other approach is to look at individual stars in galaxies and, using our knowledge of stellar evolution, reconstruct the history of star formation of each galaxy.
It is this second approach that I focus on. In collaboration with Maurizio Salaris and PhD student Emma Small, we are working on a general method to determine the star formation history of any galaxy using stellar magnitudes. Starting from a colour-magnitude diagram, our objective, non-parametric method accurately recovers the formation rate and metallicity of the stars as a function of time.

Projects for students: