Chemical Evolution of the Galaxy

(Ryan)

The tightly wrapped spiral arms seen in images of spiral galaxies are an obvious sign that galaxies undergo changes, as stars and gas clouds move around the galaxy and sites of intense new star formation are set up. In addition to their motions about a galaxy's centre of mass, stars change the chemical composition of the galaxy in which they reside, through nuclear fusion reactions in their hot interiors. Massive stars (mass > 10 times the mass of the Sun) and intermediate mass stars (between 2 and 10 times the mass of the Sun) eject much or all of their gas back into the interstellar medium at the end of their lives, thus injecting new elements which are incorporated into subsequent generations of stars. By measuring the chemical compositions and motions of stars of different ages, it is possible to piece together the chemical and kinematic history of a galaxy.

At the University of Hertfordshire, we are concentrating on the earliest phases of the chemical evolution of our Galaxy, the Milky Way. We study this by using 8 metre telescopes (especially Subaru and the Very Large Telescope) to record high-resolution spectra of extremely old stars which preserve the fossil record of the Galaxy's earliest epochs. The stars in the Milky Way can be studied in greater detail than those in more distant galaxies, and by studying the earliest phases of evolution we can learn about the Big Bang and the conditions in the newly forming Galaxy when its first stars were emerging. We are studying elements formed in a number of different astrophysical sites (supernovae, cosmic ray reactions, red giants, etc) which tell us about different stages of stellar and galactic evolution. In addition, we have a programme to improve the theoretical modelling of stars which underpins the analysis we undertake, by computing new, more complex 3D representations of stellar atmospheres.