Supernovae in Cambridge - a lovely summer internship

Throughout last July and August, I was lucky enough to spend my summer working at the Institute of Astronomy in Cambridge on a funded summer internship exploring theoretical models of early universe stars and their supernovae.

The previous January had been somewhat stressful as I spent much of it writing and sending off internship applications, however once it got to summer and I was cycling into the Institute for my first day at work, it was all worthwhile given how much fun I had doing research. The first thing I saw when I came into the Observatory building (above), where I spent most of my time working, was a series of posters dedicated to past and present women in astronomy. Jocelyn Bell Burnell, who worked at Cambridge as a young postgraduate student in the 60s and was fundamental in the discovery of rotating neutron stars but was not awarded a Nobel prize (which went to her thesis supervisors instead), was on one such poster. She has since become a trailblazer for women in astrophysics and an inspiration to many. Another poster showed Caroline Crawford, an emeritus member of the Institute who has done a huge amount for astronomy both in research and outreach, and I regularly listen to her talking about stars and planets on the radio.

My project revolved around theoretical models of early universe stars, how they evolve and how they eventually die in spectacular explosions called supernovae. The first stars in the universe around 13 billion years ago that formed from primordial gas after the Big Bang were very different to ‘modern’ stars formed more recently. The first stars, known as Population III stars, could have enormous masses potentially up to a hundred or even a thousand times that of the Sun. Their chemical compositions were very different too as they were made mainly of hydrogen and helium. Modern stars are more chemically enriched and contain elements such as metals because these are the recycled products of previous generations of stars, which burnt light elements and fused them into heavier ones. Life as we know it could not exist without metals, which in astrophysics language means anything other than hydrogen and helium. Even the air we breathe is the result of elements made from past generations of stars. Because of their unique properties, it is thought that Population III stars may be able to explode in a rare and hypothetical supernova called a Pair Instability Supernova (PISNe). These can occur when a massive star undergoes a thermonuclear explosion. At the high temperatures and densities present in supermassive stellar cores, energetic photons collide with atomic nuclei resulting in the photon splitting to form an electron-positron pair. The star remains stable for as long as the outward radiation pressure from photons escaping the burning stellar core balances with inward gravitational pressure. However, when electron-positron pair production occurs the reduction in the number of photons present reduces the outward pressure, and the core rapidly contracts as the inward gravitational force begins to dominate and culminates in an enormous, bright explosion. When the core collapses it ignites explosive silicon and oxygen burning which blows the star apart and leaves no remnant behind, not even a black hole, which is unusual. This process is shown in a sketch I made below. Early universe stars have not yet been directly observed, but it is hoped that they may be indirectly observed through their explosions as PISNe, which is challenging due to their suspected rarity, but may be possible.

My job was to replicate some stellar evolution models, and then calculate how many PISNe could be seen per year for each model. Each model had a different distribution for the masses of the stars in a hypothetical population. For PISNe to occur, progenitor stars are thought to need a mass of 140-260 solar masses, so models that produce more stars in this mass range will also result in more PISNe.This is helpful because in a scenario where, say a telescope has been observing an area of sky for 1 year and found no PISNe (as is currently the case), we can calculate which models are most likely to have caused this outcome, which in turn constrains what we know about the stars that have evolved in such a model. This reverse statistical inference process is one used by many astronomers hoping to find rare supernovae and use them to uncover the secrets of the first stars.

Most of my work was quite mathematical and theoretical, but there were other interns on the summer programme working on observational astronomy and on topics as varied as white dwarf stars, exoplanet atmospheres and black holes. Everyone was very friendly and it was exciting to be in an environment with so many other astrophysics enthusiasts, with the highlight being a talk on space travel by the Astronomer Royal, Lord Martin Rees, which was very inspiring. Occasionally I would potter around the historic site, and I found some lovely old libraries with a treasure trove of astronomy magazines and books. One such room had an old astrophysics department photo from 1973, and peering closely I saw the famous Stephen Hawking on the front row. Next to that photo was a bust of another noteworthy Cambridge alumnus the Indian astrophysicist Subrahmanyan Chandrasekhar, who famously worked out the maximum mass of a white dwarf star before it explodes while on a long boat voyage in the 1930s. Seeing such famous names and important history about the scientists who worked here was very motivating. All in all, it was lovely to be part of the inspiring Institute of Astronomy community for two months and I learnt a great deal about astrophysics. I would like to thank the Summer Research Programme for funding my project and Dr. Fialkov and Mr. Gessey-Jones for supervising me over this time.