How do we explode conventional thinking on the Milky Way’s evolution?

Researchers expose the shortcomings of stellar evolution models and theory - going back to fundamental physics to herald a new generation of stellar modelling

Glancing up at the sky on a clear, dark night reveals a familiar swathe of light – the Milky Way. Visible in the heavens since the Earth first formed, this intriguing line of luminescence is, in reality, the heart of our galaxy, as seen from one of its spiralling outer arms.

Understanding the galaxy’s structure has long been challenging. Our solar system sits on the outer edges of a vast rotating disc some 100,000 light-years across, with the other side of the galaxy obscured by its dense central ‘bulge’ of stars, gas and dust.

The mists surrounding the Milky Way’s formation may start to clear, as a team of astrophysicists at the University of Birmingham lead academic partners across Europe in a five-year asterochronometry project using asteroseismology and detailed stellar modelling to test our current understanding of the galaxy’s structure and evolution.

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Understanding our galaxy’s ‘assembly’

Backed by ERC (European Research Council)  funding and led by Dr Andrea Miglio, the team is using data from a range of sources to explore chemical composition and movement as a function of the ages of stars, as they create a timeline of events shaping the galaxy’s development; understanding the temporal sequence that led to the Milky Way’s ‘assembly’.

In a complex piece of galactic archaeology, researchers are using information from the Gaia satellite mapping the positions and movements of over a billion stars alongside data from the Sloan Digital Sky Survey’s APOGEE experiment that uses spectroscopy to measure the distribution of elements in stars and images from the recently-retired Kepler space telescope. 

One example of the kind of investigations the team will be carrying out uses chemical traces left by stars to uncover new information about a smaller galaxy, known as the ‘Gaia Sausage’, which collided with the Milky Way around 12.5 billion years ago – helping to shape the barred spiral galaxy with which we are familiar today. 

Hydrogen makes up 70% of most stars with helium contributing 28%; coupled with the remaining traces of other elements the mix yields valuable insights into the star’s life. When stars die they release different elements to the interstellar medium – for example, old stars are oxygen rich but contain little iron. Type la supernovae accrete matter from companion stars over the billion-year period it takes for them to reach the point of explosion – these contain higher levels of iron.

“The Milky Way is the only galaxy where you can determine the ages of individual stars with high precision,” commented Dr Miglio. “There are huge challenges in accurately establishing a star’s age, but we’re helped by red giants; evolved stars which have cooled and are very luminous. Asteroseismology allows us to measure the mass of these stars and obtain a proxy measurement for age – the challenge here is measuring within 10-15% accuracy.

“We can measure its chemical composition and use stars as fossils since the various elements such as hydrogen, helium – and to lesser extents iron, oxygen and carbon – are locked into the star’s structure. If we combine chemical composition with movement and the mass of a star - determined by asteroseismology - we can build a fairly accurate picture of its evolution.”

‘Seeing’ inside stars

Dr Miglio is working with a team of post-doctoral researchers to improve models of understanding to ‘see’ inside stars, and colleagues Dr. Fiorenzo Vincenzo and Dr. Ted Mackereth are using detailed information on the ages of stars to reconstruct the assembly history of the Milky Way. Red giants provide reference points that allow a galaxy-wide extrapolation giving researchers the opportunity to map properties of the stars in relation to the central galactic disc. Properties change as stars move away from the disc – with age increasing and average mass decreasing.

Recent research led by Dr Vincenzo focussed on the Gaia Sausage and highlighted a gap in the age distribution of stars occurring at the same time as the collision, suggesting that the impact caused an interruption in star formation within the galaxy. Chemical analysis also allowed researchers to more accurately date the smaller galaxy, which previous estimates had put at 2.5 billion years older. The team focussed on tracers left by iron, magnesium and silicon, but future research will incorporate measurements from other elements to build an increasingly accurate picture of the galaxy’s formation.

“Turbulence and heating caused by the merger of the Gaia Sausage with the Milky Way could have prevented the formation of stars at this time, but to confirm this we would need even more precise measurements of the ages of the stars in the smaller galaxy.” commented Dr Vincenzo. 

Analysis carried out by Dr. Mackereth and his team recently revealed that the oldest stars in the Milky Way move more rapidly in and out of the densely populated disc at the heart of the galaxy than their younger counterparts. Part of the team’s work is to train computer algorithms to infer the ages of millions of stars across the galaxy – an extremely useful application of Artificial Intelligence, albeit one which is highly dependent on using a training data set with accurate ages for the stars.

“We think older stars are more active because they were formed during a period when the galaxy was more violent, with lots of star formation disturbance from gases and smaller satellite galaxies,” he explained.

Mapping the Milky Way’s evolution

With its dynamic and chemical substructures, the Milky Way is a complex system where competing processes such as mergers, internal secular evolution, gas accretion and gas flows take place. Detailed study of the giant galaxy’s formation and evolution will require the reconstruction of a highly accurate timeline setting out its main formation events. 

Asterochronometry will determine precise ages for tens of thousands of stars, using an approach that develops novel star-dating methods coupled with a careful appraisal of ‘known unknowns’ around age based on our limited understanding of stellar physics. This will allow Dr Miglio and colleagues to use astrometric, spectroscopic, and asteroseismic data to build chrono-chemo-dynamical maps of regions of the Milky Way probed by the space missions CoRoT, Kepler, K2, and TESS. They will quantify the relative importance of various processes playing a role in shaping the galaxy, for example mergers and dynamical processes, as well as using chrono-chemical tagging to look for evidence of aggregates.

“The ERC project is about improving stellar models and age estimates; using them to learn about the Milky Way and provide observational evidence to stimulate development of a new generation of stellar modelling. Having experts in asteroseismology, galactic structure and chemo-dynamics in the same team allows us to operate as an efficient interdisciplinary unit.

“We will create stringent observational tests of stellar structure and answer some of the long-standing open questions in stellar modelling, such as the efficiency of transport processes and the occurrence of products of coalescence and mass exchange. These tests will improve our ability to determine stellar ages and chemical yields affording us a greater understanding of how our home galaxy came into existence.”

Overcoming stellar challenges

Partnership is a fundamental part of the project and astrophysicists at Birmingham are working with counterparts in research institutions across Europe. These include the Universities of Potsdam, Padova, Liège and Aarhus, as well as the Paris Observatory, University College London and the University of Surrey.      

These international partnerships will help researchers at Birmingham overcome the stellar challenges of the next decade and they aim to explode conventional thinking within the next five years. Rather than providing answers to the questions surrounding the Milky Way’s formation, the project will create the building blocks to help better understand the galaxy’s evolution.

“We want to expose the shortcomings of stellar evolution models and theory; providing evidence that we need to move to new paradigms,” commented Dr Miglio. “At the end of five years, we should have a much clearer view of the Milky Way’s assembly history.” 

Notes:

Main header image: This image shows Gaia's all-sky view of the Milky Way based on measurements of almost 1.7 billion stars. Credit:ESA/Gaia/DPAC, CC BY-SA 3.0 IGO

Vincenzo et al (2019). ‘The Fall of a Giant. Chemical evolution of Enceladus, alias the Gaia Sausage’. Journal. DOI: https://doi.org/10.1093/mnrasl/slz070

The Asterochronometry project, co-ordinated by the University of Birmingham, and funded by the European Commission, will determine accurate, precise ages for tens of thousands of stars in the Galaxy.

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