‘Imagine Indiana Jones but in space’: galactic archaeology explained
I ALWAYS GET excited when someone asks me about my PhD and my field of research, one reason is because I really love what I do, but the other reason is because there is a small chance that they’ve heard of my field before.
When someone says they’re doing a PhD in astrophysics there is a multitude of niche subfields that they could be a member of, ranging from exoplanets to studying some of the most massive and distant galaxies in the Universe. For me, my field is all about galactic archaeology.
But what exactly is galactic archaeology? One of the big questions in the study of the Universe is how galaxies form and evolve throughout cosmic time. There are approximately two trillion galaxies scattered throughout the Universe.
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So, we have plenty of test subjects to study in our quest to answer our questions about galactic evolution, but there’s one pretty annoying obstacle. They’re all really really far away. The nearest major galaxy to us is our galactic neighbour Andromeda and she’s a whopping two and a half million light years away.
Despite this galactic social distancing, we do have one very big advantage when it comes to studying how galaxies form and evolve. We live in one! Living in the Milky Way galaxy gives us a front row seat to understanding more about how galaxies like our own evolve and change. The Milky Way galaxy is like our very own Rosetta Stone that helps us understand galaxies in general.
Imagine Indiana Jones but in space, that is galactic archaeology in a nutshell. Instead of studying ancient artefacts on the Earth to understand more about the history and evolution of ancient civilisations like an archaeologist does, I study the stars in the Milky Way galaxy to better understand the history and evolution of our island universe. It’s a great mix of geeky and nerdy and I absolutely love it!
Observing stars in the Milky Way can tell us a lot about our galaxy. When we look at a star with our telescopes, we feed the light they shine down onto the Earth through a special instrument called a spectrograph. The spectrograph then breaks up the light from the star into a spectrum or a rainbow and when we look closely at this rainbow of light, we can see signs of what sort of elements exists inside of the star. We’re looking at the stars’ DNA, or its chemical abundance.
All of the stars in the universe are made up of mostly hydrogen and helium gas and we can see features in a stars’ spectra that verify this fact. For example, the big deep dip in the red part of the spectra of a star from the GALAH survey is called the H-alpha line, this tells us that there is a whole lot of hydrogen in this star! While stars are made up of mostly hydrogen and helium gas, we also find traces of other elements within stars, for example, the second line highlighted in this spectrum indicates that there is iron in this star. The abundance of these elements in a star gives us clues about the location of the stars’ birth as well as when it came to life.
In astronomy, our periodic table is very simple, you have hydrogen and helium, the building blocks of the universe that were formed in the very first few minutes after the Big Bang, and then you have everything else which are referred to as metals. These metals are formed in a variety of ways, some are formed within the blazing hot cores of stars. Heavier metals are formed from the dramatic explosions of stars, called supernovae (or supernovas if you want to start an argument with an astronomer), and the heaviest metals are formed in the explosive collision of very dense stars.
All of these elements, when they’re created by stars and their explosions, are expelled out into the universe and recycled to form newer generations of stars. So, a star with a very small amount of metals is likely to have formed in a time when the Milky Way galaxy was very young, whereas a star that has a larger quantity of metals would have come to life more recently.
Stars with very similar chemical DNA and ages are likely to have formed from the same cloud of gas but, due to the internal motion of our galaxy, those stars may no longer be together today. So, by studying a stars’ composition we can find its stellar siblings and uncover more about the stellar and chemical history of the Milky Way. In galactic archaeology we call this chemical tagging.
We can also determine if a star was not formed within our Galaxy. Galaxies all over the universe interact with each other and our own Milky Way has experienced multiple interactions with other galaxies and will continue to in the not so near cosmic future. During these galactic interactions stars and gas from the interacting galaxies can be stripped and stolen and by analysing a stars’ chemical abundance, we can determine whether or not it was cannibalised by our Galaxy.
This is what I get to do in my research as a team member of the GALAH team. One of the key goals of my PhD project is to create a detailed 3D map of chemical abundances within the Milky Way galaxy. In doing this I will be able to uncover when and where new elements have been created in our Galaxy’s history.
GALAH stands for galactic archaeology with HERMES and HERMES is the name of the spectrograph used in the survey. If you want to learn more about what myself and the team are researching, you can explore the GALAH survey website galah-survey.org and follow the @galahsurvey Twitter account.
You can also play with some of the data for yourself, if data analysis and visualisation tickle your fancy! DR2 (data release 2) is available for download at datacentral.org.au and DR3 (data release 3) will be available at the start of October. It will include spectra as well as an expanded and improved catalogue.
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