Sampling the DNA of 600,000 Stars in the Milky Way Galaxy

In November 2020, a massive amount of important scientific data on the chemical composition of almost 600,000 stellar objects in our Sun’s neighbourhood was released by an Australian-led team, as part of the Galactic Archaeology with HERMES (GALAH) survey.

The dataset, called DR3 (data release 3), was the outcome of more than 30 million measurements, which totalled a massive 500 GB of information collected by the Anglo-Australian Telescope (AAT) at Siding Spring in NSW, led by astronomers from Australia’s ARC Centre of Excellence in All Sky Astrophysics in 3 Dimensions (ASTRO 3D).

To help decipher what elements lay within the composition of each star, the High Efficiency and Resolution Multi-Element Spectrograph (HERMES) spectrograph was used, splitting incoming starlight from more than 300 stars at once, into red, green, blue and infrared components - allowing a ‘unique fingerprint’ to be captured, that would reveal each star’s chemical signature.

“It’s a bit like a galactic version of the game Cluedo,” said ASTRO 3D’s Sven Buder, a research fellow at the Australian National University - who also happens to be the lead author on the paper.

“The chemical information we’ve gathered is rather like stellar DNA – we can use it to tell where each star has come from. We can also determine their ages and movements, and furnish a deeper understanding of how the Milky Way evolved.”

This week, a supplementary release of GALAH’s DR3 was also released, with the team announcing instructions on Twitter on how to download the information, allowing members of the public, citizen scientists and other astronomers to access the massive data set of stellar spectra. The Galactic Archaeology with HERMES (GALAH) first released data publicly in 2016.

Galactic archaeology (different from space archaeology) is the study of the chemical evolution of galaxies. When stars die, the materials left over can form new stars in a process called stellar recycling. This recycling of materials leads to the accumulation of particular elements in future generations of stars. 

As such, studying this chemical makeup is similar to studying buried fossils which can tell us about the earlier conditions of the environment, lifeforms and behaviours on planet Earth. In this way, studying the chemical makeup of stars can tell us about the early conditions of galaxies and how stars form. 

This field of astronomy also allows astrophysicists to model, observe and test their theories on what happened to the very first stars in the Universe - which would exhibit extremely ‘metal’ poor chemical signatures (astronomers term any element other than hydrogen and helium a metal). 

Additionally, through galactic archaeology astronomers can learn about the evolution of globular clusters - which contain some of the oldest (and metal-poor) stars in the Universe, the historical interactions of smaller, satellite galaxies as they have been consumed and cannibalised by our Milky Way, and map the streams of gas and stars between the galaxies to help paint this historical picture of interactions which have been lasting for billions of years. 

In addition to measuring stellar spectra, another technique employed by scientists is to measure the ‘vibrating sounds’ of stars to better understand their interior compositions. This is known as asteroseismology and when combined with stellar spectra surveys provides a powerful tool for astronomers to build this picture. 

There are a number of galactic archaeology surveys around the world, including the APOGEE survey, SAFA, LAMOST and the GAIA-ESO survey, but the GALAH DR3 release is by far the largest set of stellar chemical data ever compiled. 

The GALAH survey collects the chemical information of the stars it observes through HERMES, which has four cameras/filters, (blue: 471.5 - 490 nm; green: 564.9 - 587.3 nm; red: 647.8 - 673.3 nm; and infrared: 758.5 - 788.7 nm). 

The beauty of this system is that it can capture the spectra of over 300 stars in one observation, across all cameras and filters through positioning of the fibre optic sensors by the AAT’s existing two-degree field (2dF) robot, which places each sensor on the target object with unbelievable accuracy. The instrument then surveys each object with a spectral resolution of approximately 28,000.

The wavelengths from these stars that HERMES looks at can then be translated into detailed spectra, which can then be used by scientists to analyse the chemical makeup of each star - revealing its chemical composition and story about its history. 

As part of this third release, the team has published the spectra for almost 600,000 mostly nearby stars (within 10,000 light years of the Sun), inclusive of more than 75 stellar clusters. From this, abundance rations were derived for 30 elements including lithium, carbon, oxygen, sodium, calcium and silicon - a lot of the same elements we see around us here on Earth (including the stuff that we are made from). 

The information from the GALAH surveys can be used to answer various questions (like where did all the big bang lithium go?) which has important implications for our understanding of the evolution of the Universe (lithium production, along with hydrogen and helium is a key outcome of the Big Bang model of the Universe).

“Basically, a lot of the oldest stars have burned much of the Big Bang lithium, so our measurements for this element come out lower than the amount that was initially synthesised in the early Universe,” said ASTRO 3D researcher, and fellow author, Dr Sanjib Sharma from the University of Sydney.

“At the same time, we have found that one type of star, known as evolved giants, should have burned through pretty much all of their lithium by now, but a lot of them have much more of it than we expected. The GALAH data will help us discover why.”

The GALAH survey data can also be used to find new mysteries.

“For instance, while we are mainly surveilling our solar neighbourhood, we have found more than 20,000 stars which do not have the same chemical composition or age our Sun and its neighbours,” explained Dr Buder.

“We know that roughly eight billion years ago the shape of the Milky Way changed drastically when it collided with another, smaller galaxy, which contained millions of stars. We’ve now used the stellar DNA to identify some of the prime suspects for the assault. These stowaways are so different they can only have come from somewhere else.”

As with the prior two releases of data from the GALAH survey, DR3 is also now available (link below) and will be utilised by several other projects from the science community to help build that important historical picture of our Galaxy, it’s cannibalistic habits and what happened to some of the oldest stars around. 

"Making large datasets like GALAH DR3 widely available is really important for astronomical research," explains Associate Professor Sarah Martell from ASTRO 3D and UNSW Sydney.

"Since the start of the GALAH project we have focused on building a dataset that can answer our questions about the history of the Milky Way, and also many others. I'm excited to see what our international colleagues will do with GALAH DR3."

Since its release in 2018, the GALAH DR2 has also spawned a large number of scientific discoveries surrounding the evolution of the Milky Way, the existence of exoplanets, the determination of ages of stars around the Sun, and many more science papers.

_{We acknowledge the Gamilaraay people as the traditional owners of the Siding Spring Observatory site.} 

_{Opening video credit: Dr. Amanda Bauer (Australian Astronomical Observatory)}

_{Closing video credit: Ángel R. López-Sánchez (Australian Astronomical Observatory / Macquarie University)}