A new study led by UNSW Sydney researchers into a cluster of stars 2700 light years away reveals their stages of evolution through the 'sounds' they make. This discovery will allow scientists to map the history of the Milky Way and other galaxies, accelerating knowledge in the field of astrophysics.
Dr Claudia Reyes is the lead author of the study published today in Nature. While undertaking her PhD at the UNSW School of Physics, she studied 27 stars in a cluster of stars called M67. The stars in this stellar cluster were all born from the same cloud of gas four billion years ago.
She says these stars have similar chemical compositions but different masses which made them ideal for studying evolution in real-time.
"When we study stars in a cluster, we can see their whole sequence of individual evolution," Dr Reyes says.
She says while these stars are the same age, it's their mass that gives away how quickly they've evolved. And, she adds, M67 is a very special cluster as it includes a broad range of 'giants', from the smaller, less evolved subgiants to red giants – the latter being the most evolved of stars.
The study also opens ways to learn more about what our own star – the Sun – will do as it becomes bigger and older. This is because, "the Sun was born in a cluster similar to the one we studied," says Dr Reyes.
What's the deal with clusters?
Observing such a broad evolutionary range of stars within a single cluster has never been achieved before at such detail.
"This is the first time we have really studied such a long range of evolutionary sequences, like we have in this cluster," says coauthor Professor Dennis Stello, also from the UNSW School of Physics.
"Verifying the age of a star is one of the most difficult things to do in astronomy, because the age of a star isn't revealed by its surface," Prof. Stello says.
"It is what happens inside that shows how old it is."
Because the stars in the M67 cluster are of a similar age and composition to our Sun, they can offer insights into our solar system's past and formation, as well as its future as the Sun evolves.
"Almost all stars are initially formed in clusters," Prof. Stello says. "They are basically big families of hundreds to thousands of stars born from one big cloud of gas.
"Usually, they would slowly disperse into a diffuse random selection of stars.
"But some of them are still within groups – clusters. You can see them when you look to the sky as areas with lots of stars close together, where they are still closely bound, like the cluster we studied here."
A symphony in the sky
The study allows for the precise determination of a star's age and mass based on its oscillation frequencies. The frequencies by which any star 'rings' depends on the physical properties of the matter inside of it, giving clues about its density, temperature and age.
This was the first time researchers could interrogate the 'ringing' across a cluster of stars to learn more about their interiors. They used the Kepler K2 mission as the primary way to observe, or 'listen'.
Prof. Stello says the process is like listening to an orchestra, and identifying instruments based on their sound.
"The frequency by which an instrument is vibrating – or ringing – depends on the physical properties of the matter that the sound travels through," he says.
"Stars are the same. You can 'hear' a star based on how it rings.
"We can see the vibration – or the effect of the vibration – of the sound just like you can see the vibration of a violin string."
The biggest stars have the deepest sounds. Small stars have high-pitched sounds. And no one star plays just the one note at once – each star covers a symphony of sound coming from its interior.
In space, no one can hear you scream (or sing)
Sound exists as a wave of energy, a vibration, moving through particles – solid, liquid or gas. But in space, there are no particles, which means there's no sound.
Prof. Stello says each star is like a breathing ball of gas – cooling down and heating up – with slight changes in brightness.
"It's these fluctuations in brightness that we watched and measured, to gauge the sound frequencies," he says.
As stars mature towards red giants, their frequencies change and behave differently. These changes can track their evolution.
The frequency differences between the many nodes 'played' by a star can give clues about a star's interior properties.
By studying the 27 stars in the M67 open cluster, the researchers could, for the first time, observe the relationship between small and large frequency differences in giant stars, which can now be applied to individual stars.
Understanding the Milky Way
To better understand the formation and evolution of galaxies, scientists need to know the age of all its components, including the stars.
Dr Reyes says this study will lead to the accurate identification of the mass and age of stars in the Milky Way – something yet to be achieved.
This is also important for understanding stars that host planets, as a star's properties are critical for supporting life on those worlds.
Prof. Stello says frequency signatures will also be important when modelling the future evolution of our own Sun.
"This study has enabled us to probe the fundamental physics that happens inside stars, deep into their interiors, and the fundamental physics under these extreme conditions," he says.
"This is something we still grapple with. It's important for us to build evolution models that we can trust, so that we can predict what happens not only to the Sun, but also to other stars as they grow older.
"Seeing the evolutionary phase of stars directly through the fingerprint of frequencies is what enables us to be much more certain about the 'ingredients' we put into our models," he says.
What's in the future?
Dr Reyes says their findings were unexpected.
"We discovered something new with this signature in the frequencies," she says.
Dr Reyes says we already have data from many years of studying the Milky Way and its stars.
"The next step is to go back and look at that data," she says. "To look for these particular frequencies that nobody thought to look for before.
"And we can do this by listening to the stars."
