A recent study explores how the acoustic vibrations of stars in the open cluster M67, located 2,700 ly from Earth in the Milky Way, can reveal crucial information about their internal structure and evolution.
This study, led by UNSW Sydney researchers, focuses on 27 stars with similar ages and compositions to our Sun, but in different evolutionary stages.
Let's dig deeper.
Stars, like the Sun, vibrate because of sound waves generated within them. These vibrations, called 'acoustic modes', can be detected by observing small variations in their brightness. By analysing these oscillations in the 27 stars of M67, the researchers studied two types of differences in the frequencies of the vibrations: 'large separations' (they indicate the distance between the main frequencies and reflect the overall density of the star) and 'small separations' (these are finer differences between nearby frequencies and usually reflect conditions in the core). As stars evolve from subgiants to red giants, they develop a deeper outer convective zone, a layer where stellar material mixes with motions similar to boiling water.
The study found that as these stars age and become red giants, the patterns of 'small separations' deviate from what was expected. This deviation is due to the influence of the lower boundary of the convective zone, which deepens over time as the stars evolve. In effect, acoustic vibrations allow researchers to “see” how the internal structure of stars changes over time.
This work is significant because it offers a new key to understanding stellar evolution. Traditionally, 'small separations' were thought to lose importance in stars with inert cores (such as red giants), but this study shows that they can still provide valuable information, related to the convective zone. Furthermore, the observation of a “plateau” in the vibration patterns represents a novel clue, which could become a new tool to measure the age and evolutionary state of stars. Since M67 contains stars similar to the Sun, these results also help researchers better understand the past and future of our Sun.
On a practical level, this research refines the methods of asteroseismology, a powerful tool that uses stellar vibrations to study their interior, much like seismologists use earthquakes to explore the interior of the Earth. Improving researcher's ability to interpret these vibrations means they can more precisely determine the age and composition of stars in other clusters or galaxies. This has implications for mapping the universe, understanding star formation, and even finding planetary systems, since the age of a star affects the habitability of its planets.
In essence, this research not only expands our theoretical understanding of stellar evolution, but also offers practical tools for future astronomical observations, making asteroseismology an increasingly powerful means for exploring the cosmos.
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Image: The M67 open cluster contains a population of giant, evolved stars.
Credit: Sloan Digital Sky Survey | CC BY 4.0
References
UNSW Sydney Press Release
Scientific Paper: 'Acoustic modes in M67 cluster stars trace deepening convective envelopes'
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NOTE: I report a couple of questions to which I have provided answers that may be of interest to someone.
Q: "Stars, like the Sun, vibrate because of sound waves generated inside them."
What are these sound waves generated by?
A: Sound waves inside stars, such as the Sun, are generated primarily by turbulent movements of plasma in the convective zone, a region beneath the stellar surface where heat is transported outward through the movement of matter. In the case of the Sun, the convection zone is located just below the photosphere, the visible layer.
The turbulent movements are caused by thermal energy produced in the core of the star through nuclear fusion. In the core, hydrogen is transformed into helium, releasing enormous amounts of energy in the form of radiation and particles. This energy heats the surrounding plasma, creating temperature gradients that lead to instabilities: hot plasma rises to the surface, while the colder, denser plasma sinks inward. The convection process generates oscillations and turbulence that produce sound waves.
The waves propagate within the star, bouncing and interfering with each other, and can be studied using helioseismology (in the case of the Sun) or asteroseismology (for other stars). The resulting vibrations provide valuable information about the internal structure of stars, such as density, temperature and chemical composition.
In practice, stars "ring" like cosmic bells, and we can "listen" to them by analyzing variations in light or movement on their surfaces!
Q: What exactly is the plateau that the research talks about and that you mentioned in your post?
A: The “plateau” refers to a phenomenon observed in the patterns of the frequencies of acoustic vibrations (or acoustic modes) of the stars in the M67 cluster, in particular in the so-called “small separations”. To understand it clearly, we need to take a step back and understand what these separations are and why the plateau is an important discovery.
Stars vibrate due to internal sound waves, and these vibrations produce a series of frequencies that scientists can measure. Between these frequencies, there are regular differences:
- Large separations indicate the distance between the main frequencies and reflect the overall density of the star.
- Small separations are finer differences between nearby frequencies and, in stars like the Sun, are related to the conditions of the core (for example, how dense or rich in helium it is).
As a star evolves from a subgiant to a red giant, its core contracts and the outer convective zone (the layer where the material mixes) deepens. Traditionally, it was thought that in red giants, small separations become meaningless, because the core becomes less influential than the outer layers.
The “plateau” is an unexpected behavior observed in these small separations in the stars of M67. Instead of continuously decreasing or disappearing altogether as the star evolves, the small separations stabilize at a constant value for some time, forming a sort of “plateau” in the data. This happens because the lower boundary of the convective zone, which moves inward as the star ages, begins to influence the acoustic vibrations in a new and measurable way.
In simple terms, it is as if the vibrations “feel” this deepening boundary and, at a certain point, produce a stable signal instead of constantly changing.
The plateau is a scientific surprise because it reveals that small separations are not only an indicator of the core, but can also tell us something about the structure of the outer layers of stars, in particular the convective zone. This makes it a new “hallmark” of stellar evolution.
Scientifically, it suggests that theoretical models of stellar evolution need to be updated to account for this effect.
Practically, it offers scientists a new tool to estimate the age and evolutionary state of stars: by measuring the plateau, they can understand how deep the convective zone is and therefore where the star is in its life.
