research highlight: Simulating the complicated lives of stars, from birth until death

​Scientists from the ARC Centre of Excellence for Gravitational Wave Discovery and the University of Cologne (Germany) have developed new simulations of stars’ complicated lives, boosting research on how new stars are born and how old stars die.
 
These stellar evolution simulations, called the BoOST project, can be used to predict how often gravitational waves should be detected—gravitational waves (ripples in space-time) are expected to happen when the death throes of two stars merge. The project can also help to study the birth of new stars out of dense clouds in space.
 
Not all stars are the same. Sure, they all look like tiny, shining points on the sky, but it's only because they are all so far away from us. We only see stars that are close and bright enough. The rest, we may see with telescopes.
 
If you use a telescope to measure the colour of a star, it turns out that some stars are rather red, some are blue, and some are in between. And if you measure their brightness, it turns out that some are brighter than others. This is because a star’s colour and brightness depend on its heaviness and age, among other things. It's a complex theory that has been developing since the age of the first computer simulations in the 1950's.

The UNIVAC computer gave a boost to stellar evolution research in the 1950’s. Image credit: Mario Sturny

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Today, we have computer simulations that can predict how a star lives its complicated life, from birth until death. This is called 'stellar evolution' and applies to the stars that are close enough for us to observe with telescopes.
 
But there are stars so far away that even the largest telescopes can’t view them clearly; there are stars hiding inside thick clouds (yes, such clouds exist in space); and there are dead and dying stars that used to exist once upon a time. Is there a way to study these unreachable stars to observe similarities and differences from those that we can actually see?
 
Stellar evolution simulations can help here because we can simulate any star—even the stars we can’t see. For example, stars that were born soon after the Big Bang used to have a different chemical composition than those stars that we see today. From computer simulations, we can figure out how these early stars looked like: their colour, brightness etc.
 
What's more, we can even predict what happens to them after they die. Some of them become black holes, for example, and we can tell the mass of this black hole based on how heavy the star had been before it exploded.
 
And this presents more opportunity for discovery! For example, it’s possible to predict how often two black holes merge. This gives us statistics about how many times we can expect to detect gravitational waves from various cosmic epochs. Or, when trying to understand how stars are born out of dense clouds, we can count the number of hot bright stars and the number of exploding stars around these cloudy regions. Both hot bright stars and explosions change the clouds' structure and influence the birth of new stars in delicate ways.

The BoOST project predicts how stars live their lives. These diagnostic diagrams show stellar evolution simulations of massive and very massive stars (colourful labels in solar mass units). These are stellar lives in the Milky Way (left), in the Small Magellanic Cloud (middle) and in a metal-poor dwarf galaxy (right). One line on these diagrams belongs to one star’s whole life from birth to death. Their brightness is shown to change on the vertical axis, and their apparent ‘colour’ (surface temperature, with lower values meaning red and higher, blue) on the horizontal axis. These simulations can give a boost to research on how new stars are born and how old stars die.

Lead scientist on the study Dorottya Szécsi from the University of Cologne says: ‘Much like the theory of stellar life got a boost in the 1950's from computerization, we hope our BoOST project will contribute to other research fields, because both the birth of new stars and the ultimate fate of old stars depend on how stars live their complicated and very interesting lives’.
 
“Given the importance of massive stars in astrophysics, from determining star formation rates to the production of compact remnants, it is essential that our theoretical models of stars keep pace with advancements in observations,” says OzGrav postdoctoral researcher and study co-author Poojan Agrawal.

 
Link to paper: https://www.aanda.org/articles/aa/full_html/2022/02/aa41536-21/aa41536-21.html
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