Using TULIPS to understand how stars come into bloom
“Stars look like circles in the sky but that is not the way we usually study them.” HITS researcher Eva Laplace studies the evolution of stars with the help of computer simulations. Just as most astrophysicists, she used to visualize most of her research results with abstract, two-dimensional graphs – also called plots – displaying different lines of data, for example the inner composition of stars or any other characteristics. However, she did not find this approach very intuitive and for a time it seemed to her that she was missing out on important details when looking at all the lines and numbers. “One day I just decided to come up with a plot that looks a little different and that might help me understand the information I need to see for my work. I changed some function that enabled me to look at the star as if it was right in front of me and cut it into half – and that’s when I could finally see what I was looking for all along!” And just like that the idea of the software program TULIPS was born.
Show, don’t tell
We all know the famous proverb “a picture is worth a thousand words” and it holds some truth. “Visualization is such an important tool to convey information in an understandable way.” says Eva Laplace, who joined the HITS research group Stellar Evolution Theory (SET) in November 2021. “It is particularly important for astrophysics, because it is the field where most communication about research heavily relies on visualizing complex data.” However, common methods such as producing abstractplots are not always intuitive and can even be less inclusive: “There are not only other scientists such as students, colleagues and partners who want to understand the work, but also people outside of research.” And common methods do not offer the best insights according to the HITS researcher.
Eva Laplace completed her PhD at the University of Amsterdam from 2017-2021 on the topic of the late evolution, explosion, and afterlife of stars that transfer their outer layers to a companion star. While working on her PhD, she decided to develop a software to help her – and other scientists – get a better look at the scientific data coming from simulations of stars. The result is “TULIPS”, a software tool that allows the visualization of stars in visually appealing plots and short movies. “TULIPS” stands for “Tool for Understanding the Lives, Interiors and Physics of Stars” and shows not only the beauty of stars but also honors Eva’s home at that time – the Netherlands. For her work, Eva Laplace received the “ET Outreach Award” of the Royal Holland Society of Sciences and Humanities in November 2020.
In essence, TULIPS uses the data of astrophysical simulations such as the “Modules for Experiments in Stellar Astrophysics” (MESA) and visualizes it in a more natural way. Instead of lines it displays the data in a circle. “You visualize the properties of the star as if they were the properties of a circle.” Stars are nearly perfect spheres and look like circles in the sky, so the new approach is only logical: “Our brains are wired to look at familiar objects. We see stars as circles, hence a circular visualization makes far more sense to us”.
The code is your lab
But not only the output of the software should be easy to understand, Eva also put a lot of work and effort into making the code itself as accessible as possible. TULIPS was developed in the programming language python which offers a lot of advantages: “Python is so close to our own language that you can just tell the program what to do,” Or, in other words: “Coding is like magic!” Following this dictum, Eva explains that TULIPS mainly consists of functions, or how she likes to call them “magic spells”. “If you use the right spell in the right way, then you will get the right visualization.” In short, TULIPS can be described as a package of various spells that transform abstract data such as zeros and ones into a picture or video that resembles a star and includes all the useful information such as its size, brightness, and composition.
What seems so easy was actually a lot of work: “Writing a program takes a lot of time and effort. And writing a program in such a way that others can easily use it is particularly difficult.” However, investing into code development is very important: “The code is our lab! We need to be thorough when writing it, otherwise we might not even understand our own work a couple of months later.” Learning how to properly code is therefore getting more and more crucial for scientists.
Reach for the stars – and beyond
As mentioned before, TULIPS was always intended to be used by everyone, especially students who are just starting to learn about astronomy. Further, the software can have several more applications: “You can basically use it for anything that is in the shape of a circle such as stars and planets.” The applications can even be broader than that. After further development the software might also be used for the visualization of star clusters or supernovae. Eva already got some requests from other researchers who would like to use TULIPS for their own research. “I would love to develop TULIPS further and include many more features such as the visualization of binary stars.”
According to Eva, the way the data is visualized with TULIPS helped her detect valuable information that would otherwise have remained hidden in the data. Thanks to TULIPS she realized that there are systematic differences between the structures of stars that are single and stars that transfer their outer layers to a companion star.
Of course, there have been previous artistic representations of stars but what is new is the possibility to visualize the star based on the results of detailed computer simulations. For example, it is possible to see the exact composition of stars as predicted by stellar physics. “And that is quite different from what we usually see when checking for example online articles: Stars don’t look like onions!”
Up to now, the software has been downloaded more than 2000 times. TULIPS is easy to use and is a great way especially for new students as well as teachers to learn and teach about the fascinating properties of stars.
You can find everything about TULIPS at: https://astro-tulips.readthedocs.io/en/latest/#
Isabel Lacurie wrote (28. Apr 2022):
> [ … https://scilogs.spektrum.de/via-data/files/composition_of_stars_compared.png ]
> On the left, a typical graph used by astrophysicists to show the inner composition of a massive star. The star shown here is 10 times heavier than our Sun […]
> On the right, the exact same information is shown with TULIPS.
The TULIPS graph on the right is apparently missing the information that the star represented there is 10 times heavier than our Sun;
and also missing more specifics about how much mass of the star is contributed in particular compositions (which the left conventional graph presents).
(Instead, the TULIPS graph may contain more specifics about the composition of the star at or between certain radii; which, however, could be presented in a suitable conventional graph, too.)
Indeed, based on this figure it appears that the quantitative information is lost in the TULIPS representation. For the sake of simplicity and brevity we did not mention this in the article, but this information is retained in the TULIPS plots. The radial direction of the TULIPS plots is proportional to the square root of the mass coordinate (the X-axis on the left plot). As a result, the surface area of one given colored region / isotope is proportional to its mass inside the star. In TULIPS plots used for research, circles indicating fractions of the total stellar mass are overplotted on the TULIPS plot (e.g., 25%, 50%, 75%) to help read quantitative information (see for example https://ui.adsabs.harvard.edu/abs/2021A%26A…656A..58L/abstract). In the example figure shown here, you can see that the border of the oxygen-rich region (dark green) reaches approximately half of the outer circles, which means that the mass inside this circle is about half of the total mass of the star.
Isabel Lacurie wrote (29.04.2022, 13:44 Uhr):
> […] The radial direction of the TULIPS plots is proportional to the square root of the mass coordinate (the X-axis on the left plot).
Well, thanks for spelling out this particular convention.
> As a result, the surface area of one given colored region / isotope is proportional to its mass inside the star.
(In a conventional plot, the “area under the curve” is likewise proportional; provided, of course, that the extension of its mass coordinate axis is itself proportional to mass.)
> In TULIPS plots used for research, circles indicating fractions of the total stellar mass are overplotted on the TULIPS plot
This may further help to distinguish TULIPS plots from various other round pictures of different meaning and contents; such as these.
> […] see for example [ https://ui.adsabs.harvard.edu/abs/2021A%26A...656A..58L/abstract …]
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