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Star birth: a slow and mysterious drama

A star does not just appear in the sky overnight. Its creation takes tens of thousands of years. Twenty years ago, astronomers took a picture of a star in its birth phase. The James Webb Space Telescope (JWST) could now capture that same star in much greater detail. This does not only provide beautiful images, but also a lot of new information. ‘The star is developing relatively slowly,’ says masters student astronomy Merel Reitsma who collaborated on the study.

Just like us, stars have a beginning and an end. Only their life cycle lasts billions of years. The process of creation therefore takes a lot of time. Before a star like our sun really starts burning, we are several million years ahead.

In 2011, astronomers took an image of the star-in-the-making Herbig-Haro 211-mm (HH211) with the Very Large Telescope (VLT). ‘That was one of the most prominent images the telescope had taken at the time,’ says astronomer Ewine van Dishoeck. ‘The image of that star has therefore always been in our minds.’

A glimpse into cosmic childhood

Because Dutch astronomers helped to build one of the JWST’s four instruments, they were given free observation time with the space telescope. ‘We already knew very early on that we wanted to use this to further investigate the origin of this star.’ When the JWST images of HH211 arrived in September 2021, it exceeded all expectations. ‘ESA colleague Mark McCaughrean was in tears when he showed it to us.’

The JWST image of HH211

Master's student Merel Reitsma had the opportunity to work with these images. Together with McCaughrean and Van Dishoeck, she could do research on this ‘baby star’ thanks to a collaboration project with the European Space Research and Technology Centre (ESTEC) for talented master's students. ‘By comparing the old and new information, I was able to measure how all the structures around that star have shifted over the past 20 years,’ says Reitsma. ‘And from that, we can then determine the velocity of the material.’

Low speed means small mass

And as it turns out, the star's development speed is relatively slow. ‘This star is between ten and twenty thousand years old,’ says van Dishoeck. ‘That's very young compared to the sun which is about four and a half billion years old. This star is therefore really still developing.’ The fact that the star is changing relatively slowly also says something about its mass. ‘It is not that high yet. It is really still in formation and will probably become twice as massive as it is now.’

What the exact mass of the gas ball currently is, is difficult to say. ‘What you see in the picture is actually not the star itself. It is hidden behind a huge cloud of dust in the centre. What you do see are huge jets that the star produces by blowing away everything around it. You see that kind of jets in all young stars, but we have never seen it in such detail.’

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Function of jets still unknown

The exact function of those jets is still a unclear at the moment. ‘The star is forming so there is a lot of gas going towards it,’ says Reitsma. ‘At the same time, it is also emitting a lot of gas. Possibly that has something to do with the interaction between the forming star and the magnetic field. How that happens exactly is not clear yet.’

However, astronomers do see that the jets are moving at a tremendous speed. ‘This is not Max Verstappen driving a few hundred kilometres per hour,’ laughs Van Dishoeck, ‘this is 100 km per second.’ To the surprise of the group of astronomers who studied the object, the stream did not consist of atoms. ‘We know that the jets of older stars consist mainly of atoms. In this young star, the jets are still mainly composed of the molecules H2 and CO. That was a big surprise.’ How they are produced without the added ingredients of atoms and ions is still a mystery, van Dishoeck said.

A special poster in the physics classroom

Reitsma's findings became part of a publication in the scientific journal Nature. ‘When I started, I didn't know this was going to be something so big,’ she says. ‘It is very special that I was allowed to participate in this research as a student.’ Meanwhile, Reitsma has graduated. In September, she started working as a physics teacher in a secondary school. However, the JWST photo of HH211 does get a special place in her classroom. ‘It is a nice reminder of my short but special research career in astronomy.’

Scientific article

Read the full publication Outflows from the Youngest Stars are Mostly Molecular on the website of Nature.

Text and photo: Inge van Dijck
Image: ESA/Webb, NASA, CSA, Tom Ray (Dublin)

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