07/12/2024 | Topic, English

The mystery of the young stars

With the James Webb Space Telescope and other high-precision instruments, scientists are exploring regions of space both far and near with unparalleled precision. Star clusters are turning out to be surprisingly young and previously overlooked black holes are coming to light.

by Jan Voelkel

You have aged well if you’re still good for surprises in old age. This certainly applies to our universe, which is 14 billion years old, and to our galaxy, which is around 13 billion years old. In a recent study, researchers found something unexpected when they took a closer look at a cluster of stars called IRS 13. The region is in the immediate vicinity of Sagittarius A*, the supermassive black hole at the centre of our galaxy.

»Immediate« should be understood in astronomical terms: The star cluster is located 0.1 light years away – a distance that would still require travelling from one end of our solar system to the other twenty times. The researchers had expected the stars in IRS 13 to be arranged randomly. Instead, they move in an unexpectedly orderly pattern. Two conclusions can be drawn from this regular pattern: »On the one hand, IRS 13 appears to interact with the black hole,« explained Dr Florian Peißker, astrophysicist and first author of the new study. »Secondly, there must be something inside the star cluster for it to be able to maintain its compact shape.« In order to get to the bottom of this question, the researchers chose a rather complex approach: They analysed data from the last twenty years in a variety of wavelength ranges and from different telescopes, combining them with an extensive theoretical analysis in order to categorize everything. This is how they came to their conclusion that it can only be an intermediate-mass black hole.

Defying radiation

Although IRS 13 was discovered over two decades ago, only now has it been possible to determine the individual stars in detail and to discover the black hole. Such discoveries are made possible by the technological development of instruments and telescopes such as the James Webb Space Telescope. This telescope – a joint project of the American, European and Canadian space agencies – can look deeper into space than any previous instrument.

Eventually, a coherent picture emerged. The researchers observed that the young stars and the star cluster interact with Sagittarius A* and yet remain densely packed within the star cluster. This means that Sagittarius A* is ›pulling‹ IRS 13, while the intermediate-mass black hole is pulling the star cluster in the opposite direction, thus preserving its shape.

They also found out that the stars in IRS 13 are several 100,000 years old, making them exceptionally young by stellar standards. Due to the high-energy radiation, it should not be possible for such a large number of young stars to be in the direct vicinity of the supermassive black hole Sagittarius A*. »From a certain distance, IRS 13 was captured by the gravity of the central black hole,« explained Peißker.

During this process, the dust surrounding the star cluster may have created a bow shock at its tip – similar to the tip of a ship in water. This led to a condensation of dust, stimulating further star formation. This explains why these young stars are mainly found at the tip of the cluster. »Our analyses are the first attempt to unravel a decades-old mystery about the unexpectedly young stars in the galactic centre,« said Peißker.

Time travelling with a telescope

The galactic centre, observed with the Very Large Telescope and the ALMA Observatory in the Chilean Atacama Desert. The region contains more than 10,000 stars.

The new James Webb Space Telescope not only produces precise images of regions close to us where new stars are forming, it also allows scientists to look back farther than ever before to the origins of the universe. Peißker and other scientists at the University of Cologne’s Institute of Astrophysics were involved in the development of certain instruments for the telescope. They built the parts of the detector for observation in the infrared spectrum. »The universe has been expanding since the Big Bang. The older the stellar objects, the farther away they are. The expansion also changes the wavelength of light or elements such as hydrogen. »To measure this, we look at the infrared range,« he explained.

The phenomenon is comparable to the so-called Doppler effect of an ambulance: The sound of the siren changes as it passes us and moves away. This is also the case with the spectral lines produced by objects in space. The emission line of hydrogen becomes increasingly red the longer it travels, right into the infrared range. This is what researchers are searching for to decipher how the early universe developed.

Looking back as far as possible to where it all began – the Big Bang – leads to exciting insights, but also raises new questions. For a long time, scientists assumed that the first galaxies could only form over a period of around 250 to 300 million years after the Big Bang. However, the latest observations from the James Webb Space Telescope show that a large number of very heavy and mature galaxies already existed at that time. »That’s a big mystery. We don’t know how these early galaxies formed,« said Peißker. So-called quasars stand out in particular. They are among the brightest objects in the universe. When the first quasars were discovered, they were initially thought to be very bright stars. However, this proved to be wrong due to their spectral lines. Instead, they were found to be the massive nuclei of galaxies. This is how quasars got their name: quasi-stellar objects.

»We are sitting in a galaxy that is a bit boring compared to the galaxies with quasars at their centre,« said Peißker. In the course of their research, he and his colleagues discovered a particularly massive quasar. For comparison: the black hole at the centre of our galaxy weighs around four million solar masses. The galaxy the scientists now found has three giga-solar masses – i.e. three times ten to the power of nine. »That’s a completely different dimension. The question is: How is it possible that such an object could have formed so soon after the Big Bang – around 700 to 800 million years later?« In principle, collisions and mergers could produce such objects. However, the original objects would then also have to be very massive. »So we can’t get around the problem: Some of the things we are finding now contradict current theory. That’s a big deal in science,« said the astrophysicist.

The precision and high sensitivity of the James Webb Space Telescope make new, unique observations possible.

The Lego universe

At the moment, more and more questions are arising and the answers cannot be expected soon. But this is precisely why Peißker believes it is an excellent time for science: »I think there has rarely been an era like this in astrophysics. A leap like the one we are currently experiencing is exciting.«

It was not necessarily predictable that Peißker would turn his fascination for space into a career and make it into the most renowned research groups and co-operations. After graduating from secondary school, he completed his secondary school certificate at a vocational college and did vocational training as a plant technician. Only then did he complete his university entrance qualification and was able to study physics in Marburg before moving to Cologne for his Master’s degree. »I somehow only realized late in life, at 18 or 19, that I had this great interest in science. Due to my background, I didn’t really know that you can also turn this into a profession. I have always made progress bit by bit and have mostly been able to do what I wanted to do.« Ultimately, he obtained his doctorate and recently his habilitation.

He is now a private lecturer at the institute and conducts research on every nook and cranny of the universe: on the oldest objects in the universe as well as the stellar nurseries that the star cluster IRS 13 turned out to be, in which the hidden intermediate-mass black hole has now come to light.

The pictures in this article were taken in the university’s CAVE (Cave Automatic Virtual Environment). The CAVE is three-by-three-metre cube open on one side that can be used as a 3D projection space. This allows scientists to visualize their work in a three-dimensional virtual reality environment. The assistance of CAVE team member Daniel Wickeroths (on the left) to Florian Peißker (on the right) was instrumental in visualizing the black hole.

In addition to technological developments, data processing plays a decisive role for researchers. Data is no longer exclusive and guarded like a treasure, it is available online to the scientific community around the globe. Computing capacities allow complex data to be analysed and specific aspects to be examined in detail. In this way, old data does not become obsolete but rather turns out to be important: In combination with more recent analyses, the resulting complexity allows Peißker and his colleagues to notice aspects that were originally not even considered. »I approach this data with a different set of eyes and can analyse it all over again. This opens up so many different perspectives.«

He compares this to Lego bricks that you use to build a car, for example. Of course, you can also build something that looks like a car with just a few bricks. But the more bricks are available, the more detailed and complex the structure becomes. However, a model with a lot of Lego bricks also makes the building plan more complicated. It takes longer to arrange the individual bricks and put them together correctly because there are large and small parts, more of some, less of others.

It’s the same with Peißker and the data: »I can hardly keep up with tinkering with the data blocks, but I think that’s a good thing. I really enjoy puzzling over how everything fits together. Working at a time like this is incredibly fun. I go to bed every night thinking: ›Tomorrow we’ll continue – that’s so cool‹.« All things considered, he is certain: Despite the impressive age of the universe, it will still have one or two surprises in store for us in the near future.

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