Back to the universe’s prehistory
‘Poor stars’ in the Milky Way
Most research confirms that which other research had already concluded. But occasionally, a discovery can turn existing theories on their head. Compare it, for instance, to the statement that ravens are black: the more black ravens are found, the more likely it is that they are indeed black. But when someone finds a white raven, that single discovery is enough to change all insights into the birds.
In 2022, UG astronomer Else Starkenburg and her colleagues found one such white raven. It appeared in the form of a group of extremely old stars, which hadn’t joined the Milky Way until later.
A group of researchers in Strasbourg, who were studying the way stars moved around the centre of the Milky Way, noticed them. Not all stars move around this area the same way. ‘Groups that joined the Milky Way later on still move roughly in the direction of their original orbit’, Starkenburg explains.
The same unusual movement in different stars means that these stars weren’t always part of the Milky Way. That includes this group of stars, which was dubbed C19.
But when Starkenburg and her team added their knowledge to that of the original researchers, they quickly realised something else was going on.
Few heavy elements
Starkenburg studies the amount of heavy elements in stars’ atmospheres. To measure this, she uses a special telescopic filter, which enables her to see which elements and how much of each are present in the stars.
The exact age of these extremely old stars is very hard to determine
‘We figured we’d combine the two things, since on one side we have this group of stars that travel together and on the other, our team has a lot of information on what this group of stars consists of’, she says.
During her research, she found out that each of the stars in the C19 stream was extremely low in heavy metals; they were metal-poor stars. ‘We immediately realised that we’d found something unique.’
When a star dies and forms a supernova, this creates many heavier elements. The new stars that are created during this process will contain those same elements. That means metal-poor stars must be remnants from the universe’s very beginning, when very few stars had died yet.
Globular cluster
The astronomers wanted to know where these ancient stars in C19 were from. ‘We’d been arguing among ourselves about whether it was a ripped-apart globular or a ripped-apart dwarf galaxy’, says Starkenburg.
Just like regular archaeologists, we look for ancient objects
She had been convinced from the start that it was a globular: a spheroid group of stars that aren’t part of galaxies like the Milky Way, but are formed from gas clouds. If these gases get mixed really well, this leads to a cluster of stars that all look similar, are the same age, and are close together.
‘We think they’re formed when a large gas cloud undergoes an intense star formation event, after which the globular stays intact for a very long time’, Starkenburg explains. That also applied to this particular group, until it got too close to the Milky Way and was ripped apart by our galaxy’s gravity.
But the others didn’t believe that it was possible at all. It was impossible for globulars to form inside a gas cloud with so few heavy elements, at least not at this scale.
Prehistoric relic
However, Starkenburg turned out to be right, and it was a spectacular discovery. While a few hundred stars with even fewer heavy metals had previously been discovered, none of them were part of a globular cluster – at least as far as people were aware.
Unfortunately for now, the researchers can only estimate when this particular globular was formed. ‘The exact age of these extremely old stars is very hard to determine’, says Starkenburg.
How can something that’s existed for so long now be ripped apart?
It is clear, however, that globular cluster C19 is a relic from the universe’s prehistory, making it extremely interesting to a galactic archaeologist like Starkenburg. ‘Just like regular archaeologists, we look for ancient objects that can tell us something about what things used to look like’, she says. ‘But rather than study pyramids, arrowheads, or fossils, we study stars.’
In order to study objects from that particular era of the universe, astronomers usually study galaxies with high redshift values. This is a measure of distance within the field of astronomy that indicates how far away stars are. The light that reaches us from those galaxies today started when the universe was young.
Starkenburg wants to compare C19’s data to these ancient far-away objects in an effort to study the universe’s history. The stars in C19 are potentially just as old as other objects, but are much closer. ‘We can now compare the observations of redshift stars with these close-up observations that provide much more detail.’ In other words, the C19 stars provide insight from an entirely different perspective.
Smaller galaxy
In the meantime, others are also working on the star stream. Starkenburg’s PhD student Akshara Viswanathan, for instance, recently found a star that likely belongs to C19 as well – quite far away from the cluster. It’s entirely possible more stars will be found, which will reveal more secrets of the globular cluster.
There’s another interesting thing: ‘We’re working hard on trying to determine the cluster’s orbit. While C19 is still considered a stream, it is actually falling apart. How is it possible for something that’s so old, that has existed for so long, to now be ripped apart?’
One of the likeliest theories is that the globular cluster was part of a smaller galaxy and that together, they were drawn in by the Milky Way. But a globular of that size in such a small galaxy would be particularly interesting, and makes Starkenburg wonder if other globular clusters were part of small galaxies, and whether they might be able to find similar remnants.
In any case, there is plenty of research still to be done, since the outer edges of the Milky Way are likely littered with tiny galaxies that were drawn in by its gravity. Since no one thought one of these galaxies could ever bring along a globular cluster like C19, so who knows how many more treasures will be uncovered during this galactic dig?