Issue 16 / June 2021
HE 1327-2326 mon amour
Anna Frebel in an interview with Stefan Klein
Astronomer Anna Frebel likes small lights and has little preference for the familiar. Ideal conditions for plunging into space, even if only via telescope and theory
Stefan Klein: When did you first become interested in stars?
Anna Frebel: Most definitely before I was ten. Back then I was watching Star Trek and wanted to be Captain Picard. But then I saw a documentary film where the cosmonauts in Kazakhstan were placed in a centrifuge so that they could accustom themselves to the acceleration that would take place during a rocket launch. It was at that moment when I realized that I was not suitable for being an astronaut. I was nevertheless still drawn to outer space. And what’s the next best thing? Looking at stars of course.
SK: Do you have a favorite star?
AF: When you’ve worked with stars for a long time, you develop real affection for them because you know them so well and know more about them than anyone else. My favorite star is HE 1327-2326. I discovered it in 2005. And we’re still finding out new things about it.
SK: Where exactly is HE 1327-2326? And what does it look like?
AF: In the Hydra constellation, south of the celestial equator, at about 5000 light years’ distance from us. For us astronomers it’s almost right next door. You can spot it with an amateur telescope. You might notice a slight bluish hue. But stars are actually always boring to look at. They are such tiny lights, after all, but I like tiny lights.
SK: You have an emotional connection with your research object.
AF: A little bit. HE 1327-2326 makes me sentimental because it reminds me of the good old days. When I discovered it, I was a graduate student in Australia. Those were exciting and inspiring years. I met great people, and discovered things that suddenly the whole world wanted to know about. I worked with an older telescope in the middle of an enormous national park with countless kangaroos. One time, during my longest observing run, we were threathened by a brushfire. Fortunately the fire eventually bypassed the observatory.
SK: You found HE 1327-2326 when you were searching for stars that have a particularly low iron content for your PhD thesis. Why were you looking for such stars?
AF: Because such stars are very old and tell us a lot about the formation of the universe. After all, the elements are formed by nuclear fusion within stars. After the Big Bang, the universe only consisted of hydrogen, helium and traces of lithium. Over time, increasing amounts of the heavier elements were added. The less iron a star contains, the more primitive it must be. And when we discovered HE 1327-2326, it was the most iron-poor star known. Incidentally, I almost threw out the piece of paper with a bunch of star names on it, including HE 1327-2326, as it had already been on my list of misclassified objects. Luckily, I decided, just to be on the safe side, to ask my supervisor again. I became quite aware of just how much experience I was still lacking.
SK: You were 25 years old. What exactly led to your becoming interested in the oldest stars?
AF: Chemistry, nuclear physics and stellar astronomy had always fascinated me as a teenager. Then when I was sixteen I did an internship at the astronomical observatory in Basel. At that time, the age of the universe was still only loosely determined. It was believed to be around 20 billion years old, and people were arguing about the value of the Hubble constant.
SK: That’s the speed with which the universe has been expanding ever since the Big Bang.
AF: The constant of proportionality of this expansion, to be precise. We can use it to calculate the age of the universe. There were two schools of thought and it was war to the knife. One camp claimed that the Hubble constant was 55, whereas the other camp made a case for it being 100. Today we know that the true value lies exactly in the middle. The leader of one of these camps just happened to be the director of the observatory in Basel, so I gathered something of the history of this rivalry without quite understanding its significance, which had been a central question in cosmology.
SK: It seems to me that this is one of the most important and at the same time most underestimated insights of the twentieth century: the universe is not eternal but instead it has a history. And if you wish to understand it then you have to understand not only the laws of nature but this history as well.
AF: Indeed. In 1929, the American Edwin Hubble discovered that the farther away the galaxies are from us, the faster they continue to move away from us. This revolution in cosmology was unstoppable. Two years earlier the Belgian astronomer and priest Georges Lemaître had already theorized this expansion, hence there was a beginning when everything was concentrated in one single point.
SK: Today we call this beginning the Big Bang. Albert Einstein didn’t want to believe it. He wrote to Lemaître that, “Their calculations are correct, but their physics are atrocious.”
AF: Einstein wanted a static universe. That’s why he added a constant to his equations for the theory of relativity. Ordinarily, these equations are describing the expansion of the universe but the addition of Einstein’s “cosmological constant” resulted in the expanding universe being pulled back like being held back by a rubber band. So he tweaked his mathematics in order for the universe to conform with his ideas.
SK: Einstein later regretted his opinion. He called it “the biggest blunder of my life.”
AF: Yes. In the 1950s and 1960s the first cosmological models describing an evolving universe were introduced.
SK: Nevertheless, evolution of the universe remained a contested subject. In the 1960s the important British astronomer Fred Hoyle was still making fun of the “Big Bang” – his own coinage for the controversial theory of the beginning of time. He himself was convinced that the universe was static and thus eternal.
AF: But in a groundbreaking way it was Hoyle, together with three colleagues, who explained the origin of the elements which can only be the result of a universe that also has a history!
SK: It was only in the year 2003 that discussion about the age of the universe suddenly found its end. A satellite had measured the cosmic background radiation with great precision – the afterglow of the Big Bang that still fills outer space.
AF: I had just come to Australia at that time. I still remember the conference where we were informed that the Big Bang occurred 13.7 billion years ago.
SK: Immediately after the Big Bang the universe was very simple. Actually there was only radiation – and the laws of nature.
AF: The enormous temperatures determined everything. If you understand the physics of all this, which is difficult enough, then what happened can be well understood. It even has a certain elegance to it.
SK: The universe cooled down, and after about 30,000 years the first lightweight atoms formed.
AF: The first stars formed from these atoms that drifted as gas through cosmic space. But that happened only some 300 million years after the Big Bang. We still have a long way to go before we gain a full understanding of just how this all exactly occurred.
SK: Doesn’t it seem strange to you that we know more about the first beginnings of the world than we do about the birth of the celestial bodies that surround us and the elements that we carry in our bodies and our planet?
AF: The formation of the first structures led to an incredible amount of feedback processes, and they make life difficult for us researchers. Depending on the mass of a star, different elements are made. When a star finally explodes as a supernova, these elements are hurled into space and they influence the formation of the next generation of stars. At the same time streams of gas came together to form early galaxies; those then went to consume neighboring gas clouds and smaller galaxies to grow over time. The universe is a strange place. . .
SK: . . . and to which we ultimately owe our existence.
AF: Colleagues who have simulated these processes with supercomputers have been able to make wonderful progress in recent years.
SK: Can you actually imagine these processes?
AF: In my mind, I see the universe in both 3D and in color. I find it difficult to describe. Sometimes I feel like I’m in a dollhouse and looking at all the rooms. Then I give some thought to where a bed might fit and just how the kitchen would have to be designed so that everything comes together nicely. We scientists are always looking for what is missing.
SK: And do you walk around in the dollhouse? After all, the universe was already almost 10 million light years in diameter when the first stars shone forth.
AF: The dimensions only mean that it takes a long time to go from one end of it to the other. If you can travel as fast as you want then these dimensions no longer form any obstacle.
SK: Unfortunately, the theory of relativity forbids any movement that exceeds the speed of light.
AF: In my imagination, I don’t have to abide by it. My mother used to always say that she would get dizzy whenever I told her about light years. She simply couldn’t imagine it. But it has to be said that my mother dislikes travel. She likes things to be familiar. I’m bored with familiar things.
SK: In any event, at some certain point the dollhouse was all ready for us to move in . . .
AF: Yes. When there was enough carbon present in the universe to form planets and provide means for terrestrial life to develop.
SK: To a very large extent our bodies are made from carbon. Apart from oxygen, carbon is the most common element in every organism, without carbon there is no metabolism.
AF: First of all, carbon had to get synthesized, and it was created in an astonishing way. In stars, two helium atoms come together to form beryllium-8, and this then fuses with another helium atom to form carbon. But because beryllium-8 is extremely short-lived, the process works only because of very delicate resonances between the carbon, helium and beryllium nuclei.
SK: One can imagine these resonances as in a musical instrument. If a violin consisted solely of its strings then it would barely be audible. It is only the resonating body that enables the vibrations to build up for an enormous amplification. For this to occur, however, the resonating body must be precisely tuned to the pitch levels of the strings.
AF: Carbon synthesis occurs in exactly the same way. Coincidentally, the energy levels of the atomic nuclei of beryllium-8 and helium, which fuse with one another, correspond precisely to that of carbon. Without this resonance the process would be much too improbable to produce any appreciable amounts of carbon. Fred Hoyle, who scoffed at the notion of a Big Bang, was also the first to recognize just how delicate this process is – and laboratory experiments later confirmed it.
SK: Do you believe in coincidence? Hoyle doubted it. He speculated that, “A higher intellect must have designed the properties of the carbon atom. If only the blind forces of nature were involved then the chances of finding such an atom would be minimal.”
AF: The interplay of the laws of nature is wonderful. It’s also why I’m a scientist. Even if we still don’t understand a lot of things yet, I’m still confident that everything can be described and explained – even if a correlation would seem extremely complex or unbelievable at this time. We live in a universe where there is carbon, so we can give some thought to it. In another universe, if there is such a universe, everything would surely be different. In this respect it may instead be a coincidence that we are presently talking in this very universe where the carbon atoms have built up exactly as they did.
SK: But the question remains as to whether the laws of nature can solely explain the formation of carbon. On top of that, certain conditions had to exist for the reaction to get underway in the first place.
AF: I would say that there was an interplay between history and the laws of nature. It is only at temperatures above 10 million degrees that carbon synthesis ignites in a star’s interior. That’s more than six times the core temperature of our sun. Had there not been such high temperatures in the first stars when they formed about 300 million years after the Big Bang, then you and I would not be sitting here now for the lack of carbon.
SK: Perhaps life would have emerged on the basis of silicon.
AF: I’m afraid it wouldn’t have gotten that far at all. Carbon is not only the basis for life but it was also crucial for the development of the cosmos. The firstborn stars consisted solely of hydrogen and helium. They were quite massive, twenty to a hundred times heavier than the sun, and they burned very brightly. But after only a few millions of years they exploded as supernovas. No complex structures, no galaxies, no planetary systems could form in such a short period of time. If there had only been these stars then the universe would have remained a rather boring place. And yet we owe them everything. They incubated the first carbon and other heavy elements and flung it all out into space in their explosions.
SK: So we’re made of the ashes of these earliest stars?
AF: Not only us. The enriched gases that entered space following these explosions contracted again through their gravitation. New stars ignited from these gas clouds. And carbon is an ideal coolant which helps the collapse of gas. It enabled the formation of stars similar to the sun, which burned much more slowly. Most of these second-generation stars still shine today.
SK: Like your favorite star HE 1327-2326.
AF: It has likely been in the sky for 13 billion years. In recent years we have also found stars that are even slightly older.
SK: HE 1327-2326 and its friends are almost three times as old as the sun. Is it actually conceivable that planetary systems and therefore perhaps even life itself developed around those stars at the time?
AF: Hardly. There were still too few amounts of all the heavy elements present. For instance, the iron abundance in HE 1327-2326 is a hundred thousand times smaller than in the sun. In such an environment, at most, primitive gas giants, perhaps like Jupiter, could form, although probably not even these.
SK: Too bad.
AF: And light stars like HE 1327-2326 contribute hardly anything to enrichment of outer space with metals. Astronomers designate all elements that are heavier than hydrogen and helium as “metals,” including carbon. Besides the light-weight, long-lived stars like HE 1327-2326, massive stars continued to be born, which always burned out quickly, exploded and again released elements after their deaths. Several years ago, we finally found strong evidence for how heavier elements are formed, up to and including uranium. I was nursing my son when I got a call at two o’clock in the morning from a doctoral student who was sitting at a telescope in Chile. He had found a small galaxy full of stars and all of them had large amounts of very heavy elements. We were able to show that these observed heavy elements had to be the product of a collision between two so-called neutron stars, which must have occurred before the birth of our stars. A year later astronomers succeeded in measuring the existence of such collisions of neutron stars by means of the gravitational waves they emit. It was in this way that we solved a mystery which had puzzled astronomers for more than sixty years.
SK: Life was therefore only possible when conditions settled down. What happened then?
AF: That depends on the local conditions. As the universe expanded after the Big Bang, matter that had initially been dispersed began to locally contract under its own gravity to form the first galaxies. Then the larger galaxies gobbled up the smaller ones to grow further. Matter in outer space behaves just like money that goes where money already is. The rest have to fend for themselves. When galaxies collide, huge amounts of gas are set free from which many new stars form in a short period of time. This is still happening in many places in the universe.
SK: Here in the Milky Way . . .
AF: . . . not much has happened for at least 8 billion years. Elsewhere in the universe there are galaxies which are still colliding.
SK: So the old stars tell not only about formation of the elements but the history of their particular galaxy.
AF: That’s right. Dwarf galaxies are especially interesting. These are small collections of only a few thousand stars, but unlike globular clusters they are embedded in their own “dark-matter halo.”
SK: Dark matter is something like a cosmic glue that holds stars together through its gravity. Nobody knows what exactly it’s made of.
AF: And every dwarf galaxy has its own glue, if you will. In the Milky Way and its surroundings, we know of some fifty dwarf galaxies. They’re hard to find because they’re so faint. These dwarf galaxies are comprised of very metal-poor stars, meaning that they are either very old or very primitive. We are probably talking about systems from the early days of the universe that survived because they were lucky enough that the much larger Milky Way has not yet ripped them apart.
SK: Are you assuming that these dwarfs ceased further evolution and growth at an early stage of their development? In that case one could then use them to study the beginning of galaxy formation.
AF: Exactly. Presumably these systems simply had too little mass to attract gas or swallow up other galaxies. But they themselves were captured by the Milky Way to now be hanging out at a safe distance from it, similar to a dog on a leash, and pending further notice. We have just published a study on what happens when two dwarf galaxies coalesce – then a fireworks of new stars ignites. Conversely, if a dwarf galaxy contains only old metal-poor stars, then it necessarily must be very primitive.
SK: Could the entire Milky Way have once looked like today’s dwarf galaxies but have evolved differently because it had more mass to it ever since the very beginning? Or did the Milky Way take form through a combination of many dwarf galaxies?
AF: Both things happened. The universe is hierarchical in its structure. Gravitation causes large systems to become even larger. The Milky Way began life as a dwarf galaxy.
SK: Perhaps we should actually be glad that so much destruction occurs in the universe. Without the constant collisions and explosions, then structures would never have taken shape and there would have been no life.
AF: Correct. But I would formulate things a little bit differently. We are living in an extraordinary phase. A few billion years ago the universe was not yet sufficiently developed to allow for human existence. At that time the cosmos was not especially exciting, but with all the collisions and unstable galaxies it was also more turbulent. Today, at least in our location in the universe, the conditions are stable and the expansion of space, which is constantly accelerating, is still of a moderate nature. There will come a time when space will have expanded to a point that we won’t see a single galaxy in the heavens anymore – that would be rather tragic for astronomers. Except, of course, for the Andromeda Nebula. That’s so close to us that its gravity binds it to the Milky Way forever.
SK: By the time that this great void finally occurs, the sun will have burned itself out.
AF: Or the Andromeda Galaxy will have crashed into the Milky Way. We can expect this catastrophe to occur in the next 4 to 6 billion years.
SK: Does the thought of the cosmos being so transient worry you?
AF: We still have a bit of time till then. But yes, the cycle of destruction and new creation will eventually come to an end. When outer space has expanded far enough then no more structures can form. I do find it somewhat sad that everything will be over at some point.
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Images: © Maurice Weiss