“We are all made of stardust”
LMU astrophysicist Andreas Burkert studies origins – the origin of the Universe and the origin of life. Next week, he will begin a new Research Focus at the Center for Advanced Studies (CAS) by discussing the links between these problems.
The title of the new Research Focus is “Stardust” – which evokes impressions of Hollywood. What do you mean by it?
Burkert: We humans are both residents and products of the Universe – and the material of which we are made was created by a natural process. In the beginning, immediately after the Big Bang, the expanding Universe contained only the two simplest atoms, hydrogen and helium. All the other elements that make up our planet and ourselves were forged in the interior of massive stars, and when these stars exploded at the end-point of their evolution, this material was strewn into space as stardust.
Let’s start with these stars, which have “squandered” their riches in the form of stardust. Why do stars explode?
A star is a massive sphere of hot, luminous gas. It is this heat that generates the radiation that we perceive as starlight. Gas pressure and radiation exert an outward force that is balanced by the inward gravitational force. So the star is in equilibrium and does not collapse or explode. The energy emitted comes from nuclear fusion in the center of the star, beginning with the fusion of hydrogen atoms into helium. Helium in turn serves as the building block for the production of heavier elements, such as carbon, nitrogen and oxygen. These are the basic building blocks of life, and they are generated naturally in this fusion process. As more and more of the lighter elements, the star must use progressively heavier elements as fuel to maintain its stability. Eventually, the star’s fuel is used up. When that stage is reached, massive stars do not simply collapse to form exotic black holes or neutron stars. Instead, they violently explode and eject large fractions of their outer layers – including the newly produced heavy elements which are the building blocks of planets and life – into space. What’s left behind is the spent core of the star. The Earth and everything on it, including ourselves, owes their existence to such exploding stars. We are all made of stardust. That also means, however, that every star at some point reaches the end of its active life and expires as otherwise there would not exist any stardust – and that includes our own star, the Sun.
What then happens to the ejected stardust?
Galaxies like our Milky Way are gigantic and the vast volume of space between the stars is filled with large amounts of interstellar gas. This gas is a mixture of primordial hydrogen and helium and “contaminated” with grains of stardust. Because galaxies are so large, gas and dust is very diffusely distributed. In regions however where the force of gravity is dominant, gas can be compressed to a high density, forming a so-called molecular cloud. Giant molecular clouds of up to a million solar masses are the birthplaces of new generations of stars. And around young stars, the left-over gas collects into a so-called protoplanetary disk. Then the dust grains trapped in this disk also begin to agglomerate.
What drives this compression?
Gravity. Interstellar matter collapses under the force of its own weight. But the gas does not always fall radially onto the central star. It can also have a lateral component of motion. This leads to what is called angular momentum, which is in turn responsible for the formation of a flat disk around the central star. Angular momentum is a combination of the rotation of an object and it’s radial extent and physics tells us that angular momentum has to be conserved. The smaller the radius of the object the fast it has to rotate to keep the angular momentum constant. This is the same effect as that experienced by an ice-skater who performs a pirouette: If you begin to rotate with your arms outstretched and then gradually draw your arms together your rate of rotation increases. The faster the rotation, the higher the centrifugal force becomes, which pulls you outwards. Eventually the force of gravity and the centrifugal force balance out and the rotating object traces out a stable circular path. If all circular orbits are within one plane, which is the case if the initial cloud had a well defined rotation axis, the sum of these circular orbits produces a protoplanetary disk.
Does the composition of the matter in these disks vary randomly?
The evolutionary development of stardust is a fascinating process: In the earliest phase of the evolution of the Universe, only the primordial elements, hydrogen and helium existed – there was no stardust. Then over the course of billions of years, ever more stardust collected in the interstellar gas, as a by-product of the formation, evolution and explosive extinction of stars. At some point, the concentration of dust grains in the disks that formed around later generations of stars became high enough to allow for the formation of planets. But where did the first planets develop? And in which environment did the Sun originate? Our Sun is now relatively far – 26,000 light years away – from the center of our galaxy, the Milky Way. But it probably formed in a region much closer to the galactic center, where the concentration of stardust could have been high 4.5 billion years ago – and then slowly drifted outwards. Another fascinating puzzle at present is the following: What combination of conditions is most likely giving rise to planets – and potentially to life – and where can they be found? What are the chances that life could have evolved elsewhere in the Universe? Is it in fact inevitable that life forms in our Universe? And what kinds of biomarkers can we hope to find – in the form of spectroscopic signatures in the atmospheres of exoplanets – that could be unambiguously interpreted as signs of extraterrestrial life? One possibility is ozone. Ozone molecules give rise to a highly characteristic spectroscopic signal, which can be clearly distinguished from others. If ozone were to be detected in the atmosphere of some exoplanet or another, would that provide a strong hint that the planet might harbor life?
Are there other basic conditions that can be defined as prerequisites for the evolution of life?
We used to assume that all planetary systems look much like our own Solar System. Meanwhile, nature has shown us that this is emphatically not the case. Planetary systems are much more diverse and complex than we once imagined. We generally assume that, if life is to evolve, a planet must possess liquid water. Therefore, it must be far enough away from its star that the water doesn’t boil off, but close enough to ensure that it does not freeze. So the planet has to be located in what is called the habitable zone. Thousands of stars with planetary systems have now been discovered, and some of them have Earth-like, rocky planets in the habitable zone. Such exoplanets are the most promising candidates to focus on in the quest for extraterrestrial life. The Milky Way comprises some 100 billion stars, and there are 100 billion galaxies like it in the observable Universe. In light of these numbers, it appears very likely that life does exist somewhere out there, although we have not yet been able to detect it.
Biological evolution was preceded by a phase of chemical evolution.
Some relatively complex carbon-based compounds found on Earth have been shown to have originated in the dusty environment of the Solar System’s planet-forming disk or even earlier. How one gets from molecules like these to the first living systems is a question that we plan to study intensively in Munich over the next few years. The basic idea is that the process got underway when certain chemical compounds acquired the ability to catalyze their own reproduction, by interacting in cyclical reaction sequences under non-equilibrium conditions. We hope to demonstrate that it is possible, via such a ‘hypercycle’, to generate self-replicating molecules that can store and transmit information. The informational molecules that evolved on Earth have been able to conserve this information for 4 billion years through an enormous number of replication cycles. Otherwise, the process would long since have come to a halt and we wouldn’t be here. Myriads of corrupted copies have been eliminated by selection, but enough biologically meaningful information has survived, thanks to the emergence of error-repair mechanisms, which have been improved and optimized by evolution. The search for environments in which prebiotic Darwinian chemical evolution could have potentially taken place has now focused on areas where physicochemical conditions are kept very far from equilibrium. Marine hydrothermal fields, where heated gas bubbles from vents on the seafloor, are perhaps the most promising possibility. Based on the work of LMU biophysicists, the kinds of processes that led to the precursors of living could have taken place in such a milieu. As part of an interdisciplinary network, we will to set a dust and sequencing laboratory in which we can simulate such processes.
Thus completing the link between stardust and species, so to speak?
One could put it that way. But what we really want to understand is a fundamental phenomenon, which we refer to as emergence: The whole is more than the sum of its parts. Consider a very large number of components that interact with each other. These interactions can give rise to an organism whose form and behaviour no longer exhibits any resemblance to the building-blocks from which it was made. Take me, for instance. I am made up of about 100 billion cells, but you could not guess what Andreas Burkert looks like, what he is thinking about or what he likes and feels just by looking at one of his cells. So the really exciting questions are how emergent structures like a human being can be generated – and why the Universe has the potential to assemble something complex and entirely new from a collection of much simpler constituents, in a hierarchical fashion. Equally exciting is the prospect of being able to discover extraterrestrial life in the not too distant future. That would be comparable to the situation in which Christopher Columbus found himself when he stumbled on the American continent. One thing is clear: If we ever discover unequivocal signs of life elsewhere, then we are no longer the measure of all things. We are just another part of the whole. We would remain unique, but we would not be alone.
Prof. Dr. Andreas Burkert holds the Chair of Theoretical and Computational Astrophysics at the Munich University Observatory (Universitätssternwarte München, USM), which is part of LMU.