Nothing lasts forever, and this applies even to stars. Massive balls of plasma go through different life stages depending on their size, either ending up as red giants and white dwarfs, or being torn apart in the most powerful explosions ever found in the universe: supernovas. For a bright celestial body to end up in such a spectacular way, its mass must be eight times greater than the mass of our Sun.
Before it dies, its immense gravity squeezes the atoms in its core together to form new elements, turning hydrogen and helium into iron, among other things. Jenny Vige of the Technical University of Berlin is targeting this element. Using the latest analytical methods, she can distinguish between terrestrial iron and stellar iron, thus looking back millions of years. Feige was a guest of the Austrian Academy of Sciences (ÖAW) at the symposium “Disasters in Earth History” of the ÖAW Committee on Earth Sciences.
Killer gang swinger
Supernovas occur constantly in space. What effect does this have on the Earth?
Jenny Figi: It depends on the distance at which the supernova occurs. There is what is called the killing distance, which is 30 light years. If the explosion occurred within this lethal distance, it would have catastrophic consequences because we would then have this material ejected directly onto the ground. This would destroy all macroscopic life. But 30 light-years isn’t that big on a cosmic scale, and there aren’t many stars within that range, and none of them will explode in the foreseeable future, so there’s no need to worry about that.
We can reconstruct our solar system’s path through the Milky Way, as it repeatedly flies past the remnants of star explosions.
What about more distant stars, do you notice anything when they explode?
coward: Yes, in principle yes. There are different types of radiation that can reach us. Cosmic radiation, for example, high-energy particle radiation, but also electromagnetic radiation such as X-rays or gamma radiation. They can all have different consequences. Our ozone layer could be destroyed, which could result in more ultraviolet rays from the sun reaching the Earth, which of course increases the risk of cancer. But climate changes can also be a consequence of this, for example through increased cloud formation.
On the other hand, your research focuses on the dust that is created during a supernova – how can you imagine that?
coward: Some of this dust actually ends up on Earth, but it is very difficult to detect directly. Its size in space is only a few micrometers and then burns up in the Earth’s atmosphere. Therefore, we cannot sift stardust grains from Earth and distinguish them from terrestrial dust. But we can recognize it by the signatures of its counterparts.
Radiant star atoms
Does supernova dust contain other atoms?
coward: Yes, it contains above all a special iron isotope, called Fe-60. These are iron atoms that contain a number of neutrons in their nuclei that differ from regular iron. On Earth you usually only find iron nuclei containing 28 to 32 neutrons, while iron-60 contains 34 neutrons. This isotope is formed mainly in supernovae. It is also radioactive and decays with a half-life of 2.6 million years.
In fact, some of the dust from star explosions ends up on Earth.
How can you distinguish between isotopes of regular iron?
coward: This takes a very long time, because in every gram of sediment that we examine, we only measure a few iron 60 atoms, which are actually very small. It also has the same mass as regular nickel. So you first need a series of chemical purification steps, and then the sample is separated using a huge particle accelerator and mass spectrometer. There is currently only one accelerator in the world that can do this, and it is in Canberra, Australia.
Where do you find these iron isotopes?
coward: Basically everywhere, but in very small quantities. Deep-sea sediments are particularly interesting to us, because here the material gradually flows down and is deposited very evenly in layers. The deeper the layer, the older it is. In our latest project, we are also studying sediments found in the Atacama Desert, where layers similar to those found in the deep sea are formed. If you find iron-60 in such a layer, you can calculate when it deposited and which star might have exploded.
Traveling through cosmic bubbles
How do you know which iron star 60 comes from?
coward: We can reconstruct our solar system’s path through the Milky Way, as it repeatedly flies past the remnants of star explosions. Space is not an empty void, as one always thinks, between the stars there is the interstellar medium of gas and dust. This is constantly being mixed and pushed together by stellar explosions. This creates bubbles that can be assigned to specific supernovae. Our solar system currently lies inside such a bubble, the so-called local bubble. They are so large because they were caused by several explosions.
You have received a seed grant of €1.7 million from the European Research Council (ERC) for your work – what do you plan to do?
coward: On the one hand, we want to move forward with the above-mentioned work in the Atacama Desert and see if it is more suitable for our research than deep-sea sediments. Hopefully we can better resolve the signals from supernovas in Saharan dust. We will also examine so-called micrometeorites, which are small remains from asteroid impacts in our solar system.
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