Average chemical composition of the universe to be the same as that of the sun
This is the “South Pillar” region of the star-forming region called the Carina Nebula. Like cracking open a watermelon and finding its seeds, the infrared telescope “busted open” this murky cloud to reveal star embryos tucked inside finger-like pillars of thick dust. Credit: NASA Enlarge
All of the chemical elements that are heavier than carbon, the oxygen we breathe, the silicon that makes up the sand on the beach, were produced inside stars through nuclear fusion and released by stellar explosions called supernovae. By measuring the chemical composition of the Universe, scientists are trying to reconstruct the history of how, when, and where each of the chemical elements so necessary for the evolution of life were produced.
Very generally speaking, there are two ways that a supernova explosion can take place, and the proportion of chemical elements that are produced depend on the supernova type. Lighter elements, like oxygen and magnesium, originate mainly from the explosions of very massive stars, more than 10 times the size of our Sun, at the end of their lifetimes. These are known as “core-collapse supernovae”. Smaller stars instead usually end their life cycles as “white dwarves”, a small fraction of which can explode as a “thermonuclear” or “type Ia” supernova if they later accrete matter from a companion star, causing the white dwarf to become unstable to the pull of its own gravity. Heavier atoms like iron and nickel mostly come from this latter type of supernovae. To make up the chemical composition of our Solar System, for instance, we require a mixture of roughly one thermonuclear for every five core-collapse supernova explosions. JAXA research fellow Aurora Simionescu wanted to find out whether the average chemical composition of the Universe was similar to that of our Solar System, or whether our local neighborhood was, after all, a special place.
Actually, perhaps counterintuitively, the answer to this question is best found not by looking at the stars themselves, but rather looking at the intergalactic space. That is because most of the normal matter in the universe, and thus also most of the metals, are presently not contained in stars, but rather in a very hot, diffuse gas that fills the space between galaxies, and is so hot that it shines in X-ray light. The brightest X-rays come from so-called clusters of galaxies, the places in the Universe where the galaxies are packed closest together.
“I’ve found this idea fascinating ever since the first year of my PhD: X-raying the chemical content of our Universe”, says Aurora Simionescu. But back then, almost 10 years ago, it was very hard to obtain reliable measurements of the metal abundances except for the very densest, brightest parts of the intergalactic medium, due to a lack of X-ray photons and high background noise. So we could only really probe the chemical composition of roughly the central one-thousandths of the typical volume of any given galaxy cluster.
JAXA’s Suzaku X-ray satellite dedicated a great amount of observing time, collecting data over many weeks, to address this problem. The first such deep observations, targeting the brightest system, the Perseus Cluster, allowed remarkably detailed measurements of the iron abundance in the intra-cluster medium on large scales. However, information about chemical elements predominantly produced by core collapse supernovae was still missing.