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Stars: The origin of all chemistry

It’s a chicken and egg situation: without stars there would be no chemistry, and without chemistry there would be no stars. A few minutes after the big bang all matter in the universe was made from the simplest element, hydrogen. So how did all the exotic elements and complex molecules we know as chemistry come about? As time went on, some of the hydrogen atoms clumped together and eventually grew into the first stars. These stars then became the element ‘factories’ that fed the growing universe. 

The production of elements other than hydrogen is a side effect of the way stars fuel themselves. Once the clump of hydrogen forming the star achieves sufficient density, this crushes the atoms, creating both heat and pressure. With enough crushing, the star fuses hydrogen atoms together, producing the bigger element helium and energy. This creates even more heat and an outward pressure, stopping the young star from collapsing in on itself. The star will continue to fuel itself like this until its core hydrogen is used up and converted to helium. 

From this point, a star can evolve in a number of ways. One of these ways is to expand to become a red giant. As it turns into a red giant, the star starts to fuse helium atoms, creating bigger elements, up to iron. Iron is at a crossover point in the periodic table. To fuse together elements heavier than iron requires energy to be put in, so this would suck energy from the star rather than fuelling it. As it gets to the point of using up its helium, a red giant will  expand very quickly and puff its outer layers off into space. All the elements it has produced, helium, carbon and nitrogen mainly, get mixed back into the dust of outer space where they will form new stars or planets. 

The more dramatic way a star can evolve is to explode into a supernova. Along with the supernova explosion the star will create a massive pulse of energy. The energy of this event is enough to fuse together elements larger than iron, so all of these (like zinc, mercury and uranium) will have been forged in a supernova event. As well as producing all the bigger elements, supernovae have other effects on the space around them. The pulse sends out a shockwave that disrupts any dust clouds in the vicinity. This disruption can lead to clumping in the dust, which itself can lead to the formation of new stars and planets that are enriched with the elements the supernova has just given off.

There’s still a lot of work to find the origin of iron and the heavy elements in our solar system,’ writes Dr Penny Wozniakiewicz of Laurence Livermore Laboratories. ‘We are still wondering if they came from the supernova that started the collapse of a molecular cloud to form our sun and its planets, or from another event.’

Cosmochemists, like Penny, use the relative amounts of different element isotopes to find out how old the solar system is. Isotopes are elements with varied amounts of neutrons in their nuclei but same number of protons. They are chemically identical, but the difference in mass affects stability, meaning that over time some will change to other isotopes. Using some of the meteorites that fall to Earth, cosmochemists examine the relative amounts of starting isotope and how much of it has changed to a different isotope over the time since it was formed. By knowing how quick the change should be, they can work out the age of the meteorite, and figure out where it came from. 

The other branch of science that explores chemistry in the universe, astrochemistry, analyses light from distant objects to discover what they are made of. This technique, known as spectroscopy, is now so powerful that it can examine exoplanets orbiting other stars. Among other things, spectroscopy has shown that some planets are rich in water vapour and methane. 

A key question in astrochemistry research has been to discover how the complex molecules we see today came about. The molecule benzene, a simple ring of six carbon atoms and their associated hydrogen atoms, has attracted a lot of interest. This is because it can be thought of as the simplest building block for more complicated molecules, called polycyclic aromatic hydrocarbons (PAHs). It is thought that most of the carbon in the dust surrounding stars is locked up in these PAH molecules.

When we talk about the complex chemistry of the universe, it is important to keep in mind that, despite the universe being over 14 billion years old, its elemental abundances are still about 74% hydrogen, 24% helium and only 2% everything else. Tracking where the heavier elements are and studying their chemistry reveals much about the origins of our planet, our solar system and our universe. 

Helen Maynard-Caseley is a freelance scientist

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