by Sophia Nasr
We have all heard the famous quotes “We are made of star stuff” from astronomer Carl Sagan and “We are not figuratively, but literally stardust” from astrophysicist Neil deGrasse Tyson. Now, let’s get into exactly what they mean, and how it is that we have come to be made from the stuff of stars.
Let’s begin with the two most common elements in the Universe: hydrogen, followed by helium. 13.8 billion years ago, the Big Bang gave rise to our Universe. The Big Bang created all the matter in the Universe after undergoing a series of events. The hydrogen nucleus is just a proton, so it was the first of the nuclei created, and the most abundant. Through nuclear reactions, hydrogen then formed helium nuclei. Very rarely, fusions led to the formation of lithium, and even less so, beryllium. This is pretty much all that was formed in the early Universe, because anything heavier requires more heat, and the Universe cooled quickly as it expanded with time. A point of interest is that all the hydrogen in the Universe today was formed when it was young—it is a relic of the Big Bang. Today, hydrogen and helium dominate the elements in the Universe at 98% by mass (about 73% hydrogen and 25% helium). The remaining 2% constitute the other elements.
The hydrogen produced in the early Universe then went into forming stars, stars that would fuse this hydrogen into heavier elements. Stars are like big nuclear fusion houses in which lighter elements are constantly being fused into heavier ones. A star begins by fusing hydrogen into helium. The lowest mass stars in the Universe run out to fuel once all their hydrogen has been fused into helium, leaving them with an inert helium white dwarf core. Medium sized stars like our Sun will fuse the helium into heavier elements like carbon and oxygen. Once all the helium is depleted, they are left with an inert carbon core and shed their layers in a planetary nebula, with a white dwarf remaining at the center.
Stars heavier than five solar masses are ones of particular interest—these are the stars that facilitate the formation of the heaviest atoms. Such stars will continue fusing carbon into even heavier elements. If a star is massive enough, it will go on fusing elements into nickel, which decays into iron, and become a red supergiant. Because it takes energy to fuse iron into heavier elements, iron is the end of the line for the star. At this point, the star will explode in a violent supernova that leaves behind either a neutron star or a black hole, depending on its mass. This explosion produces temperatures so high that it ignites nucleosynthesis again, producing elements as heavy as gold, platinum and silver, to name a few. Spewing elements the star once contained, and ones newly formed, the supernova essentially spreads out the necessary ingredients for life.
Elements spewed from supernovae enrich molecular clouds that condense under gravity, eventually forming stars, and possibly planets that orbit those stars. Such star systems will have all the ingredients required for life, thanks to stars fusing elements in their cores, and supernovae further fusing these elements into even heavier ones. On Earth, a few billion years combined with favorable conditions, the right distance from the Sun, and the required ingredients for life, resulted in a planet beaming with life. We can take “stardust” to be all atoms aside from hydrogen (which we know was exclusively formed during the Big Bang). Our bodies are composed of 60% water. Water is only about 11% hydrogen by mass. When looking at the rest of the stuff we are made of (which includes “stardust” and some hydrogen), we find that the human body is 93% stardust by mass—the remaining 7% is hydrogen, a relic of the big bang. In this way, we are all quite literally stardust, or star stuff (with a bit of Big Bang stuff).
Taking into account that this process is undergone everywhere in a universe as vast as the one in which we live, does it not follow that our universe may be beaming with life? The very stuff we are made of is spewed all over the universe, and becomes part of other molecular clouds that form star systems, systems that will be enriched will all the necessary ingredients for life… Life elsewhere seems almost inevitable, doesn’t it? I certainly think so.