To make planets and people you need dust. That’s what we’re made of. Dust. But mixed and tended in ways so remarkable our bodies give that dust passion and voice. Dust comes from aging and exploding stars. Supernovae are the most spectacular but not the only dust-makers. As stars like our sun age, they puff up into red giants and blow their atmospheres into space, enriching it with materials manufactured within the star’s belly such as carbon and silicates. These and supernova-spew become the seeds to grow future generations of stars and planets.
As one generation of stars dies and gives rise to the next, the universe becomes ever richer in dust. The very first generation of stars didn’t appear until around 100 million years after the Big Bang and were made of only the simplest elements: hydrogen, helium and a touch of lithium. But no dust. In their deaths as supernovae, interstellar grit made its first appearance.
Astronomers recently used the Atacama Large Millimeter/submillimeter Array (ALMA) to detect a huge mass of glowing stardust in a galaxy seen when the universe was 600 million years old or only 4% of its present age. ALMA studies the sky in “submillimeter light” which occupies a slot between infrared light, which we sense as heat, and radio waves. Cold dust clouds where new stars are born appear black and obscure and visible light but open up to reveal their contents to the prying waves of submillimeter light.
ALMA beamed in on one A2744_YD4, the youngest and most remote galaxy ever seen by the array. Remote means we’re seeing the galaxy as it was long, long ago when it was just a pup. That’s why astronomers were surprised to find that this youthful galaxy contained lots of interstellar dust — dust formed by the deaths of an earlier generation (s) of stars. ALMA also made the most distant detection of oxygen here. What’s wild about the dust discovery is how little time it took for the first generation of post-Big Bang stars to grow old, explode and release the dust we see.
“Not only is A2744_YD4 the most distant galaxy yet observed by ALMA,” comments Nicolas Laporte of the international team of astronomers who made the observations, “but the detection of so much dust indicates early supernovae must have already polluted this galaxy.” Today, after many generations of stars, this dust is plentiful and a key building block in the formation of stars, planets and complex molecules. But in the early universe — before the first generations of stars died out — it was scarce.
One of astronomy’s holy grails is determining just when the first generation of stars formed. The detection of dust in the early universe gives us new information on when the first supernovae exploded and therefore the time when the first stars lit up a previously young and dark universe.
The team estimates that A2744_YD4 contained about 6 million times the mass of our sun. In the young galaxy, that dust was being converted into stars at a quick pace with an estimated rate of 20 suns per year compared to just one solar mass or sun per year in the Milky Way. At that rate, it would only take about 200 million years for all the dust we see to form, so we’re seeing the galaxy just after it formed.
Fast forward 13 billion years later to the present. You, me and all of Earth are the products of that first generation of stars. By studying their ashes, we’re exploring our origins. Every discovery that takes us further back in time also takes us every closer to finding that first generation.