Twinkle, twinkle, little star,
How I wonder what you are!
Up above the world so high,
Like a diamond in the sky.
The lyrics above are from an 1806 poem by Jane Taylor; she could have saved herself some time by simply analyzing the star’s spectrogram. How can we know the content of stars that are thousands of light years away? The answer is deceptively simple: You can see what they are made of by literally, looking at them.
Stellar spectroscopy is a science discovered a few decades after Taylor’s poem. Quite simply, every star’s light shows its elemental content when broken down spectrographically. Our sun is a typical “Population I” star, about 4.6 billion years old, and here’s what its spectrograph looks like:
The black lines are called absorption lines, and they display the metallurgical content of the star. It is, in essence, a star’s “bar code,” and it is as distinctive as a human fingerprint. The stars that preceded our sun would have similar, but not identical characteristics, as will stars that eventually arise out of the death of our sun. I’ve used the term “omnis cellula e cellula” before; it means every cell comes from a previous cell. Here’s a variation on that same theme: “omnis stella e stella,” every star comes from a previous star.
The old marketing refrain suggests that a diamond is forever. Well, it might be less romantic, but you know what is genuinely forever? Hydrogen atoms. In the first stellar generation after the Big Bang the only material elements that existed in the universe were gases of hydrogen and helium. The first stars to light up the early universe were deep blue pure hydrogen stars. If you ever want to start your own universe, all you’ll really need is hydrogen and gravity; given enough time the rest will follow. There was no metallicity in the early universe — in other words, no hard materials or any element higher than Three on the periodic scale. The pure stars of the first generation seem to have lived only briefly, burned brightly, and would have been devoid of iron. It was this first generation that created and expelled the first heavier-than-helium elements into the universe, contributing these materials into the next generation of stars.
In the words of Dr. Phil Plait: “A star is basically a machine for turning lighter elements into heavier elements.”
There are cycles to the universe. Stars form, they live out their lives, they die, they blow off winds and they explode, seeding their material into gas clouds which then form new stars with heavier elements in them, which will repeat the cycle again. So if you want to think about it that way, the universe is the ultimate recycler.
— Phil Plait, How the Universe Works, s04e01, Discovery Communications, 2015
Thirteen billion years of fusion, nucleosynthesis and occasional beta decay has only altered the composition of the material universe by two percent. An overwhelming ninety-eight percent of the universe is still hydrogen and helium today. Let’s say this again: there are 92 naturally occurring elements on the periodic table — two comprise an overwhelming ninety-eight percent of the whole, while the remaining 90 elements make up only the last two percent. Oxygen, carbon, nitrogen, everything you can see and feel and experience on the earth has emerged out of this fractional two percent of atomic mutation. Stars are hydrogen furnaces, and every atom from helium to uranium is a product of that creative kitchen. Every supernova seeds the next stellar nursery, and as each generation of star is further enriched with carbon, nitrogen, oxygen, silicone and iron than the generation that preceded it, each generation becomes “dirtier.” Hence, the less metallicity in a star, the earlier it is likely to have formed.
Enter SMSS J031300.36-670839.3. Not the name I would have chosen, but I guess there’s already a Ralph. In 2013 this innocuous star within our galaxy, some 6000 light years away and visible only in the Southern Hemisphere, caught the attention of Stefan C. Keller and the SkyMapper Southern Sky Survey. It registered the smallest amount of iron ever detected in a star; there appears to be 10,000 times more iron in the earth’s core than there is in this star, and the star is a million times the size of our earth (source: Anna Frebel, http://afrebel.scripts.mit.edu/www/1726-2). This spectrographic analysis discovered what is probably the cleanest and oldest star yet found in our sky, its age estimated at perhaps 13.6 billion years.
As Keller himself remarked: “What we’re able to do with this star is, for the first time, say that there was only one star that preceded it.” SMSS0313 seems to have formed from the debris from one of the very first stars in the universe. In case you’ve ever wondered, the image above is the most ancient star we’ve found in the sky.
Theoretical models predict that the truly first … stars [also known as ‘Population III’] were all massive, such that all of them would have died a long time ago. [This SMSS0313] star is just a whisker away from the elusive Population III, but preserves the conditions in the early universe. Thus, we are getting here as close as one can hope to the moment of first light.
— Professor Volker Bromm (http://newsoffice.mit.edu/2014/researchers-identify-one-of-the-earliest-stars-in-the-universe-0209)