Monday, November 18, 2013

We are Stardust. No...Seriously.

We are stardust                     
We are golden                       
    We are billion-year-old carbon
                         - Joni Mitchell
A Giant Hubble Mosaic of the Crab Nebula
Crab Nebula: A Supernova Remnant
Sometimes my fellow skeptics get on my nerves. So many of them focus too much on negatives--on what's false, rather than what's true. They get so fixated on debunking falsehoods and quackery that they forget to talk about how amazing the real world is. Don't get me wrong...nonsense certainly needs to be exposed, and naked emperors should assuredly be mooned. But still, there's a balance to be struck. Why spend all your time talking about what you don't believe? Why define yourself negatively--in terms of what you reject?

Since I spent my last post talking about astrology and other things I reject, in this one I want to tip the scales in a more positive direction and talk something not only beautiful and wondrous, but scientifically sound to boot. I want to talk about the stars--the stars of the astronomers, not the astrologers. Astrologers have always been fascinated by stars, and rightly so, but I think they miss what's really amazing about them. It's not that the stars influence our personality or destiny in any direct way. They are, after all, giant balls of incandescent plasma, and even the closest are almost inconceivably far away. There's zero scientific reason to think they influence our daily lives--they don't care about some race of featherless bipeds on a faraway planet, and they couldn't do anything about it if they did. But that doesn't mean we have no connection to them. The stars aren't about us, however much we might want them to be, but we are about them. Joni Mitchell was basically right--we really are stardust.

To be more specific, most of the atoms in our bodies, and almost all the elements in the periodic table, were forged billions of years ago in the interiors of stars. Many of the heavier elements were created in stellar death throes, in the titanic explosions known as supernovae. All these processes of element creation are still going on today. The elements that weren't formed in stars (mainly hydrogen and helium) have an even more impressive provenance--they were formed in the Big Bang itself, 13.8 billion years ago.

When scientists are asked what is the most amazing scientific fact they know, many of them talk about how we are made of stardust (star-forged atoms, technically). I tend to agree with them. It's an idea as stunning as anything I've ever heard, and unlike astrological ideas, it's almost certainly true. Here's the basic story scientists have pieced together.

Less than a second after the universe began, protons, neutrons, electrons, and various other particles had formed. As the infant universe expanded and cooled, between the first 3 and 5 minutes, protons and neutrons clumped together to form what would become the nuclei of hydrogen (which has a single proton), and helium (which has two). Conditions were such that this created 6 helium nuclei for every 76 hydrogen nuclei. After about 380,000 years, the radiation that had been knocking electrons away from nuclei cooled enough to allow atoms to form. The hot, glowing universe went dark, but this radiation can still detected today in the form of weak microwaves that permeate space. Astronomers call it the Cosmic Background Radiation--the afterglow of the creation of the universe.

The early universe, then, was a homogenous sea of hydrogen and helium atoms (along with a little bit of lithium). If it had stayed that way, we wouldn't be here--two elements don't make a rich-enough atomic alphabet to create complex structures like, say, us. But as things kept expanding and cooling, the weakest force of nature--gravity--began to assert itself. Matter attracted matter, and great clouds of hydrogen/helium gas started to collapse into spheres. When the pressure got high enough at their core, hydrogen nuclei were squeezed together into helium nuclei. Nuclear fusion reactions had ignited, and the dark universe lit up once again, this time with the light of countless stars. (At larger scales, of course, stars and other matter had formed galaxies, which grew as "small" galaxies combined into larger ones.)

The Cat's Eye Nebula (A Planetary Nebula)
After these primordial stars had burned for a few million or billion years (depending on their mass), a core of helium formed at their center.* The more massive the star, the hotter it burned, and the faster the helium core formed. When it did, helium started to fuse into carbon, which would one day become the backbone element for life. But the carbon core couldn't support the weight of the star, so it started to contract, while the hydrogen and helium around it burned even hotter. This caused the star to puff out its outer layers, forming a huge, cool(er), red giant star. Eventually this behemoth would blow away its outer layers entirely, forming a luminous cloud called a planetary nebula. The core would remain as an intensely hot, small, dense star called a white dwarf, which might be smaller than the earth but more massive than the sun.

Things got a little more exciting in bigger stars, several times as massive as the sun. Here again, hydrogen burned to create a core of helium, which then burned to create a core of carbon. This time, though, temperatures got high enough (hundreds of millions of degrees) for carbon to start fusing into neon. And on it went--neon fused into oxygen, oxygen into silicon, and silicon into iron. Now there was an iron core surrounded by all these other elements, still burning furiously in concentric layers. Once the iron core over. As tough as iron might seem to us, it couldn't support the weight of all those layers. The iron nuclei were torn apart, and then the protons combined with electrons to form neutrons. The core of the star collapsed and then rebounded, sending a shock wave shrieking through the outer layers of the star--blasting them apart in a supernova. Supernova explosions can release more energy in a few months than our sun ever will in ten billion years; causing the dying star to glow as bright as a small galaxy. The explosion was energetic enough to create all the remaining elements of the periodic table, and then blast them into space. All that remained of the core was a neutron star--a ball of neutrons as dense as an atomic nucleus but as big as a city, spinning several times per second and spewing jets of radiation from each pole.

These dying stars enriched interstellar space with clouds of elements beyond hydrogen and helium, capable of combining into an enormous variety of molecules, which could in turn combine into all kinds of complex, intricate structures. The new interstellar clouds of heavy elements and molecules then began to collapse again, forming a new generation of stars like our sun, born with a full suite of heavy elements. Around many of these stars, little balls of metal, rock, gas, and ice formed. We call those planets. Many of them have a wide variety of elements--potential building blocks for complex structures. On at least one of these planets, that potential was realized in the form of carbon-based life, which formed shortly after the Earth cooled. It's been evolving and diversifying ever since, creating millions of unique life-forms, from slime molds to blue whales, from giant sequoias to seahorses. Lately, it even created a race of odd, upright, talkative apes; quarrelsome creatures, but clever, too--clever enough to discover that they're made from the dust of exploding stars.


* Helium was actually first discovered on the sun, based on characteristic spectral lines in the light it emits. It wasn't discovered on earth until later. It's named for Helios, the Greek god of the sun.

Postscript: I've heard the Joni Mitchell's song Woodstock probably a thousand times in my life (mostly the Crosby, Stills, Nash and Young version), and I never noticed that line about "billion year old carbon" until I looked up the lyrics today. Nice work, Joni.

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