You came from a star — but which one?
Ourselves and all the matter that is around us were mostly made in a star, many millions of years ago. That star is not our Sun. So where did we start?
If you have chuckled at the terms ‘mostly’ or ‘a star’, feel free to skip the part 1 of pop-science. Next I’ll briefly talk about the current r-process in supernovae models, Neutron stars, the JWST telescope and why does this search matter! Note: all links on pt1 are easy to grasp youtube links. The actual citations start from pt2. I’m not a physicist but ask me anything here or DM me on twitter.
Part 1: You are stardust!
Everything is made of matter and/or energy. The matter we speak of is, in a very broad sense, are all made of atoms of elements like Oxygen (O), Carbon (C), Gold (Au), Iron (Fe), Europium (Eu)! Just after the Big bang, we got Hydrogen (H), Helium (He), and a very little Lithium! Everything else were made in the stars later on. Well, except the superheavy synthetic elements we’ve made in our labs like Mendelevium (Click that link if you’re a Friends fan!)
Stars burn the H and He in a process called (Stellar) Nucleosynthesis. The atom as you may recall, has a nucleus of one or more Protons (p) and zero or more Neutrons (n) and there are electrons (e) revolving the nucleus but electrons aren’t important. The nucleus of H is1p, He is 2p2n, Li is 3p3n, O is 8p7n, Fe is 26p30n, Au is 79p118n .. and so on. Only thing to consider here is the number of protons in a nucleus. Adding more neutrons to a nucleus gets us something called ‘isotopes’ of the same element. Once we add a proton to a nucleus, only then we get a new element with totally different properties. So, Nucleosynthesis is a way to add protons (!) to the nucleus of lighter elements in order to get heavier elements. Now adding a proton to a nucleus is not easy because as protons are positively charged, they repel each other with a very strong force called coulomb force. Stars do it by a massive amount of heat energy and trillions of PSIs of pressure. Proton-proton collision (and β+ decay) or neutron formation is out of scope here, but just know that this happens in a process called Thermonuclear Fusion.
So stars can mash more protons to nucleus and we get infinite variety of elements right? Not so fast! The force that makes fusion possible comes from its gravity, i.e: the mass of the stars. As you increase mass, gravity increases which mashes the particles inside stars together, and effectively getting denser inside and shrinking in volume. But if this process is unguarded, i.e: if stars let gravity take over, then it’ll collapse into its own core. The force that stops this is the extreme heat energy from the thermonuclear fusion reaction from the nucleosynthesis we’re discusing about. This is why at one point if we try to make heavier elements than Iron (Fe 26p30n), The star’s gravity needs to be so huge that it’ll start collapse within itself. This process is called a Supernova!
Part 2: The r-Process in Supernovae and heavy element formation
The Supernova is a violently beautiful thing. It also is the reason we have all the elements in the universe heavier than Iron! We’ve glazed over the complex stellar nucleosynthesis above which used fusion to get heavier elements until Fe. But if TNF stops at Fe, how do we have about roughly 68 more elements? The answer is the r-Process (rapid Process) [Takahashi et al.] and s-Process (slow Process) [Hoyle et al. , Ward et al.] of neutron capture. For us muggles, neutron capture process is like randomly throwing a lot of spaghetti at a crowd in India and hoping it’ll stick to some Italian tourists’ face. Why this weird analogy? Because cross sections of nuclei are mega-tiny compared to their atoms, A heavy atom with multiple electron rings outside roughly compared to a size of a football field has a nucleus as a size of the pea at the center — kinda like finding an Italian in India even though the earth is full of full of people!
We throw neutrons at atoms, most will pass through the vast void and only a very few will stick to the tiny cross section of the nuclei. The cross section of the seed nucleus is kind of an important measurement for the success of the r-Process actually. We’ll not dwell on s-Process much as they aren’t relevant for Supernovae (stellar only) and the few elements are formed from the s-Process are relatively inconsequential and the theory isn’t that murky like the r-Process. For a very long time it was understood that during a supernova, a massive burst of neutrons are thrown everywhere into the heavy nuclei everywhere. in roughly 30 to 400 (mean 150) microseconds, the neutrons that stuck to the nuclei will have a beta decay (λβ) and will convert into a proton, thus forming a new heavier element. There are three general steps to understand this process:
- Gathering nuclear data especially for the heavier elements. Starting naively from the Shell structure, we go increasingly meticulous study into various aspect of the nuclei — the Trap and Mass measurements (Cyclotrons, Time of Flight, etc), the Beta decay (CERN isolde experiment) [Lorusso et al.], the beta delayed decay rate measurements with spectroscopy, Neutron capture rates etc. Why, we need to understand the stuff at hand — i.e: the nuclear properties of them reveals the clues into how they may be synthesised.
- Computational Models: Once we know how can we make bread, we run some computer simulations to discover/theorise what are the possible proportions of salt, water, flour, baking powder can make what type of breads! What happens if we add raisins (ew) or some banana maybe? The Computational models for the r-Process from all these nuclear data then tells us what to look for at the stars!
- Astronomical observations: The big donut will be to actually observe an r-Process in motion! We can start again in the lab with gamma ray radiation and then higher resolution spectroscopy to detect possible aftereffects of very short lived isotopes of the r-process elements in supernova remnants. Metal poor stars [Cowan et al.], abundance of heavy elements (Europium as std) in native galaxy and other outer galaxies. The Abundance pattern in various parts of the galaxies gives us better clues into what happened around what past events and we can theorise maybe why. Another very important observation in this century has been Kilonovae ejecta. Gravitational wave was theorised before but very recently was proven by the amazing LIGO apparatus, thus we now know, we have events such as Kilonova where a two neutron stars (or Black holes) crash into each other into a unimaginably violent, final embrace [Barnes et al.]. (We also all pray that the Strange quarks doesn’t get out of these massive quark-gluon plasma bottles or else universe is screwed! Just Kidding!)
Part 3: The problem with Gold
Supernova theory fits, as it essentially has everything — the heavier seed elements form a massive star that was ejected with great fury and they get a few stages of neutron bursts that was inside the said star at a very rapid pace. Until we had more data. Uh-oh!
As our experiments got more and more sophisticated, we started to understand that Supernovae alone aren’t all that plausible for the much heavier elements like say — Gold, given the abundance pattern! The amount of gold in the universe (extrapolated from data in Earth is massive but it should be that abundant! The gold we find on earth is tiny and only came from asteroid bombardments on only a mere million years old baby Earth. Most of Earth’s gold is buried in its core and we have an obscene amount of it. But how! Nucleosynthesising Au should be hard! How do we have so much gold!
All the new data about the r-Process models [Kajino et al. ] indicates that we should look for other sources of that sudden neutron burst needed to fit the abundance variation data found in interstellar spectroscopy for heavy elements. Why? You remember throwing spaghetti at the crowd? Turns out we need much more spaghetti (neutrons) if we need to hit a lot of Italians in an Indian crowd! But supernovae doesn’t have that many number of neutrons! The immediate solution is to find something violent that is also made of neutrons — neutron stars! (They aren’t quite ‘made of’ neutrons but let’s leave it at that!)
So we need neutron star but how do we get the neutrons out? Easy get two! Pit them together rotating around each other in ever narrowing orbits until they collide in to a Kilonova and gives away massive amount of neutrons in the process. There’s a sliiight problem with this! Binary neutron stars aren’t all that common!
In all the simulations for the Kilonove ejecta proposals fro heavy element nucleosynthesis, the amount of these events were estimated at best case scenarios at hundreds of millions (A x10⁸) while the actual number in a Galaxy in its lifetime could be as low as 10⁴. There are a group of scientists are arguing these exotic events could be too farfetched [Benoit et al.] It’s legit to ask why such a big difference? Because we don’t know! And we are about to (hopefully) in a few years with our shiny new telescope!
Part 3: Gravitational Waves and James Webb Space Telescope (JWST)
Here’s that guy again! Einstein once said (to no one, pop-sci has dumbed this down to this level): SpacetimeIsAGiantRubberSheet ™ and any object with mass causes a dip in it— a gravity well. The denser the object, the deeper the gravity well. Now if two very dense objects like neutron stars (which are essentially almost black holes who didn’t pass the bar exam!) rotate violently aorund each other, they create ripples in spacetime that travels very very far (millions maybe billions of light years). We kinda knew this in 70’s [Schramm et al.] but actually detecting one was simply pipe dream building an experiement that is sensitive enough to detect GW will be extremely susceptible to all other kinds of noise from everywhere Terrestrial and otherwise. Well we (not me) did it! With the Laser Inferometer Gravity-Wave Observatory (LIGO), we now have conclusive proof of gravitational wave and what monstrous hellish events cause them [Ligo paper on Kilonova observation with about a hundred authors!]
This is great. And we can use this to look at particular parts of the sky to observe these merger events. But even Hubble Space Telescope isn’t that powerful to see these events. What do we do? Build another space telescope of course! And that’s exactly what we (again, not me, never me) did!
As of today, the amazing JWST has it’s folded sunshields and radiators stretch out (read the greatest shade in twitter history!) and the secondary mirror assembly extended (Update — Primary mirror is now fully unfolded yay!). After an agonising two weeks since its launch, we now have a telescope in space and the humming sound you hear is the collective sigh of relief from the Astrophysics community! All Nominal! This absolute monstrous feat of engineering is going to gather infrared lights from the depth of early universe so whenever we can detect a Kilonova g-Wave, we can tell it to look at the source to actually witness what is going on in those events and how feasible it is to have our heavy metal factory there! Neat-o!
We’ll soon know whether exotic events like neutron star mergers are the reasons behind all these nice shiny metals we have or is it some twist like the magneto-rotational instability in core collapse supernovae that enrich the amount or seed nuclei in a two stage r-process to synthesis large amount of heavy elements with little neutron supply [Nishimura et al.]. IN any ways, this is an exciting time for science!
Part 4: Cool — but why?
Do you ever wonder who you are? What is your purpose in life?
“It’s just the strangest thing — I’ve seen your face somewhere!
An early evening dream, a past life love affair!
Do you know me at all?”
There is no particular purpose of life in a greater scheme of things. You’ll eventually die off of self inflicted or natural causes. So will the human race, and every other living thing in the universe. In the end, after the last neutron stars cool off, the universe will be a dead cold graveyard. It is called the heat-death and by the laws of thermodynamics, it is indeed inevitable.
There is no point. Whatever you do — it doesn’t matter in the long run. There is no point helping the needy or planting trees or build anything in particular. There is no meaning of life. If you die now, nothing particular will change. Your loved ones (if there are any) will miss you and then they’ll die off eventually and you’ll be forgotten forever. We all will be. There is absolutely no grand reason to exist.
Yes, this is totally pointless but so is the universe and although as a singular sentient being you are insignificant; you and every grass blades, and raindrops, and blood, sweat, feces, flowers, suns, moons, stars, and your mothers eyes — every single thing around you including this screen on which you’re reading this, are the universe. No, I’m not talking about in any abstract philosophical term but quite literally — form the molecular, physical sense — you my friend, are the universe. This was the whole point of this article. You’re stardust. You literally are the universe. And you’re trying to understand where does your journey start.
This way you keep feeling, perceiving, affecting, and understanding the world around you until you die because when you’re trying to experience the universe, quite literally, the universe is experiencing itself.