Tuesday, March 21, 2017

A Heart to Heart Talk

Several billion years ago, a bright red star the size of Earth's orbit beat like a heart in a spiral arm of the Milky Way galaxy. Already billions of years old, this star had long since fused all the hydrogen in its core into helium. Eventually, the star grew hot enough that the helium ash could begin to burn, slowly transforming the core to carbon and oxygen. When helium in the core finally ran out, the billion year balance between gravity and radiation that every star battles to maintain gave way, and the core contracted and grew hotter. Feeling the heat, the outer envelope expanded and cooled, and a red giant was born.

This giant soon found a new, but ultimately short-lived balance in a period of its life known as the asymptotic giant branch (AGB) phase. Now, a thin shell of hydrogen surrounding the core grew hot enough to burn, producing a new layer of helium that settled onto the core. After tens or hundreds of thousands of years, the helium layer grew hot and dense enough to start its own fusion cycle, leading to a brief helium shell flash. In those moments, the star's brightness would jump by a factor of a thousand before returning to its quiescent, hydrogen-shell burning stage. This was the slow beat of the giant's heart.

Credit: Lithopsian
How do we know this story about a giant, pulsating star that died long before ours was born? We have observational and theoretical evidence that stars like this exist. With telescopes, we have found stars with masses comparable to our own that are tremendously brighter but cooler (on the surface). To be so bright yet cool, such stars must occupy a very great volume. We have also built models of stellar evolution by observing many different stars and figuring out how ones that look different might just be the same kind at different stages of life.

But what about this specific red giant from billions of years ago—how do we know about it? What lets us peer into its heart? Well, we don't know its name or where the cooling remnant of its core is now, but we do know this star was part of a lineage, inheriting the cosmic dust from previous stars and passing it on to us, but transformed. In the roiling convective envelope that surrounded the core of this red giant, there were atoms of iron built by some older star's fusion.

Iron is the endpoint for fusion that can power a star. For all elements with fewer protons than iron, smashing them together at high enough temperatures and densities liberates more energy than is required to do the smashing. But this doesn't work after iron, because you've got so many positively charged protons squished into such a small space that they strongly resist any further squishing. You can still do it, but you're losing energy. Nevertheless, this type of fusion does happen in the outer layers of dying stars, draining a bit of the star's energy with each reaction.

This process of building up elements in stars—known as stellar nucleosynthesis—was first described comprehensively in a famous astrophysical paper known as B2FH (after the initials of the four authors). In it, they gave a detailed account of the nuclear physics required to produce all the elements we see in nature. Spectrographic analysis of our star and ancient meteorites that existed in the early days of the solar system has largely confirmed that elements do exist in the proportions dictated by stellar nucleosynthesis.

But let's get back to the iron in that giant. Here, a type of nucleosynthesis known as the s-process was dominant. One way to build new elements is to bombard atoms with neutrons. Every once in awhile, an atom will capture a neutron and become a radioactive isotope of whatever element it is (as determined by its number of protons). Eventually, beta decay will turn one of the neutrons in the nucleus into a proton, which then bumps that atom up to the next element in the periodic table. This process starts with iron and ends with bismuth.

As you can see, there are two reactions going on here: neutron capture and beta decay. Because of this, the rate at which these reactions occur determines the eventual abundance of elements we see. In AGB stars, neutron capture happens much more slowly than beta decay, which means that we will eventually see a ladder of elements building up from iron rather than more and more weird isotopes of iron.

Let's look at one element in particular to see how this whole thing works. The element thallium has 81 protons and shows up in nature with either 203 or 205 total nucleons (protons+neutrons). 204 nucleons is unstable and decays with a half-life of less than 4 years. That means there is a branching point when thallium reaches 204 nucleons. From there, it can undergo beta decay and become lead with 82 protons, or it can capture another neutron and remain thallium. About 70% of thallium is the 205 kind, while 30% is the 203 variety. (There is more thallium-205 because lead-205, which you get to by thallium-205 or lead-204, is unstable over millions of years and eventually decays back to thallium-205.)

Credit: R8R Gtrs
By experimentally determining how likely thallium is to capture a neutron and how quickly it decays, we can infer how often atoms of thallium in that red giant were being bombarded with neutrons. Knowing the density of neutrons in the AGB star tells us what nuclear reactions were creating neutrons and consequently how hot the core of that star was and what elements it was composed of. It turns out that the abundances of elements we see would require a range of neutron fluxes, which is part of how we know that AGB stars undergo pulses of helium fusion before returning to hydrogen-shell burning.

Because AGB stars are about as large as Earth’s orbit but of comparable mass to our sun, their gravity is not strong enough to contain their extended envelopes. This means much material is lost, becoming a "planetary nebula" and eventually dispersing into interstellar space. That includes the products of nucleosynthesis, which come to pollute cold, giant molecular clouds.

About four and a half billion years ago, one such polluted cloud became unstable and collapsed. Out of that collapse was born our sun and solar system. As Earth formed and mixed together the metals that could withstand the searing heat of our young star, atoms of thallium got locked up in minerals of copper and lead and zinc.

Eventually, humans came along and started extracting pure thallium to do things with it, such as performing experiments that could give us insight into the hearts of long-dead stars. A week or two ago, some pure thallium-203 was bombarded with protons until it became lead-201, which has a half-life of 9.4 hours. The lead decayed into thallium-201, which has a half-life of 73 hours. Because of that short lifetime, the thallium must be prepared and used quickly. This specific batch was mixed with hydrochloric acid to produce thallium chloride, which was then put into a solution and packaged for use.

Four days ago, that radioactive thallium was injected into my veins. Because thallium behaves a bit like potassium as far as cells are concerned, sodium-potassium pumps in the membranes of cardiac cells take in the thallium. These pumps transport ions of sodium and potassium, creating a voltage that gives cardiac cells the electricity they need to beat. Cardiac cells that are working well have functioning pumps and will take up the thallium; cells that aren't won't. To make sure the thallium was well circulated in my heart, they had me run on a treadmill until I got to 160 bpm.

Thank you, Frinkiac.
To see where the thallium in my blood ended up, a camera took pictures of the gamma rays streaming out of my body. But gamma rays present something of a problem. In a normal camera, a lens focuses light rays onto a surface to form an image. In telescopes, we mostly use mirrors to bounce light in the direction we want. This doesn't work with gamma rays, however. Their incredibly short wavelength means that for everyday materials, they will either be absorbed or transmitted, but not redirected. When a photon is simply absorbed without any optics, information about where the photon comes from is lost and you no longer have an image.

Astronomers have devised many clever techniques for getting images from x-ray and gamma ray sources, one of which works for looking into hearts, too. You can preserve the image of a source by creating a very small aperture for light to pass through—a pinhole camera. On the other side of that pinhole, you have a detector. Because you can trace just a single line from where a photon hits the detector to the pinhole, you know what angle that photon came in at and thus know what the original source of the image was. The downside to a pinhole camera is that almost all of the light is blocked. To get around this, you can create an aperture with a very specific shape that lets in more light but leaves a distinct "shadow" on the camera. Using computational techniques, you can than reconstruct the original image.

Credit: Alex Spade
The camera they used rotated around me for eight minutes, producing cross-sections of my heart at different angles that were later combined to form a 3D image.

I don't yet know the results of that test (although I suspect I am okay), but I am comforted by the thought that the thallium used to peer into my heart can also peer into the hearts of long-dead stars, to give a glimpse of another world, an incomparably gigantic furnace burning at hundreds of millions of degrees that does its part in seeding the galaxy with the elements necessary for chemistry and life. I am also comforted to know that I am a part of that lineage, that my carbon was produced in another dying star, that the hydrogen in my water is nearly as old as the universe itself. I hope this specific agglomeration of carbon and water persists a bit longer, but I am happy nonetheless that the universe is eternal and spectacular and knowable.