Making beer and weighing stars

Science these days is often all about interdisciplinary work. It’s seldom just biology or just geology, and so it wasn’t surprising that the most recent gathering of Astronomy on Tap Seattle had a heavy dose of chemistry. It was for a good cause, though, as Trevor Dorn-Wallenstein, a second-year graduate student in the University of Washington Astronomy Department, gave a talk titled “An Unbeerlievable Tale” explaining how the universe made us beer, and a glass to put it into. The event happened July 26 at Peddler Brewing Company in Ballard.

It turns out you only need five elements for beer:

  • hydrogen
  • nitrogen
  • carbon
  • oxygen
  • phosphorous
Trevor Dorn-Wallenstein

Trevor Dorn-Wallenstein talks about celestial beermaking at Astronomy on Tap Seattle July 26, 2017 at Peddler Brewing Company in Ballard. (Photo: Greg Scheiderer)

We had the hydrogen about a millionth of a second after the Big Bang.

“It’s not until 10 seconds after the big bang that we can smoosh a proton and a neutron together and have deuterium and have that deuterium last,” Dorn-Wallenstein said. “Once we have that deuterium though, we’re off to the races.”

If you add a proton to the deuterium you make helium 3, or add a neutron and make tritium. Add the missing nucleon to either and you’ve got helium 4. It’s not in beer, but we’ll need it later. Much later. We have to wait about 1.5 million years, until stars start to form and start fusing new elements. Stars about two times the mass of our Sun can fuse hydrogen into helium, then toward the end of their life cylcles do what Dorn-Wallenstein called the “triple alpha reaction.” They smash three helium atoms into carbon, and add another helium nucleus to make oxygen. When the star reaches its red giant phase these elements blow off with the stellar wind.

“We’ve pollulted the interstellar medium with hydrogen, with carbon, with oxygen,” Dorn-Wallenstein said, “three of the things we need to make beer.”

Higher-mass stars, say ten times the mass of the Sun, can fuse things such as neon, titanium, silicon, sulfur, magnesium, aluminum, and calcium.

“It can unlock all of these additional stages of nuclear fusion,” Dorn-Wallenstein said.

Nickel beer night

The process leaves behind a stellar core of nickel 56 which decays into iron 56.

“Iron 56 is the end of the line for a star,” Dorn-Wallenstein explained. “There is no physical way to get energy out of an iron 56 nucleus. You cant fuse it with something else, you can’t fission it and turn it into two more things, you get nothing out of this nucleus. That’s a problem for a star.”

The outer part of the star collapses onto the core and explodes into supernova, blasting all of the elements it has made out into the interstellar medium.

“The environment around this supernova explosion is so energetic that you can make pretty much anything you want,” Dorn-Wallenstein said. “Pick any element in the periodic table that’s heavier than iron—it’s probably made in a supernova.”

Nitrogen is conspicuously missing from the list, and it is kind of hard to make. Dorn-Wallenstein said we get a little bit, but not enough, from supernovae or at the end of a smaller star’s life.

“The only way to produce enough nitrogen is via this thing called the carbon-nitrogen-oxygen, or CNO, cycle,” he said, explaining that this is how stars produce helium from hydrogen. Since nitrogen takes longer, it builds up in stars. In the universe at large there are about four or five carbon atoms for every nitrogen atom, but in a star that’s doing the CNO cycle there are more than a hundred times more nitrogen atoms than carbon.

“Via this process of converting hydrogen into helium, we actually make nitrogen as a by-product,” Dorn-Wallenstein said.

The beer glass

We’ve got the ingredients for beer. Where do we put it?

“It turns out the most complicated thing that goes into a beer is the glass itself,” Dorn-Wallenstein said, noting that your mug is mostly silicon dioxide, with a bit of sodium oxide, aluminum oxide, calcium oxide, and trace amounts of potassium, magnesium, iron, titanium, and sulfur. All of that stuff came out of a supernovae.

We have all we need for beer. Now we just need a planet to form, simple life forms like yeast to emerge, wheat and hops to grow, and someone to mix it all into a barrel and let it sit for a while.

“Look at that; we’ve made beer,” Dorn-Wallenstein concluded.

Weighing stars

The second talk of the evening at Astronomy on Tap Seattle was given by Dr. Meredith Rawls, who spoke about “Weighing Stars with Starquakes.” Rawls employs asteroseismology—your word of the day!—to figure out the mass of stars.

Rawls

Dr. Meredith Rawls discussed a new method for determining the masses of stars at Astronomy on Tap Seattle. (Photo: Greg Scheiderer)

Rawls noted that one way to calculate the mass of a star is to observe binary systems. We can measure the blockage of light as the stars orbit each other, and the Doppler shift that occurs when they do. Combine those two measurements and you get a reliable measure of the stars’ masses.

The drawbacks, according to Rawls, are that not all stars are part of binary systems, and that this method is slow and uses a lot of limited telescope time. Rawls gets around this by using asteroseismology, measuring the oscillations, or starquakes, that occur in a star’s interior. They actually ring like a bell, though you can’t hear it because space is a vacuum, and the frequency is too low in any case. Like a bell, the more massive the star, the lower the frequency of the oscillation. You can’t see the oscillations because they’re inside the star, but they change the star’s brightness. This is something that can be observed, and astronomers chart brightness changes against the frequencies of the starquakes and see how they line up with other properties of the star.

“You fit a bunch of curves to a bunch of wiggles and you try to convince yourself you’re not making it up,” Rawls quipped. The method can give clues about a star’s surface gravity, density, and temperature, and with gravity and density you can calculate mass.

Does it really work?

Rawls said they like to study red giant stars for a couple of reasons: that’s the eventual state of our Sun, and red giants brighter and easier to see. After figuring masses of many red giants with asteroseismology, they went back and calculated them again using the binary method. Then they compared the two.

“Oh, crap!” was Rawls’s reaction upon seeing how they matched up. “It’s not one-to-one. I broke science!”

In fact, the masses calculated through asteroseismology differed from those returned by the binary method by about 16 percent, on average. It turns out that big, red giant stars are not quite so simply just huge versions of our Sun.

“They have their own weird convection stuff going on,” Rawls explained. “There’s different stuff happening in different layers of the star that isn’t quite the same as what happens inside our Sun, and it’s just complicated enough that you can’t compare them one to one, even though it would be super handy if you could.”

What do they do to reconcile the differences between the two methods?

“We have to apply empirical corrections in order to get accurate masses,” Rawls explained. In other words, “We have to fudge it a little bit! But it’s consistent. It’s fine, it’s fine. Totally works. Not a problem. Don’t worry about it,” she laughed, adding that asteroseismology works just great for smaller stars like the Sun.

“It’s actually really useful, even though sometimes it doesn’t always work perfectly, because you can measure a lot of stars’ masses really fast,” she concluded.

Astronomy on Tap Seattle is organized by graduate students in astronomy from the University of Washington.


Some photos from recent Astronomy on Tap gatherings, and videos of the July talks:

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