Baby’s first pulsar cake

Gather ’round, folks, as I tell you the tale of a pulsar cake. It is a tale full of wonder and magic and sugar.

Every year at Goddard, there’s a big poster party where astrophysicists mingle with planetary and Earth scientists. Most days of the year, we completely forget the other group exists, except to grumble when they get more funding than we do. Or when they put a paper called “Radar Love” up in the hall by the elevators and you can’t walk by it without getting that song stuck in your head. Oh there it is again. Awesome.

Part of the poster party is a “Science as Food” competition, and we decided to enter this year. We went through a couple of different ideas before settling on a pulsar cake. First, a disclaimer: I am not a pulsar scientist, so I apologize if I get any of the following wrong. Anyway, here is the diagram we were working off of:

A pulsar is a rapidly spinning neutron star which emits radiation. The yellow circle in the center of that image is a neutron star. Neutron stars are some of the densest objects we know of; I think a common example is if you take the Sun and compress it until it’s as dense as a neutron star, the whole thing would fit inside a typical US city. They are created when a massive star gets to the end of its life and — skipping some details and subtleties here — starts collapsing under its own weight. The star’s atoms are then subjected to so much pressure that the electrons and protons in the atoms combine into neutrons (and neutrinos), and what you’re left with is a neutron star. If the neutron star has a strong magnetic field, then as it spins, it emits light, and it does so at different wavelengths.

In the image above, the yellow circle is the neutron star. The green cones are the emission in radio waves. The thin, curved lines are magnetic field lines. The bright purple haze is our best guess as to where the gamma rays come from. This image is actually a still capture of a sweet looking video (Figure 7 on this page). If you want to have more fun with pulsars and see what sorts of pulsars Fermi has detected, try the Fermi Pulsar Explorer.

Now that you’ve had some time to admire the science, it’s time to talk about food.

We decided to make a hemisphere cake, set on a rotating cake stand. (We hot glued a large circular IKEA mirror to the cake stand to give us more surface to work with.) After all, pulsars spin. We went with the same vanilla cake and Swiss buttercream frosting that we used in the Fermi four-year cake, because we knew the vanilla cake would give us something nice and dense to work with. We thought about purchasing a dome-shaped cake pan, but since no one really wanted to buy a dome-shaped cake pan, we made two layers instead: The bottom layer was baked in a 10-inch round cake pan, and the top in a Pyrex bowl. If you decide to recreate our efforts and make your own pulsar cake (or, I suppose, any hemispherical cake), make sure you grease and flour the Pyrex cake well, and use a maximum temperature of 300 deg F.

Despite our best efforts and intentions, the top of the cake remained stuck to the bowl when the rest of it was wheedled out. We gently detached the top and plopped it on the rest of the dome. (Frosting hides all sins, after all.) Then we wrapped the pieces in plastic wrap and stuck them in my freezer for a few days.

Speaking of frosting — we doubled the buttercream recipe for a 9-inch cake, which should have called for 6.5 sticks of butter, except my colleague D was so entranced with his first time making a meringue buttercream that he forgot to count and put in all 8 sticks of butter, which he didn’t realize until I asked him for the extra stick and a half of butter. It still tasted delicious though.

The hemispherical shape proved tricky to create, mostly because I made the rookie mistake of not trimming off enough. I’ve found that, if you really want your cakes to have nice shapes, you have to be OK with trimming a lot off. Otherwise your flat cakes end up domed, and your dome cakes sort of flat at the top.


Luckily, we had made a lot of buttercream (thanks to D’s aforementioned miscounting), and ultimately I was able to coax it into a moderately hemispherical shape.


Close enough.

There had been some discussion as to how we should color the cake. J’s officemate helpfully chimed in with, “Just make the surface look like a cracked plasma.”


After much discussion, we came to the conclusion that a “cracked plasma” looks like pieces of sugar glass. We made some poured sugar and tinted it red (and put in a bit of pomegranate flavoring), then cracked it and placed it on the surface of the “neutron star.” The poured sugar solution is basically the same as the pulled sugar solution (described in the next paragraph), except poured out into a thin sheet and cooled.

We decided to use pulled sugar to make the magnetic field lines, so that they’d be sufficiently light but still sturdy and decently hard to break. We set about making pulled sugar, which we had practiced a few times, but it turns out that making pulled sugar is reeeeally hard. There are lots of “recipes” out there, but essentially, you’re heating a sugar solution until the hard crack stage (approx 305 deg F) and then folding air bubbles into it as it cools. Glucose (corn syrup), maltose, or other sugars are generally added as well to prevent the sugar from recrystallizing. Cream of tartar, vinegar, or other acids are often added in small amounts to induce the sugar molecules to break up. I say all this, but I’m afraid I can’t cite any sources. I’m sorry for being a sloppy scientist.

We ended up using a recipe from this forum, mostly because it had easy-to-remember amounts. Based on the varying ratios that other sources suggest, it seems to me that the exact amounts are not important, so long as you 1) have enough water in which to dissolve the sugar, 2) add less corn syrup than your sugar (like a quarter), and 3) use like a teaspoon of your acid. Adjust accordingly if you’re making a tiny or enormous batch. Heat the sugar and water to 285 deg F, add the corn syrup and acid (and coloring, if you like), then heat to 305 deg F. (The sugar solution will hover around 220 deg F or so for a while as the water boils off; be patient, and the temperature will start to increase again.)


Since we only had a small surface to work with, we made a small batch of sugar at a time: 8 oz granulated sugar, 4 oz water, 2 oz corn syrup, 1/2 teaspoon cream of tartar. It is recommended that you get your hands on a marble or granite slab to work with, as the slab will absorb a lot of the heat from the hot sugar and let things cool down faster. It is also recommended that you invest in a Silpat or alternative. Otherwise, this might happen …


… and you might be stuck stabbing at pieces of hardened sugar with a knife until you can get it all off.

Anyway, let’s assume you’re smarter than we were the first time around (really, it was my fault, I insisted we could just grease the slab and we’d be fine). You should also make sure you wear some gloves! Partly because you don’t want to get your gross oils and bits of skin into the sugar, and also because everything is super hot and the gloves will help a little. You will in all likelihood get burned by a piece of sugar stuck to your glove. If that happens, peel the glove away from your skin so that you don’t maintain contact with the molten sugar. (I mean, you should really take the glove off, but the immigrant in me decided she would rather get minor burns on her hands than waste a perfectly good plastic glove. I stand by my decision.)

OK, so assuming you’ve done everything right — you’re still working with molten sugar. As the sugar slowly cools, you’ll want to maneuver it around on the slab like some sort of floppy dough. Just watch some YouTube videos of it. It’s amazing to watch the master sugar artists work with it, and there’s nothing quite like seeing the sugar become opaque and shiny as you incorporate more and more air into it.

At this point, most of the sugar work was done. However, we were still missing the areas of gamma ray emission, marked in the first image as a purple haze. We decided to use cotton candy in a bright purple/magenta color to display these regions. Now, the problem is, cotton candy is generally not sold in bright magenta. Our initial solution was to make cotton candy by hand, as various people on the internet — including Alton Brown! — said it was possible. We even set up a makeshift cotton candy collector using some old textbooks I dug up.


That’s a lie, I didn’t have to dig them up; I knew exactly where they were, and even if I didn’t find these ones in particular, I still would have been able to easily grab any of about thirty other thick hardcover textbooks, because that’s just how I roll.

Anyway, people on the internet will tell you it’s possible to make cotton candy entirely by hand.

These people are dirty liars.


You WILL make a mess. You WILL end up flinging hot sugar everywhere, and it WILL solidify into rock-hard little droplets. You will end up with a pile of hard, craggy, sharp birds nests of sugar. You will most certainly not get airy, cottony fluffs of sugar. You will then curse a lot, and you might ultimately decide to bite the bullet and purchase a cotton candy machine off of Amazon.

Coincidentally, that is exactly what happened to us. J bought the simplest, cheapest cotton candy machine she could find, and once it came, she turned into our resident cotton candy expert.


We began by testing out granulated sugar. There was some talk on the internet that the best sugar to use for cotton candy machines is “floss sugar.” However, we were not entirely convinced that this is not just a marketing ploy, so we used granulated sugar, which worked perfectly fine.


In order to get a nice purple cotton candy color, J mixed sugar with some gel food coloring and then set it out to dry for a few days. We ended up with a nice purpleish color of cotton candy.

Unfortunately, we did not account for the fact that the weather on the day of the poster party was abnormally hot for January — highs in the upper 60s, and humid as Maryland always gets. What started off as nice fluffy cotton candy in the bag quickly became thick lint-like pieces that slowly dissolved as the day progressed.


And this, folks, is baby’s first pulsar cake. In our heads, we had envisioned a grand creation of sugar mastery. In reality, we created some sort of warped spider with a lint problem. Some people seemed a little taken aback by our technicolor creation. However, once the elements were explained to them (very few people there were pulsar scientists, after all), they all seemed to understand and appreciate our cartoony representation.

In fact, we won the contest! … against one other entry. Nonetheless, we were awarded with a plaque with our names on it, two bags of candy, and a gift card to Amazon which we are putting into the Fermi cake fund.

I’m slipping a copy of that plaque into my thesis.


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