The entire universe in blog form

Aug. 23 2014 8:30 AM

Indonesian Volcanoes and Galactic Vistas

Coincidences are funny.

I recently got an email from astrophotographer Teoh Hui Chieh, who was sending me several photos and a time-lapse animation she took near the volcano Mount Bromo in Indonesia. The shots are lovely, as you’ll see in a moment.


That very same day, about an hour or two later, I got an email from astrophotographer Thierry Legault, who was sending me several photos and a time-lapse animation he took near the volcano Mount Bromo in Indonesia. The shots are lovely, as you’ll see in a moment.

But what the what?

Yup. Coincidence. They don’t know each other, but they were both in that location around the same time, and shot remarkable images of the land- and skyscape.

Here is Hui Chieh’s time-lapse animation:

And here is Legault’s:

There’s no losing here; both are gorgeous. I love the shots of the volcano (Mount Semeru) venting steam as the stars whirl above it, and the lights from cars illuminating the fog below. If I saw such a scene in a movie I would have thought it fantasy! But it exists.

The image at the top of this post is from Hui Chieh. I’m so used to seeing star trails in the Northern Hemisphere, it took me a moment to realize that shows the south celestial pole! That’s impossible to see north of the equator, but this scene is at a latitude of about -8°, just south of the equator, so the southern sky’s pole is just barely above the horizon.

You can explore more of Hui Chieh’s photography on Flickr, and Legault’s (who has been on this blog many, many times) at his website.

As for the coincidence, well, they happen. It’s a big world, with countless things happening all the time. People who say, “I don’t believe in coincidences” (usually in the movies, when it so happens that events were not coincidental) in general just don’t have a good grasp of large number statistics. Given world enough, and time, unlikely events occur. Like having two world-class photographers send me pictures from the same spot at the same time. I’ll take it.

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Aug. 22 2014 8:00 AM

The Ring Nebula, Still Mysterious After All These Years

Of all the planetary nebulae in the sky, none is more celebrated than M57, the Ring Nebula. Lying about 2,400 light years away toward the constellation of Lyra, it’s bright enough to be seen in small telescopes, and when long exposures are taken, quite a lot of detail comes out.

Astrophotographer Rob Gendler knows his way around a digital astronomical image. He has been making a habit of creating incredible photographs using multiple observatories, both in space and on the ground, professional and amateur. He took observations from the Hubble Space Telescope, the Large Binocular Telescope, and the monster 8.2-meter Subaru telescope, and combined them to make a stunning image of the Ring. I literally gasped out loud when I saw this:

Ring Nebula
The glorious Ring Nebula, one of the finest examples of a dying star in the sky. Click to greatly embiggen.

Photo by NASA, ESA, and C. R. O'Dell (Vanderbilt University) and Robert Gendler


(You really want to see that in full-res because wow.)

This extraordinary image combined visible light observations with ones taken in the infrared, well outside what our eyes can detect. Generally speaking the inner regions are emitting in visible, and the outer ones (shown in red) are infrared.

Back in the day, it was thought that the Ring was a simple shell, a thick soap bubble of gas cast off by a dying star, illuminated by the star’s fierce ultraviolet emission. Over time, though, we’ve realized it’s more complicated than that. Far more complicated.

Astronomers argue today over the details of the three-dimensional shape of this gas cloud. One group thinks the inner parts are barrel-shaped, and we’re looking down the long axis. Another model is that the inner ring is a much squatter, flatter barrel shape, with elliptical lobes of material poking out the ends. Both models argue that the “flower petals” are lobes of material, like balloons, pointing in slightly different directions, and the very outermost ring is a thin spherical shell, probably gas that was originally outside the star that got snowplowed as the gas from the star expanded and slammed into it. If we could see the Ring from the side, it would look very different; more like the Ant Nebula or possibly M2-9.

ring nebula model
This is a schematic showing one of the models of how the Ring Nebula must really be shaped. What we think of as the ring itself is just a short, barrel-shaped structure in the middle of a larger cloud, and we're looking down the barrel. The caption made me laugh, but really, this is simplified. Nature is complicated.

Diagram by C. R. O'Dell et al, from the paper

It was fascinating to read those papers; I studied planetary nebulae for my master’s degree (and in a limited extent for my Ph.D. as well). Determining their 3-D structures is maddeningly difficult, because we have limited information with which to figure them out. For example, looking end-on at an American football makes it look like a circle. Without knowing the angle we’re viewing it from the real shape may be hard to ascertain. The same sort of thing happens with planetary nebula. You have to really examine subtle details to tease out what the nebula is actually doing.

But I love this! Here is one of the loveliest and best-examined objects in the sky, and yet we’re still trying to figure out its exact shape. The thing is: We can. It’s possible. We just have to keep using new methods to observe it, use all the tools we have to dissect it, and lay out all these pieces of evidence to reassemble them into a picture we can understand.

Images like Gendler’s help this along, giving us a deep overview of the Ring. Over time, we’ll get to know this gorgeous example of a dying star even better, and from there gain a better understanding of how stars like our own Sun will die. It’s all part of science, and the joy of trying to grasp the Universe.

Aug. 21 2014 11:30 AM

Impact for a Lifetime

Talking to the public about science and rationality is an odd way to spend your life.

It can be maddening, when you go against those who oppose reality: homeopaths, anti-vaxxers, global warming deniers, young-Earth creationists. For those of us who love the Universe the way it is, going up against that tsunami of nonsense is a grueling and unending task.


Worse, in some ways, is wondering if what you’re doing is actually helping. Sure, I get traffic on my blog, or retweets, or “likes” on various social media. But those metrics are ephemeral, fleeting. Are there any indications of a more permanent effect?

Yes. Yes, there is. I recently got a note from a woman named Kristin Ormiston on Facebook. I know this sounds a bit self-serving, but bear with me. She had this to say:

Hi! I just wanted to message you to tell you I think you're amazing and inspiring! And I was encouraged to send you these pictures of the tattoos on my ankles, inspired by a quote of yours.

The pictures are these:

skeptical tattoo
"Teach a man to reason; he'll think for a lifetime."

Photo by Kristin Ormiston, used by permission

Oh. Wow. If you’re looking for some sense of permanence in what you do, someone getting something you said tattooed on herself is a pretty good sign.

The art in her tattoo represent queen chess pieces, with lotuses on top. As she told me, “The queen chess piece has the most freedom of any piece on the board. Intelligence and seeking knowledge is what I consider true freedom.”

That’s a pretty good quote, too.

The quote from me—“Teach a man to reason, and he’ll think for a lifetime”—is from my “Goals of Skepticism” talk I gave a few years ago. It’s had a bit of limelight; it was a Reddit banner, featured on a coffee mug, and used in a lovely Symphony of Science video (where I also give the quote’s context).*

I liked the phrase when I came up with it, but had no idea it would resonate so strongly with so many people. I’m glad it did, and I still stand by the idea. After all,

The mind is not a vessel to be filled but a fire to be kindled.

Plutarch said that, nearly 2,000 years ago. We’re still learning, still struggling, to do as that phrase teaches, but the fact that we still know it after all these centuries is a sure sign of its impact.

*As I mention in those earlier posts, the sexist phrasing of the quote bugs me. But it’s based on the aphorism, “Give a man a fish and he’ll eat for a day; teach a man to fish and he’ll eat for a lifetime.” I already was changing the phrasing, and if I changed “man” to “person” it seemed to me that the connection to the original aphorism would be lost. In the end I kept the original phrasing to keep the connection apparent.

Aug. 21 2014 8:00 AM

Dating a Star ... a Few Hundred Thousand, in Fact

Globular clusters are too cool. For one thing, they’re gorgeous. I have proof!

IC 4499: A globular cluster’s age revisited
The spectacularness of the globular cluster IC 4499. Click to enapiarianate.

Photo by ESA/Hubble&NASA

That is IC 4499, a tight ball of tens or hundreds of thousands of stars located roughly 60,000 light years away. This image was taken by Hubble, and besides being spectacular, it also was used to nail down the age of the cluster, which until recently has been a bit controversial. This is another reason globulars are cool …


Getting the age of the cluster is possible because globulars have a very helpful characteristic: The stars are all the same distance away. That means if a star is brighter than another in the cluster, it really is more luminous. That makes comparing the stars directly to each other easier.

At first it was assumed that all globulars are very old—as old as the Milky Way itself, 12 billion years or so—and that all the stars in each were born at the same time. But it gets a bit more complicated. Some, it turns out, clearly have stars that are old, mixed in with ones that are younger. The thinking is that these clusters are more massive, could draw in more gas over time, and then could have a second bout of star formation after the initial one.

The trick to getting the age for a cluster is that stars age at different rates. More massive stars burn through their nuclear fuel faster, so they run out before their smaller, more miserly brethren. When that happens the core of the star contracts and heats up, and the outer layers respond by inflating hugely, like a hot air balloon. The heat from the interior gets spread out through the much larger surface area, so weirdly the star gets much brighter but also much cooler. We call it a red giant (or a red supergiant if the star is particular massive).

IC 4499: A globular cluster’s age revisited
Detail in the cluster (taken from the right side of the image above). Note how many faint stars can be seen ... and far more distant background galaxies can be seen right through the cluster!

Photo by ESA/Hubble&NASA

That’s the key. If you measure the stars’ colors, the ones that have run through their fuel and turned (or are currently turning) into red giants become very obvious. Theoretical models are pretty good at showing just how old the stars are that are right at that point in their lives, so that in turn must be the age of the cluster.

IC 4499 has always been a problem here. It has an intermediate mass between the lower-mass globulars that have a single population of stars and those heavier ones with two stellar populations. Knowing its age would be very helpful to nail down the difference between the two. Different studies have come up with different ages for it, with pretty large uncertainties, too.

The good news is that the Hubble observations easily cover the stars that are starting to turn in IC 4499, and the telescope’s ability to accurately resolve all the stars really nails down the age: IC 4499 is 12.0 ± 0.75 billion years. It’s old.

This helps. Astronomers like to study extremes, since that tells us what physics is doing at the edge of what it can do. But we also need to figure out intermediate cases, too, if we’re ever to have a fully filled-in picture of what’s really going on in the Universe. IC 4499 is another piece of that puzzle for which we’ve managed to find its place.

Aug. 20 2014 11:45 AM

Our Curiosity

I haven’t written about our laser-eyed nuclear-powered red-planet roaming friend in a while. But the Curiosity rover recently celebrated its second year on Mars (which is really just over one Martian year on Mars). It’s still rolling along—literally—heading for its ultimate target: Aeolis Mons, aka Mount Sharp, the 5.5 kilometer-high peak in the center of Gale Crater, Curiosity's landing site.

Of course, it’s finding a lot of fun souvenirs along the way, and doing amazing science. Caltech put together a nice video as a retrospective of the past two years, as well as a look forward to what it will do soon.


It’s narrated by a couple of folks who might sound familiar, too. I won’t give it away (but it’s on the YouTube page show notes).

Nice. I have a public lecture I give about Curiosity, and it’s one of my favorites to give of all time. The human effort that went into building, launching, landing, and using this machine is nothing short of Herculean, and still it marches on. It’s a triumph, and I quite seriously choke up during the talk every time. I’m so proud of what we can do.

Congratulations to everyone who is involved with Curiosity and also to those who are building the 2020 rover too. Whenever I see Mars in the night sky, I see it as more than just a red dot: It’s an entire world, and one we’re just now starting to explore.

Aug. 20 2014 8:00 AM

Did I Say 30 Billion Tons of CO2 a Year? I Meant 40.

Every now and again a global warming denier will say that humans aren’t putting much carbon dioxide into the air, and it’s less than a lot of natural sources. I’ve pointed out that in fact, humans throw about 30 billion tons of it into the atmosphere every year, 100 times as much as volcanoes do. I got that number from a paper published a few years back.

Well, I just found out that paper is out of date. Guesss what the more accurate, current number for the human-made CO2 pollution put into the air every year is?


40 billion tons.

Yeah. 40. As in billions of tons. 40.

That number comes from an assessment made by Le Quéré et al. in a paper measuring the total carbon budget for the planet in 2013. I found out about this new, updated number when I wrote about the launch of NASA’s Orbiting Carbon Observatory, or OCO. I mentioned the older number in the post, and got an email from David Crisp, OCO’s science team leader (!) correcting me.

It’s almost impossible to grasp what 40 billion tons means. A cubic kilometer of water weighs a billion tons, but it’s hard to imagine a cube 3.5 kilometers (2.2 miles) on a side—the equivalent amount of water weighing 40 billion tons. The largest class of aircraft carrier weighs about 100,000 tons, but picturing 400,000 of them still strains the imagination (not to mention vaporizing them into the air every year).

It’s a vast amount.

Funny, though, it’s small compared with the total mass of the Earth’s atmosphere, which is a staggering 5 x 1015—5 quadrillion— tons! By mass, this extra CO2 is only about 0.0008 percent of the Earth’s total air.

But it adds up. By volume, CO2 is about 400 parts per million (ppm) of the atmosphere, or about 0.04 percent. How much are we adding? Looking at the Keeling Curve, which measures the concentration of CO2 in the air, we’ve added about 20 ppm in the 10 years from 2000 to 2010—that’s 2 ppm per year.

keeling curve
The Keeling Curve, measuring atmospheric carbon dioxide. The trend is pretty obvious.

Graph by the Scripps Institute of Oceanography, UC–San Diego

So, that 40 billion tons of extra carbon dioxide we dump into the air every year is accumulating at a rate of 2 parts per million when you look at the entire atmosphere.

That sounds pretty small. But is it?

I wish. For some things, it doesn't take a lot to make a huge difference. As doctors will tell you, dose makes the poison. Four hundred ppm sounds like a small amount, but this concentration of carbon dioxide in our atmosphere is enough to raise the average temperature of the Earth significantly … and we’re putting more of it into the air every year.

Forty billion tons worth. That’s why the planet is heating up. That’s why the climate is changing. That’s most likely why we’re seeing more extreme weather, hurricanes getting stronger, droughts drier, forest fires more intense.

This is our fault, and it’s also most definitely our problem. There is hope … but we need to get our heads out of the sand together and take action. Now.

Aug. 19 2014 11:54 AM

Venus and Jupiter, Together

Over the past few mornings, the fourth and fifth brightest objects in the sky have been getting cozy. Venus and Jupiter are very close together just before sunrise, and have made for some lovely photo ops.

The planets orbit the Sun in roughly the same plane; if you looked at the solar system from the side it would look fairly flat. To us on Earth, in that plane, the planets all stick more-or-less to a line that circles the sky, which we call the ecliptic. It’s not perfect; some planets have orbits that are slightly tilted, so it’s not exactly beads on a string. But sometimes they do pass pretty close together, and it makes for a striking sight.


Jupiter is currently on the other side of the Sun from us, more than 900 million kilometers (almost 600 million miles) away. Venus is also on the other side of the Sun from us, but much closer: 240 million km (150 million miles) away. Jupiter is a far bigger planet, but Venus is closer and more reflective, so it appears the brighter of the two.

The photo at the top of this post is a part of a larger portrait of the pair as they appeared on the morning of Aug. 18, 2014. It was taken by Jerry Lodriguss in New Jersey, from a cranberry bog (which explains the light mist covering the ground). He also took the photo below:

Venus and Jupiter
A closer view reveals Jupiter isn't alone with Venus; two of its moons can be seen as well (if you have a zoom lens, binoculars, or a telescope). Note the airplane and contrail to the left.

Photo by Jerry Lodriguss, used by permission

You can see two moons of Jupiter (Io and Europa) just above it and to the right, as well as an airplane zooming by to the left. Make sure to take a look at a time exposure Lodriguss took of the two rising; they appear as long, colored streaks moving up from the horizon

Due to the complicated dance of orbits of the two planets (and our own Earth, changing the perspective on the pair), Venus is moving toward the Sun in the sky, rising later every morning, while Jupiter is slowly pulling away form the Sun, rising a bit earlier. The two are still close together for the next few days, so if you’re an early riser it’s worth getting up just before sunrise to look for them. Simply look east, but you’ll need a good view of the horizon; they’re not very high up.

Oh—so if they’re the 4th and 5th brightest objects in the sky, what are the first three? The Sun, of course, and the Moon … and No. 3 is the International Space Station, though to be fair it is only sometimes brighter than Venus. No matter; while the ISS is fun to watch, there is something about looking at a planet in the sky and knowing it’s an entire world, hanging there for us to see. It’s very much worth an early morning to catch a glimpse.

Aug. 19 2014 8:00 AM

Titanic Weather

In the Northern Hemisphere it’s summer. That means warm sunshine, gentle breezes, lake water evaporating and forming lovely, puffy clouds …

Oh. Did you think I meant here? No, no, no. I meant Titan!


Titan is Saturn’s largest moon, and it’s actually bigger than the planet Mercury. It has a thick atmosphere of nitrogen, with a surface pressure half again as much as Earth at sea level. It’s cold there, about -180° C (-290° F), cold enough that water ice is hard as rock, and methane is a liquid on the surface.

In fact, methane there is like water here: It can be a liquid or a gas, depending on the local conditions. One of the Cassini mission’s biggest surprises, and totally cool discoveries, is the presence of lakes of liquid methane near the moon’s north pole. Lots of channels line the surface, too, indicating that methane must evaporate from the lakes, condense as clouds, then rain out, running along the channels.

And now, with the onset of northern summer, Cassini has taken images of clouds forming over a lake named Ligeia Mare (Ligeia was one of the mythological sirens), and these images form one of the most amazing animations I’ve seen from the outer solar system:

Wow. The lakes are the dark regions, and the clouds form over the western edge of Ligeia Mare, blowing east (right). The winds are moving at about 12-16 kph (7-10 mph); breezy but not too bad. The wind chill is like minus a bazillion, but if you’re Titanian I’m sure it would feel refreshing.

These clouds have been anticipated by planetary scientists for some time, and have frankly been overdue. In late 2010, a huge storm swept over the equatorial region of Titan, and for some reason cloud formation has been suppressed since then. These clouds may mark the return of normal weather on the ridiculously huge moon.

Titan storm
Said storm, the white arrow-shape on the left. The front was thousands of kilometers across.


It’s really odd to think of a moon having weather, especially one well over a billion kilometers from the Sun, where the temperature would nearly liquefy the oxygen out of our own air. But there you go. And it’s not just weather, but really in many ways it’s quite Earth-like. Methane lakes, a methane cycle, lots of organic compounds lying around … it’s no wonder a lot of scientists take the idea of exobiology seriously on Titan. Cassini has sent us a lot of amazing data, and a lot of tantalizing clues. I’d love to send a dedicated probe there, something that could land, move around, taste the local flavors. A few years ago it would’ve been the last place I would’ve thought to look for extant life, but nowadays it’s near the top of everyone’s list.

Aug. 18 2014 11:33 AM

The Fiery Final Descent of a Spaceship

On Aug. 17, 2014, the uncrewed Cygnus resupply ship Janice Voss burned up in Earth’s atmosphere, as planned. An astronaut on the International Space Station took this amazing shot of the cargo ship as it re-entered, breaking apart as it burned up.

Aug. 18 2014 8:00 AM

How Do You Weigh a Helium Balloon?

My friend Jenny Lawson—aka The Bloggess—is weird.

This isn’t casting any aspersions! She would be the first one to admit it. In fact, she does, constantly, on her blog. If you read her book, Let's Pretend This Never Happened, you'll find this assessment is ironclad.


For further evidence: Just the other day, she was in a conversation with her husband, Victor.* She asked him, “What weighs more, five pounds of helium, or five pounds of cheese?” She wrote up the exchange on her blog, and tweeted a link it. I laughed when I saw it, and immediately replied:

She replied:

To which I made the obvious rejoinder:

It only occurred to me recently to wonder about filling the Swiss cheese holes with helium.

But in reality, I do know the answer. This question is an old variation on the riddle, “What weighs more, a pound of lead or a pound of feathers?” The answer is neither: They both weigh a pound. But I remember hearing this riddle when I was but a wee lad, and being momentarily baffled. We humans sometimes confuse weight and density; lead is very dense, but a pound’s a pound the world around. A pound of feathers would take up a lot more space, but it would still weigh a pound.

But in the case of Jenny’s question, we get even more confused. After all, helium floats! If you had a balloon full of helium, and tried to weigh it on a scale, it would float away. If you could somehow tie it to the scale—and you had a scale with the decidedly odd characteristic that it could measure numbers less than zero—it would say the balloon has negative weight!

But that’s not really the case. Here’s the answer: A balloon filled with five pounds of helium would weigh exactly the same as a five pound block of cheese.

How can this be? Ah, let me explain.

What we think of as weight is really a force—the acceleration due to gravity acting on our mass. Mass is a property of matter that is independent of weight. I have a mass of about 80 kilograms, and I would have this same mass on the Earth, the Moon, or the surface of a neutron star. It’s basically telling you how much matter makes up me.

cheese balloon
It's surprisingly difficult to find a stock photo of a balloon and cheese.

Photo by Shutterstock/Olga Ivanova

On Earth, with its one Earth gravity, I feel that mass as a weight. The Earth pulls down on me, and if I step on a scale it says I have a weight of about 175 pounds. If I were on the Moon, with 1/6th the Earth’s gravity, I’d weigh just shy of 30 pounds. My mass is still the same (80 kilos), but there’s less gravity pulling on me. I weigh less!

All matter has mass. Even helium. So let’s say I have some amount of helium—borrowing from Jenny, call it 2.3 kilos. On Earth, that would weigh five pounds. Earth’s gravity still pulls on that helium, and given that mass, the force works out to be five pounds.

But the difference between helium and cheese is that helium is very low density, lower than the air around it. That makes it buoyant. Buoyancy is a force too, like gravity, but it works in the opposite direction: It pushes things up. It only makes objects float if they are less dense than their surroundings, like a boat on water, or a helium balloon in air.

Buoyancy works due to displacement. If I take a chunk of iron a centimeter on a side (the size of a six-sided die) and drop it in water, it sinks. The amount of water it displaces (pushes aside due to its volume) weighs less than the iron does, so down the iron goes.

But if I spread the iron out, make it bowl-shaped, it can actually displace a lot more water. If it pushes out the amount of water equivalent to its own weight, then it’ll float! The water it pushes out wants to flow back under it, and that is a force pushing up on the iron. When the water displaced weighs the same as the iron, the two forces balance. The iron floats. That’s how big steel ships float; they’re spread out, and they displace their own weight in water.

In the case of a balloon, the helium inside the balloon weighs less than the same volume of air the balloon displaces. This air outside pushes on the balloon, and up it goes!

Jenny Lawson
I asked Jenny for a picture of her holding a balloon. She sent me this. I love her.

Photo by Jenny Lawson, aka The Bloggess. No balloons were harmed in the making of this photo. Presumably.

So a helium balloon has two opposing forces acting on it: gravity pulling it down, and buoyancy pushing it up. Buoyancy wins, so you can’t really weigh a balloon, even though it does have weight.

Weird, eh? Think of it this way: If you could put a balloon with five pounds of helium in it in a vacuum chamber, then there’d be no buoyancy force pushing it up. Only gravity acts on it, so if it sat on a scale it would register as five pounds!

Tadaa! See? A five pound balloon weighs the same as a five pound piece of cheese. It would just be less tasty.

Still, I started thinking … how would you weigh a helium balloon? How could you know it weighs five pounds if you can’t weigh it on a scale?

I can think of a few ways. (Note: For all these methods, I will ignore the weight of the balloon material itself, which is hopefully small compared with five pounds.)

For one, you could look up the density of helium in a balloon, then multiply it by the volume you’d need so that it weighs five pounds. In air, helium has a density of 0.2 kilograms/cubic meter, so you’d need about 11 cubic meters of helium to weigh five pounds, which would be a balloon about three meters across. That’s a lot of helium. In reality it would be a bit smaller, since the pressure inside a balloon is higher than the air around it, but eh. Close enough.

Jenny Lawson and Phil Plait
Jenny and me, in the days before cheese and noble gases would so affect our lives.

Photo by Phil Plait

I thought of another way, which is to mount a scale upside down on the ceiling and let the balloon rest against it. But then I realize that won’t work; the force of buoyancy pushing it up is actually greater than its weight, that’s why it keeps floating up when you let it go outside (which sometimes leads to another inexorable force of nature, the force of a child crying).

But then I realized you could let a balloon go outside. It’ll float up, getting higher and higher, where the air is thinner (less dense) until eventually the weight of the air displaced by the balloon is equal to the weight of the helium in the balloon. At that point it will stop rising. You can then determine the altitude of the balloon (perhaps using a few spotters with known locations, and a bit of trig as they measure the balloon’s angle in the sky). Then look up the known density of air at that height, and the volume of the balloon (oops: measure that before you let it go) to get the weight of the air displaced. That’s equal to the weight of the helium! Well, more or less; the balloon will expand as it rises, but in principle you could find the helium weight this way. It’ll also be very high up, probably a few miles, but in principle this would work.

Still, that’s pretty complicated. A much easier and practical way is to buy a big tank of helium. Set it on a scale and weigh it. Now grab a balloon (a big one) and start filling it. When the scale says the tank has lost five pounds, you’re done! Note: My wife thought of this one, and then scolded me for making this too complicated. I suspect my own marriage is much like Jenny’s.

There are other ways, too, but they get a bit impractical (like cooling the helium until it’s a liquid, no longer buoyant in air, and then putting it on a scale; The problem with this is helium liquefies at 4 Kelvins, or -269° Celsius, which is -452° Fahrenheit. That’s a tad pricey and difficult of a procedure, as well as incredibly dangerous to do at home).

But the point is, it’s entirely possible to have a five pound balloon of helium. Mind you, it’ll lift a lot more than five pounds; a balloon that size can lift about 25 pounds. You could tie the cheese to it and it would still float away! That’s a waste of a balloon, and cheese. I don’t recommend this either.

So there you go, Jenny. I told you I knew the answer to this, because science. In fact, I even tweeted it:

*I sometimes wonder if she makes these conversations up for the sake of humor, but then—having met them both—I rid myself of that outlandish notion.

Update, Aug. 18, 2014 at 14:30 UTC: I originally said buoyancy only works on objects less dense than their surroundings, which was somewhat sloppy of me. It works on any object surrounded by a less dense medium, too; it's just that in that case the object won't float. I tightened up the language here to clear that up.