Obama said scientific decisions should be based on facts, not ideology. He said advisers should be selected based on their credentials and experience, not their politics.

Now, there are very vocal opinions on both sides of this issue, and as you would suspect, I think that in matters of scientific research, every scientist must live with their own conscience, but that science exists to serve humanity, and the research we’re doing will doubtlessly serve to have a positive impact on mankind. Once one major cure or treatment comes out of stem cell research, whether it’s for cancer, Alzheimer’s, or diabetes, I think that the debate over stem cell research will subside when it’s clear that the benefits to doing it can be so substantial.

But instead of looking at it from an economic perspective, or even from an ethical perspective (I’m still tired from doing that last week), I’d like to look at the deeper, underlying issue: whether people can come closer to realizing truths about existence from either faith or science. I also think it’s worth asking yourself, at the end, whether there’s even a right or wrong answer to this.

So rather than talk about stem cell research, I’m going to address perhaps the most interesting question of all: the very question of our Universe’s existence. My friend Brian has a cousin who is a baptist minister, and today’s question comes from him, via YouTube:

Did the Universe have a beginning? …it seems that most cosmologists do believe that the Universe had a beginning. If this is true, and you believe this, then please explain how you, in your mind, resolve the idea of something coming from nothing, uncaused.

Now, let’s start with a little scientific information about the Universe itself. We turn our eyes, telescopes, detectors, instruments, and brains towards the tiniest subatomic particles and to the farthest reaches of the heavens to learn about it, and to listen to what it tells us about itself. (Semantics: I’m going to define our Universe, for this discussion, as consisting of every single particle and every little bit of space and time ever conceivably connected to us, either visibly or invisibly.) Here’s a basic rundown:

1. The Universe is big. It isn’t infinite, though, it’s finite in size, at about 94 billion light years from end-to-end.
2. The Universe is old. If you took the longest-lived thing ever on Earth and gave it 10,000 lifetimes in a row, it would be only about 2% as old as our Sun and Earth are. The Universe is “only” about three times as old as our Solar System is.
3. The Universe has a lot of stuff in it. About 1,000,000,000,000 stars in each galaxy, and about 100,000,000,000 galaxies in the whole Universe. Considering each particle of matter and energy as a separate entity, there are about 10⁹⁰ particles in the entire Universe.

All of this, combined, tells us that our Universe is tremendously large, tremendously old, and full of an incredible amount of stuff. It is truly vast. However, all of it is finite, including the amount of information in it. Does that imply that there’s an intelligent force outside of it that created it?

No. It doesn’t tell us that this isn’t the case, either. But it does tell us something profound about science; specifically, it tells us something about the theoretical limits of science. Let me give you an analogy, the exploding grenade:

If you watch the individual fragments of a grenade during (or even after) an explosion, because you know the laws of physics and how a grenade works, you can figure out where the grenade exploded, how powerful the explosion was, and what the grenade was made out of just based on what you see. You can even tell, if you’re extremely careful and understand the physics of grenade explosions really well, how quickly and in what direction the grenade was moving when it exploded.

But what you can’t tell, based on looking only after the fact, is how the pin got pulled, and by whom or what.

So, my contention is that if we want to know about the Universe, the best source we have is to look to the Universe itself, and see what it tells us. But if we want to know what caused the Universe, although there are things we can definitely learn a number of things about it, our total amount of possible scientific knowledge is limited by the amount of information available. For instance, we are mounting evidence and may be able to someday prove that cosmological inflation caused the Big Bang to happen, and created our Universe. But then you could ask, “what caused or created the inflating Universe that gave rise to ours?” Again, we can come up with some ideas about it, and possibly some signatures to look for (although there are presently no good ideas), but at some point, you run out of information. It isn’t that something came from nothing; it’s that something came, period. We simply don’t have enough information to say that it came either from nothing or from something else. And we certainly don’t have enough information to determine what the dynamics were that caused it to come into being in the first place.

When you have no information, science is useless. And at that point, all you have left is logic and reason, and those are your only weapons against the darkness of ignorance. Science can get us far, can get us so much further than we’ve ever been before, but even science has its limits. And hence, to the many atheists that read this site, I encourage you to respect the religious beliefs of others while steadfastly standing by the scientific truths that we have discovered to be valid and factual.

We ought to all be above petty bickering here, as I think everyone is seeking to understand our Universe’s very existence, and hence we must use all of the available tools. I encourage you to recognize that religious studies can be a logical, rational pursuit as well, and that both science and religion have limits to the truths they can uncover. This one in contention here — what brought the Universe forth into existence — is still obscure to all of us.

Well, our reader Richard is way too clever to just believe what I have to say. He took a look at what neutral hydrogen actually does in terms of absorbing visible light, and looked at this image I posted:

And perhaps you’ll notice the same thing that he did: the amount of light that gets absorbed is tiny compared to the total amount of light! So what he wants to know is really reasonable (and I’m paraphrasing here):

How does neutral hydrogen, which only absorbs very select frequencies of light, block out all of the light coming from distant stars and galaxies?

Well, there are three major effects that allow hydrogen to absorb pretty much all the light you were going to see, and if you want a really technical explanation, I recommend you go here. Let’s go through all three of them, remembering that hydrogen will only make these tiny absorption lines if we didn’t have these three effects:

1. Hydrogen gas moves. Because atoms don’t stay still, they move. The atoms that move towards the light absorb a slightly lower frequency of light, the ones that move away from it absorb a slightly higher frequency. From a distance, the gas looks stationary, but in reality there’s always plenty of it moving both towards and away from a given object:

This is important, because it causes these “narrow absorption lines” to broaden. The faster the gas moves, the broader the lines get. So for gas that moves very quickly, a tiny absorption line can kill a huge amount of the spectrum:

2. The neutral gas absorbs the light along the entire journey. Astronomers often measure how far away things are by their redshift, meaning that the Universe is expanding, we know its expansion rate, and so if you measure how fast something is moving away from you, you know how far away it is. This is incredibly useful. But it also changes the frequencies that get absorbed. As the light travels to you, hydrogen gas in different places absorb light, but then the light gets redshifted:

So if I’ve got hydrogen gas at three different spots along the light’s journey to my eye, it’s going to make three different sets of absorption lines:

The combination of these first two effects, broad absorption lines happening at many different redshifts, is enough to render these distant galaxies invisible. But there’s a third effect that we see, too, that could also play a role.

3. Gas also scatters light. In addition to absorption, neutral gas is very good at scattering light. You’re probably used to seeing clouds do it in our atmosphere:

But plain old neutral gas can scatter light in space the same way! And just like clouds, they obscure everything behind them. Take a look at this galaxy, with a huge cloud of neutral gas (mostly hydrogen) in between us and them:

Fabulous! I mean, really, this is science in action! And so the next time someone wants to know how something as simple as hydrogen gas can block out anything in the Universe, you’ve got not just one, but three reasons to hit them with!

Well, we kind of know what’s going on around us now, and thanks to powerful telescopes, we can see way back to what the Universe looked like back when it was much younger. The farthest galaxies in this picture, for instance, come from when the Universe was only 700 million years old or so:

In our calendar analogy, that’s looking back when the Universe was only about 5% of its current age, or January 19^th-ish. Want to see farther back? Want to see the Universe on January 10^th, or January 4^th, or even earlier? Of course you do; I do too!

But we can’t. At least, not with normal light. And I fear that we’ll never be able to. You wanna know why? Let’s take a look at the Eagle Nebula for an explanation:

See how there are a bunch of stars, but that there are also these dusty-looking pillars obstructing our view of what’s behind them? That’s neutral hydrogen gas. When there’s just a little bit of neutral gas, like in our atmosphere, we can see through it, no problem. When there’s a lot of it, we can’t see through it. So if you look at Mars and Jupiter with a telescope, you can see the Martian surface, because the atmosphere is so thin, but you can’t see the surface of Jupiter, no matter what you do.

Well, for the last 13 billion years or so, the Universe has been practically devoid of neutral hydrogen; it’s all ionized because of all the intense energy from starlight and galaxies. But before all the stars formed, a lot of this hydrogen was a neutral gas, and blocks the light coming from behind it.

Even though each atom of hydrogen only absorbs a tiny amount of a very specific wavelength of the light, it has to pass through millions of light years of this gas to get to us. And this is too much for even the brightest of lights, and so we don’t know how to see earlier than January 19th in the Universe.

Or, in other words, the Universe has been mostly clear with tiny patches of clouds every day since January 19th, but before that, every day was foggy, and it took 19 days for the fog to clear.

So will we ever be able to do it? Will we ever be able to see what the Universe looked like through this fog of neutral hydrogen gas? Perhaps, but it will take a new technique, and I don’t know what that is. But now you know one of the toughest challenge for astronomers: to see into this fog, and learn what the Universe looked like when it was a toddler, an infant, and even a newborn.

And if this isn’t enough astronomy for you (is it ever, really?), check out the 91st Carnival of Space, highlighting 25 great space stories from this week! Happy Friday!

In Hanover, Germany, there’s an experiment called GEO600. These are two perpendicular lasers, and they shoot out for thousands of feet, get reflected, and come back to their original location to make an interference pattern.

Now the reason this is important is because gravitational waves cause ripples in space in a certain way. These perpendicular lasers are particularly sensitive to what gravitational waves do, and the interference pattern will shift in a very particular way if gravitational waves pass through them. This is the same idea that’s behind the upcoming LISA mission.

Now, GEO600, like every laser interferometer we’ve ever built, has not seen any evidence for gravitational waves. But it has seen something that it can’t explain, and that’s always interesting for an experiment.

It found some extra noise, above and beyond what can be predicted/explained by things like the vibrations of the Earth, temperature fluctuations, or instrumental noise. What does this look like? Whenever you do your experiment, you do your best to understand what noise you expect to see, and then you look for deviations from this. GEO600 saw something like this:

So there are two possibilities now: either there’s a source of noise they haven’t figured out, or something physically interesting and novel is causing this. Now, historically, whenever experiments are done, it’s almost always unexpected noise that causes something like this to happen. But once in awhile, there really is a new effect that we have going on.

It’s very important to state, clearly and unambiguously, before we go any further, that this may simply turn out to be noise. This may not be a physical effect at all, and that no other similar experiments (such as LIGO) see these effects.

But if it is a physical effect, Craig Hogan of Fermilab has come up with an extremely interesting possible explanation. He says that this excess noise could be a sign that our Universe has an extra dimension to it. How does this work? Let’s think of a hologram:

A hologram has all the information you could ever want about the dimensions of a 3-D object, but it has it all in two dimensions. For instance, you could tell some object’s (or someone’s) length, width, and depth just from looking at a 2-D hologram. All of the information is encoded in there.

Well, our Universe may be the same exact way. We know about our 3 space dimensions and our 1 time dimension. But we may have more dimensions of space than we know about; many interesting theories have them. One possible consequence is that these extra dimensions could cause extra “blurring” of our 3 regular space dimensions at very small lengths.

Now, this is very interesting, because the noise we see in the GEO600 experiment causes the laser light to blur on scales of about 10^-16 meters and below. This is smaller than the size of a single proton, but amazingly, our technology is sensitive enough that we can detect it! But is this blurring due to extra dimensions? Let’s see what the people connected with the experiment say about Craig Hogan’s idea:

However Danzmann is cautious about Hogan’s proposal and believes more theoretical work needs to be done. “It’s intriguing,” he says. “But it’s not really a theory yet, more just an idea.” Like many others, Danzmann agrees it is too early to make any definitive claims. “Let’s wait and see,” he says. “We think it’s at least a year too early to get excited.”

The longer the puzzle remains, however, the stronger the motivation becomes to build a dedicated instrument to probe holographic noise. John Cramer of the University of Washington in Seattle agrees. It was a “lucky accident” that Hogan’s predictions could be connected to the GEO600 experiment, he says. “It seems clear that much better experimental investigations could be mounted if they were focused specifically on the measurement and characterisation of holographic noise and related phenomena.”

So it looks like this is worth further investigation, but it’s way too early to draw any definitive conclusions. But it’s a possibility, and for something as grand as this, for something that would forever change the way we view our Universe, I think it’s worth investigating further, and so does the entire GEO600 team.

What do you think?

If the Universe is infinite in size but contains a finite amount of matter, then the average density must be zero, and we living in a region where it’s not would have to be a very special region of the Universe indeed. The assumption that we occupy a special place in the Universe has had a very bad history, and it seems to go against everything that we can observe now.

First off, let’s look at the history of thinking we occupy a special place in the Universe. There was the oldest school of thought, that the Earth was the center of the Universe.

A nice idea, perhaps. Except if you wanted to explain what caused the motions of planets, stars, and the Sun and Moon, you needed gravity. And the Sun is much more massive than Earth. Things worked much better if you put the Sun at the center of our Solar System:

Ahh, and things were better. So then we thought the Sun was the center of the Universe, for a few hundred years. But we learned about other stars, and how they had the same properties as our Sun. In fact, many were larger, hotter, and intrinsically brighter than our Sun. Instead of our Universe being the size of our Solar System (several hundred million kilometers), we quickly discovered it was at least thousands of times bigger than that. Moreover, the Sun wasn’t even the center; the Sun was about 25,000 light years away from the center of this great collection of stars:

And so people decided that our Milky Way, our galaxy, must surely be the center of the Universe. And just as sure as they were that the Earth was unique, and that the Sun was unique, most people thought that our galaxy was the only one. But all that changed in the 1920s, when Edwin Hubble found that all of these objects, known at the time as spiral nebulae, were actually other galaxies, with billions of stars of their own! We now know that there are actually billions of other galaxies out there, and our local group, with the Milky Way, Andromeda, and a few smaller spiral galaxies, is a pretty insignificant cluster, considering there are ones like Coma (below) with thousands of huge galaxies in it:

So historically, we see that in all the ways we’ve thought of, we find nothing special about us, our place in the Universe, or the area around us.

But let’s get to Stephen’s main question: is the region of space that we live in, that we call “The Universe”, anything special? Some facts about our Universe, to get you started:

1. Our Universe had a beginning. We call this the Big Bang. It happened 13.7 billion years ago, and so today, 13.7 billion years later, we can say that’s how old the Universe is.
2. Because the speed of light is finite, and the age of the Universe is finite, the size of our Universe is finite. Let’s be clear, it’s huge, but it isn’t infinite. The current distance to the edge of our Universe is about 46 billion light years.
3. Even though the Universe is 13.7 billion years old, it is bigger than 13.7 billion light years in size. Why? Because it’s been expanding this whole time, so things that we see now (the light from 13.7 billion years ago) have kept moving away from us, so they’re more than 13.7 billion light years away now.

One big source of confusion is that the Universe is expanding. Yes, it is expanding. But it’s expanding the same way a balloon expands. If you lived on the surface of a deflated balloon and inflated the balloon, you would see everything else expanding away from you. So it goes with our Universe: what we call our Universe is just one section of this balloon.

Now, the big question: is our section of the balloon, what we call the Universe, special? As far as we can tell, no. But, to be completely honest, we can’t see any other sections of the Universe that are outside of the 13.7 billion years we’ve had to see, just the 46 billion light years in every direction centered on us. Someone in a distant galaxy would see a different section of the Universe, just like someone living on a different dot on the balloon would be able to see a different part of the balloon.

As far as we can tell, everything we observe shows that every other part of the Universe that we can see is, more or less, just like ours.

Want some scientific details? There are some tests we can do to see whether the things that happen slightly outside of our Universe are similar to our Universe. As far as we can tell, they’re not significantly different, as this recent paper shows:

(If you’re wondering, the green dotted line is the only thing here that matches up with the data that we have; everything else is ruled out.)

So, as far as we can tell, our section of the Universe is just about the same in density as all other sections of the Universe. It has the same matter density, and the same amount of dark energy everywhere, too, as far as we can see. The average energy density of the Universe is small (about one proton per cubic meter, give-or-take), but definitely not zero.

And that’s perhaps the most spectacular find: that not only are the Earth, the Sun, the Galaxy, and the local group nothing special, but our Universe itself appears to be nothing special! We don’t know what lies beyond our Universe (since, you know, it’s not in our Universe for us to see), but whatever it is, and however far it goes on, it isn’t much different from us!

Let’s make a simple analogy: start with a 100 Watt light bulb.

Now, you can put that light bulb anywhere in the Universe, and you’ll know how far away it is. How? Because you know how intrinsically bright it is, and you can measure how bright it appears, you can calculate how far away it must be. It’s as simple as this:

Well, guess what? This works all over the Universe. You find something that you know its intrinsic brightness, you measure how bright it appears, and you can figure out how far away it is! Well, there’s one special type of object that is the same everywhere in the Universe: a type Ia supernova. They all work the same exact way. Let’s show you:

Start with a white dwarf star. Many stars (including our Sun) will end up like this. When all the nuclear fuel of a star is used up, the core simply collapses and the star sheds its outer layers. What we see at first is a planetary nebula:

but the hot gas of the nebula dissipates after several thousand years. The white dwarf star, at the center of many of these nebulae, remains behind. This is the fate of our Sun. However, unlike our solar system, which only has one star, many star systems have two or more stars in them. If one of those stars is a white dwarf and the other one isn’t, something very neat can happen:

The white dwarf star, if it’s close enough, because it’s so dense, can start stealing mass from the other star! It can do this for a long time, but not indefinitely. As the white dwarf gets more and more massive, the pressure in its center increases. At some point, when the white dwarf reaches a mass of about 40% more than our Sun, the pressure gets too great, and starts to destroy the atoms in the center of the star:

And the collapsing atoms release a tremendous amount of energy, resulting in a type Ia supernova explosion!

Because it’s the same exact physics every time, these have the same brightness every time, and we can use them just the way we use a 100-Watt light bulb! You measure their brightness, figure out their distance, and for the last part, you measure how quickly they’re moving away from us! From this data, you can figure out what type of Universe we live in: open, closed, flat, or accelerating. Guess which one we see?

Closed is the red line, open is the green line (which is far enough away from the data at about 2-3 Gpc that it doesn’t work), flat is the black line, and the accelerating ones are purple. Which one works best? The accelerating one, definitely. We can make more complicated models, but they all need something to explain this. And that’s how we know that there’s dark energy in the Universe. Any questions?

Imagine the Big Bang the same way you would imagine a grenade exploding. After a big explosion, everything moves outward, away from the center. But our Universe is different from a grenade in that grenades are little with very tiny masses, but the Universe is huge and incredibly massive, with an estimated mass of about 10²³ Suns! So just like a grenade, everything begins by flying apart, but unlike a grenade, gravity is so important that it tries to pull the entire explosion back together. Can it? Let’s look at the three obvious options:

1. Gravity Wins! Even though the Universe starts off expanding incredibly rapidly, if there’s enough mass and energy, gravity will pull everything back together again, resulting in a Big Crunch. A neat idea, but we need an awful lot of matter and energy to make it happen. What if we don’t have enough?

2. Expansion Wins! If there isn’t enough mass and energy, the expanding Universe just goes on forever. Gravity tries to slow the expansion rate down and manages to do so a little bit, but the Universe keeps on expanding, with gravity unable to stop it. Galaxies move farther and farther apart, the average density of the Universe drops asymptotically to zero, and the temperature of everything begins to freeze. This is called either the Big Chill or the Heat Death of the Universe. So what’s the third possibility?

3. Goldilocks? Instead of the Universe getting too hot (Big Crunch) or too cold (Heat Death), the Universe, like Goldilocks, could get it “just right,” and neither recollapse nor expand into an abyss. We don’t have a name for this case, but I like to call it the Big Coast, where the expansion rate asymptotes to zero, but never reverses and recollapses.

So those are the three classic fates of the Universe. The Big Crunch is what we call a closed Universe (shaped like a sphere), the Heat Death gives us an open Universe (shaped like a horse saddle), and the Big Coast gives us a flat Universe (shaped like a flat sheet).

What do we see? None of these. Instead, we find that for the first few billion years, the Universe looks like it’s doing the Goldilocks case, asymptoting in its expansion, and looking like it’s going to coast forever, like a flat Universe.

And then the fun starts. The unexpected happens. Something starts noticeably pushing galaxies farther apart! The expansion rate between any two galaxies in the Universe increases, like there’s some mysterious, repulsive force between them. If there’s an extra force (remember that F=ma?), that means there’s an extra acceleration in the Universe, and so something we didn’t predict at all happens:

The Universe expands faster and faster, and eventually all the billions and billions of galaxies we know of will disappear from view, leaving only us and Andromeda. Why is this happening? Well, folks, that is the mystery of dark energy. To be continued…

Well, the fine structure constant is made up of a combination of three other fundamental constants: the electron charge, Planck’s constant, and the speed of light. One of these must change, at least a little bit, as a function of either energy or time. Well, yesterday, a scientist named Lorenzo Iorio published a paper on the limits of changes in the speed of light. This is really interesting for a number of reasons. Ready?

If the speed of light was faster in the past, the Universe is much younger than we think it is now. It also means that it’s possible that there was no inflation. Since our understanding of the Universe hinges on the constancy of the speed of light, this is an important thing to measure.

It may be possible for other constants to change over time, too. I know of one person who things that Planck’s constant changes over time as well. Her theory is that the speed of light and Planck’s constant both change proportional to (the age of the Universe)^(1/3). Although I disagree with her conclusions, it is certainly a possibility.

Or, that is to say, it was until this recent paper came out. If you watch the planets move in orbits around the Sun, the point of closest approach, or their perihelia, changes over time in a very predictable way.

This change, observable in Mercury, Venus, Earth, and Mars over multiple centuries, is very sensitive to the speed of light! So you predict the motion of the planets based on the laws we know, and then you look for anomalies, which will give you evidence that the speed of light is changing.

These measurements are incredibly precise over the last century or so, and the conclusion is reached that over that time, with more than a 99.7% certainty, the speed of light has not changed at all.

Phew! So it’s still possible that the speed of light changed in the past at some point, but this piece of research indicates that, as best as we can tell, it isn’t changing now. So you can keep looking for evidence of changes, past and present, if you like, but I’m convinced that it’s 299,792,458 m/s now, that it always was in the past, and that it always will be in the future.

That’s why we do the experiments, though, because you never know what new evidence is going to come up! And speaking of what comes up, the new Carnival of Space is up, and the Martian Chronicles is hosting. How appropriate that I just wrote about terraforming Mars! Have a great weekend, folks!

1. The Cosmic Microwave Background:

Size = the entire known Universe, about 47 billion light years. This leftover radiation from the big bang comes to us from everywhere in space, and has taken the entire age of the Universe to reach us. So this is the largest thing; a sphere the size of the observable Universe, 2.725 Kelvin everywhere, with slight variations (hot shown in red, cold shown in blue).

2. Galaxy Clusters and Superclusters:

Size = up to 250 million light-years. (The largest known one is the Sculptor Supercluster.) These are clusters of galaxies, often with hundreds or even thousands of individual galaxies, with masses often approaching 10¹⁶ solar masses.

3. Individual Galaxies:

Size = up to 6 million light-years across. (That distinction belongs to IC 1101, over 60 times larger than our Milky Way.) We have over 100 billion galaxies in our Universe. 100 years ago, we thought we might be the only one. How small must we seem now to those who lived so long ago?

4. Star Clusters:

Size = usually about 10-20 light years across. Some types are more impressive than others. The gorgeous image above (thanks SDSS), the open star cluster M34, contains about 100 stars, but globular clusters can contain as many as a few million stars, like Omega Centauri.

5. Planetary Nebulae:

Size = about 0.5 to 3 light years across. When huge stars die, they go Supernova, and blow off the outer layers of the star. The inner stuff collapses to form a neutron star or black hole, while the outer stuff expands in a great shock wave, emitting light for hundreds of thousands of years. Eventually, it cools down and is no longer visible; this allows it to recollapse and trigger new star formation, eventually becoming an open star cluster! (It’s the circle of life, but for stars.)

6. Solar Systems:

Size = about 10 billion kilometers (or about 0.001 light-years). This gets all the planets, planetesimals, and comet-like objects all the way out to (and including) the Kuiper belt. This appears to be true for other solar systems as well as our own.

7. Individual Stars:

Size = from 200,000 km to about 8 million km. These vary tremendously in brightness, and it takes an estimated 400 billion to make up our Milky Way galaxy. Keep in mind, these are only living stars that are burning nuclear fuel.

8. Gas Giant (Jovian) Planets:

Size = about 20,000 km to 140,000 km. These are large, typically diffuse balls of gas. Interestingly enough, Jupiter is about the largest gas giant possible; if you start getting more massive than Jupiter, your planet actually starts to shrink in size due to gravitational compression.

9. (tie) Rocky Planets / White Dwarfs

Size = about 3,000 km to 20,000 km. When stars like our Sun run completely out of fuel, the only way they have left of generating light and heat is by gravitational cooling. They shrink to the size of about Earth, and slowly shrink further, emitting light until they cool completely and have no more energy left. They will then become completely dark, and are known as black dwarfs.

10. Asteroids (includes minor planets):

Size = up to 1,000 km, but they can be much smaller. The largest known one is Ceres, but most of them are so small that their gravity isn’t strong enough to pull them into a sphere.

11. Neutron Stars:

Size = from about 3 km to 20 km. These things can be more massive than our own Sun, and yet the largest are about the size of the Washington, DC beltway. Note, from the image, that they can flare up in the X-ray part of the spectrum and emit huge bursts of light.

12. Comets, Meteoroids, Micrometeoroids, and Dust Grains:

Size = typically 1 km or smaller, although a few comet nuclei can be as large as about 40 km. These are tiny conglomerations of matter that simply never made it into larger things; they remain small, rocky (and possibly icy) objects to this day. Examples range from Saturn’s rings to comet trails that result in meteor showers. But even though individual objects can be as small as millimeters, there are things that are even smaller in space.

13. Black Holes:

Size = infinitesimal. That’s right, these things are singularities, as far as we know, with masses as large as over 10 billion Suns concentrated into a single point, smaller than a proton. We can often see huge jets of energy caused by the matter interacting with the highly curved spacetime around it, but they’re called black for a reason: because no light can escape them.

Yes, I think this is badass! Of course, there’s a lot more out there than just outer space, and so for your enjoyment, I am reproducing the xkcd comics linked to above, “height” and “depth”, which show a lot more than just astronomical objects. Enjoy!

Galaxies, on the other hand, are made up primarily of stars. (At least, the portion of them that we can see is!)

So my quiz question for you is this:

Are there more cells in the typical human body or more stars in a typical galaxy?

The numbers are tremendous for both of these. Let’s start with humans. We start out as a zygote: a single fertilized egg cell.

From that, we grow up into full-grown adult humans, containing (are you ready?) 75 to 100 trillion cells! Now, things are even more interesting than that, because the cells that make up your skeleton and internal organs, your nervous system and your skin, your blood vessels and brain, only number about 4 trillion.

So where are the rest of them? Moving through your body, keeping you alive and functioning. Your body contains about one trillion white blood cells of various types, about two trillion platelets, and a whopping 30 trillion red blood cells! This means every time you donate a pint of blood, you’re losing 4,000,000,000,000 cells!

There are also about 40 trillion bacterial cells that live in your body (mostly your intestine), aiding in digestion. All told, that’s over 75 trillion cells in your body; fully half of them were made by you. That’s going to be tough to beat.

Let’s look at a typical galaxy now, like our own, or perhaps our nearest neighbor (above), Andromeda. Our Sun is not typical of stars in galaxies, it’s actually pretty bright. Only about 10% of stars are as massive as our Sun (a G-type star), which means 90% of them are dimmer, cooler, and smaller.

In fact, when you look up at the night sky, of the hundreds or even thousands of stars that are close to us (within about 30 light years), you know how many are type O? Zero. Know how many are type B? Also zero. Know how many are type A? Four.

Most of the stars that are there are not the stars you think of, much like the red blood cells and digestive bacteria in our bodies are not the cells that you think of when you think of our bodies. But when we count up all the stars in a typical galaxy, we get a pretty large number, too: 400 billion. That’s still nowhere close to the 75 trillion cells in a human body, but that’s the answer for typical galaxies. What about the largest galaxy in the Universe, though, can that compete?

In the center of one of the hugest galaxy clusters we’ve ever found, Abell 2029, lies a galaxy 6 million light-years across, or more than 60 times the diameter of the Milky Way (at only 100,000 light years). The number of stars in this galaxy? Estimated to be just over 100 trillion. The galaxy itself, the largest known galaxy in the Universe, is IC 1101:

So there are, give-or-take, as many cells in your adult body as there are stars in the biggest galaxy in the entire Universe. Remember that the next time someone tells you how insignificant you are: it takes 75,000,000,000,000 cells to make something as insignificant as us!