Starts With A Bang! » Astronomy Ethan Siegel's blog/video blog about Cosmology, the Universe, and everything else Sat, 04 Apr 2009 20:12:38 +0000 en Naming the new ISS module: a suggestion Thu, 26 Mar 2009 21:20:20 +0000 ethan As many of you have heard, NASA has had a public vote to help name the new node of the International Space Station — node 3 — shown here in its full glory:

Although the name Serenity for the new node got 70% of the vote on the NASA site, that’s totally misleading. Because someone started a write-in campaign to get the module named after himself:

And the name Colbert beat Serenity by over 40,000 votes! Before you shout, “curse you, Colbert” (I already did), I bring up the sad fact that NASA has said the results are not binding, and that this dubiously-qualified megalomaniac may not get his name on the module due to a technicality.

But if NASA had a sense of humor (or any sense of increasing positive publicity), they would listen to my advice:

Name the node COLBERT.

But pronounce it KOHL-burt.

Trust me, he’ll hate it. Loathe it. Perhaps even have someone on his show to throw one of his patented tirades at. Because it won’t be his name, but it completely follows the expressed will of the public. And that, my friends, is the way to make democracy work.

The Most Energetic Mystery in the Universe Wed, 25 Mar 2009 20:05:43 +0000 ethan When we look out at galaxies, we find the most energetic particles we’ve ever found anywhere in the Universe coming from their centers. Why?

Because as far as we can tell, all galaxies, at their centers, have huge, supermassive black holes! When matter (like a star, globular cluster, intra-galactic gas, etc.) gets too close to one of these black holes, it gets ripped apart, and settles into a disk around the black hole. This disk is called an accretion disk:

Like everything in a strong gravitational field that moves, these particles radiate (give off high-energy photons), fall in towards the center of the black hole, and sometimes get accelerated and shot out of the galaxy!

For a galaxy like ours, with a black hole a few million times as massive as our Sun, we can get extremely energetic particles out: up to 1018 eV, which is 70,000 times more energetic than the LHC!

But our black hole is kind of a commoner — a weakling, even — when you look at other galaxies. There are huge galaxies out there, such as active galaxies and quasars, where instead of a few million times as massive as our Sun, their black holes are billions or even tens of billions times as massive as our Sun:

Well, in theory, the energies of these particles can be thousands or even tens of thousands of times higher than what our galaxy can produce! We’re talking about energies of 1022 eV, which is not only insane, it’s impossible!

Why? Because there’s a maximum energy that particles traveling through the Universe can have. There’s a bath of leftover light from the Big Bang permeating the entire Universe: the CMB. If you smash a particle with too much energy into one of these CMB photons (which is unavoidable as you travel millions of light years), it causes these high-energy particles to slow down until they’re below the “speed limit”. (Okay, it’s an energy limit, but it’s really close enough.)

And unlike cops that pull you over, the light that fills outer space slows you down until you’re below the cosmic energy limit: 5.7 x 1019 eV. Well, do we see a cutoff there?

Maybe. The AGASA experiment says no, there is no cutoff, but the Pierre Auger Observatory says yes, there is one. Who’s right? On one hand, we’ve definitely seen events where we’ve measured more energy than should be allowed. It may mean our theories need revising, or it may mean that there’s something super-energetic happening in our own backyard that we don’t know about. Or — on the other, more boring hand — perhaps we’ve just done a bad job of measuring things at very high energies. Whatever the case, explaining these events that exceed the cosmic energy limit of the Universe is, in fact, the most energetic mystery in the Universe!

A Little Sun in Your Life… Dire Consequences? Fri, 20 Mar 2009 20:18:05 +0000 ethan I really get a kick out of reading The Straight Dope. What started as a weekly column in a Chicago newspaper has grown into a nationwide phenomenon and a small empire, and is often full of fascinating questions and extremely well-researched and knowledgeable answers.

But doesn’t anyone there know to contact me if you’ve got an astrophysics question? Today’s column, which will be nationally syndicated, declares that they cannot answer the following question:

If the pink grapefruit sitting in my fruit bowl spontaneously turned into a grapefruit-sized sun, what would happen to my flat, London, and the rest of the world? If I put it somewhere safe, could I enjoy not paying for central heating? Or would it end life as we know it by melting through my floor, into the African textile shop, through the subway system, and finally to a fiery chasm in the middle of the earth where it would make all volcanoes erupt and kill everything, before coming out the other side and changing the way all the planets spin?

Well, I may be no Cecil Adams, but I can certainly answer this one. Let’s take a look at how our Sun actually works, and then scale it down to be grapefruit-sized.

The Sun is a giant ball of mostly hydrogen gas. It’s extraordinarily massive — about 300,000 times as massive as Earth — and tremendous powerful. The Sun gives off what most people would call an unfathomable amount of energy, but in scientific notation, it’s about 4 x 1026 Watts, which is at least fathomable, although it’s absolutely tremendous. This means that even at the surface of the Earth, 150 million kilometers away, that means we have 129 Watts of solar energy striking us over every square foot that receives sunlight.

But what’s truly exciting, at least for me, is the way the Sun creates its energy. To understand it, we have to go down to the tiniest atomic levels, and look at the hydrogen atoms themselves:

Because the pressure at the center of the Sun is so high, due to the gravitational pressure of having 300,000 Earths pushing in at the core, the nuclei of these hydrogen atoms, protons, get pushed together with a tremendous force. The force is so large that it causes these nuclei to fuse together, in a process called nuclear fusion. With a little math, I can figure out that in order to create the amount of energy the Sun gives off, it has to fuse together about 3.6 x 1038 atoms of hydrogen every second! That little atomic reaction, happening over and over, trillions of times every nanosecond in the Sun’s core, is what produces all the heat, light, and energy we’ve ever received from the Sun.

Now, the person who asked this question wanted to know about having a little stable grapefruit-sized Sun.

Well, here’s a big downer for you: this thing ain’t gonna be stable. If you want to have nuclear fusion going on at the core of this grapefruit-sized ball of hydrogen, you’re going to need a tremendous amount of pressure to push the atoms at the center together. There are only two ways to handle it that we know of, and neither one of them will give you an answer that you’ll like, although they’re both fascinating.

The first way is to artificially increase the pressure, like lasers (shown here) would do. Practically, you wind up getting less energy out than you have to put in to increase the pressure to obtain fusion, which is another disappointment. Scientists working on this call it inertial confinement fusion, and so far, it has never yielded more energy than it took to get it going. So this way looks like a dud. But there’s another way to get nuclear fusion…

You can increase the pressure tremendously — albeit for a very short time — by setting off a small explosion around the hydrogen core, compressing it and causing ignition. There’s a problem with this way, too. The resulting fusion reaction is uncontrolled and runs away, igniting everything. This is commonly known as a hydrogen bomb.

Either way, you give off a tremendous amount of energy, your initial “grapefruit” gets blown apart like a super-powerful exploding grenade, and depending on how much of the hydrogen in there actually managed to fuse would determine what happened. If you scaled down the Sun so that the grapefruit worked on exactly the same scale, you’d only get about 100 million atoms fusing together, or about 100 microJoules of energy. Enough energy to push the hydrogen gas away, but not even enough energy to light a match. But, if you managed to fuse the entire grapefruit into helium, you’d get the energy equivalent of a 160 kiloTonne explosion, or about 11 times the energy of the atomic bomb dropped on Hiroshima, in your fruit bowl.

Without the entire mass of the Sun to insulate this nuclear explosion from the rest of the Solar System, this grapefruit-sized ball isn’t going to last long. Either way, you’re better off getting a heat lamp for your desk, and paying your electric bill.

Book Review: The Hunt for Planet X Fri, 13 Mar 2009 19:49:33 +0000 ethan Pluto, try as some people may to belittle it, is too beloved to simply go away. Even anti-Plutonians are fascinated by it. So to celebrate the state of Illinois’ very first Pluto Day, since Pluto was discovered exactly 79 years ago today, I present to you a review of Govert Schilling’s newly released book, The Hunt for Planet X: New Worlds and the Fate of Pluto.

This is one of the most detailed books about the Solar System, its history, and the neighborhood at and around our Sun. Schilling starts in the 18th Century, when the Solar System was “well known” with its six planets: Mercury, Venus, Earth, Mars, Jupiter, and Saturn. He tells fascinating versions of the discovery of Uranus, including its naming and its impact on those interested in Astronomy, a great version of the discovery of Neptune, a personal favorite historical story of mine, and spares no detail in telling about the important people and personalities involved.

There’s also a very intricate history given of the asteroid belt, including of a time when the asteroids Ceres, Pallas, and Vesta were all called planets. But the star of this book is no star at all, but the small, icy, distant world, Pluto.

The book talks about Clyde Tombaugh and the discovery of Pluto, omitting practically no detail, and continues to go on about the discovery of its major moon, Charon (and later the theory of its formation), and its two far smaller moons, Nix and Hydra. Throughout this, the reader gets a genuine feeling of the good fortune that goes into such a discovery, as well as the myriad of hours and patience that being a good observational astronomer requires. Amateur astronomers, especially, will find much to identify with in this book, including a kinship with many who share their passion for the heavens.

The last great thing that I really enjoyed about this book was its discussion of the Kuiper Belt and the many objects that have been discovered there so far. It really keeps in perspective that the first Kuiper Belt object other than Pluto and Charon was only discovered in 1992, and yet already, we know that there are many other icy worlds just as important to our Solar System as Pluto:

The book is very well illustrated throughout, with some wonderful pictures in both color and black and white, of the astronomical objects themselves, the astronomers who discovered them, and on special occasions, the actual astronomical data that were used in their discovery.

There are some negatives to this book, and I would be remiss if I didn’t tell you what they are. There is a sense of despair when he discusses theories that didn’t pan out, such as the Nemesis theory (that the Sun has a super-long-period binary companion), the search for a fifth very massive giant planet, and the idea of Vulcan (a planet closer to the Sun than Mercury). This is also not a quick, easy read. Because the book is so dense with details, it often takes quite some time to digest all the information that’s being related. This book could have also used an editor, as every story in the book is given the same weight, whereas many are clearly both more important to the book overall and also are simply more interesting. And this is a personal distaste, but the sheer amount of time and space devoted to the naming of Solar System objects is way out of proportion to their actual importance. Finally, there’s hardly any mention at all of planets around other stars, which would certainly not be out of place in this book.

But to anyone really interested in Pluto and in understanding our Solar System in general, this book contains all the latest, up-to-date scientific information, including on Neptune’s Moon, Triton, and on things as esoteric as what you can learn from an occultation. This book will probably be obsolete when the New Horizons mission gets to Pluto in 2015, but until then, you won’t find a better, more comprehensive source of information that’s accessible to a non-scientist about the Solar System. Overall, to anyone interested in learning about some of the lesser-known worlds in our Solar System and the stories of their discovery, The Hunt For Planet X is for you.

Happy Pluto Day!

Why is Venus the Brightest? Wed, 11 Mar 2009 16:59:00 +0000 ethan Sometimes, when you look up in either the early, pre-dawn morning or early, post-sunset evening sky, there’s one point of light that outshines all the others. It’s usually relatively close to where the Sun was, and unlike most points of light you see in the night sky, this one doesn’t twinkle. I’m talking, of course, about the planet Venus:

Yes, it isn’t nearly as bright as the Moon, but it’s certainly much brighter than everything else you can see. Well, it was only a matter of time before someone wrote in and asked why. Reader Dan asks:

Can you explain why Venus is so bright in the sky right now? I don’t think I have ever seen it so bright in my life.

Venus, as we’ve talked about before, is covered in a very thick layer of greenhouse gases. This makes its surface extremely reflective, so that about 70% of the sunlight that comes in to Venus gets reflected as visible light.

But Venus is also extremely close to Earth. In fact, Venus gets within about 40 million km of Earth, which is less than 1/3 of the distance to the Sun. At its farthest, Venus is about 250 million km from Earth. Why? Because Venus and Earth both orbit the Sun; sometimes they’re on the same side as one another, and sometimes they’re on opposite sides. When they’re both on the same side, Venus has its closest approach to Earth:

But I’m going to force you to think about the geometry of this a little bit. One side of Venus faces the Sun, and gets illuminated; the other one is dark. Imagine that the Earth is far away from Venus. What will the planet Venus then look like from Earth? It will be mostly “full”, and the closer to being in direct opposition to Earth, the more “full” Venus will appear. But the more “full” it appears, the further away it is, and so it should also look smaller. Take a look at these shots of Venus, through the same telescope, when it’s far away from Earth:

When it’s more full, it’s also smaller. But when less of it is illuminated, it’s closer to us. Take a look at what Venus looks like when it’s closest to us:

So when Venus is close to us, it looks like a crescent, but it looks like a very large crescent, and when it’s far from us, it looks like a disc, but a much smaller disc. Which of these two things is more important for the brightness of Venus?

Well, we can find out. You see, Dan is right; Venus is nearly at its brightest right now. Have a pair of binoculars? Well, about 11 days ago, somebody did, and photographed Venus and the Moon together in the sky. Here’s what they saw:

Venus is at its brightest when it’s a crescent! It turns out that being closer to us means everything. Astronomers use magnitudes to measure brightness, and the smaller your number is, the brighter you are. This is useful for stars, where very bright stars are typically 0 or 1 (the very brightest, Sirius, is -1.5), and the dimmest ones visible with the naked eye are 5 or 6.

What about things that aren’t stars, though? Not surprisingly, the Sun is the brightest, at -26.7, and the Full Moon is second, at -12.6. But Venus is third! When Venus is a crescent, it’s -4.6, and when it’s full (but far away), it’s -3.8, which is interesting! Why? Because when it’s full, you can’t see it during the day, even under optimal conditions. But right now, when it’s a crescent, you can! Check out this photograph by John Harper of Venus during the day:

It’s a crescent, just like we said it should be! So that is why Venus is the brightest thing in the sky, and that’s why it’s brighter now than it usually is. Get out your binoculars and have a look! And if you do it tonight, look on the other side of you too, because the Moon is full and it rises at 8:30 PM! Enjoy!

Pluto is a Planet — in Illinois! Sat, 07 Mar 2009 00:32:16 +0000 ethan I was going to write about something else today, but when I saw this story, I simply couldn’t resist. Apparently, the Illinois state legislature not only declared that Pluto is a planet like the other eight:

They also declared that March 13, 2009 is going to be the very first Pluto Day, in honor of the discovery of Pluto on March 13, 1930, by Illinois-born Astronomer, Clyde Tombaugh:

People talk about how small, insignificant, and far away Pluto is, and they’re right. But what they’re missing is just how lucky Tombaugh was to find it. Sure, there are many, many Kuiper Belt objects out there, and we know of lots of them at this point:

But after the discovery of Pluto, people searched, in earnest, for the 10th planet. It took a loooong time for the next Kuiper Belt object to be found. And when it was found, you know what it was?

Pluto’s moon, Charon! Do you know how long it took, after the discovery of Pluto, to find Charon? Almost 50 years! So yes, we know of many other Kuiper Belt objects now, and some of them are even more impressive than Pluto.

But Pluto holds a very special place as:

  • The first and only planet in our Solar System (even if it was only temporarily a planet) discovered by an American.
  • The very first Kuiper Belt object discovered.
  • The first trans-Neptunian object discovered.
  • And as being the 9th planet for about 70 years.

You know what I think about this? Good for Illinois. Good for them! This raises awareness of Astronomy, gives them something to be proud of and a great historical achievement to celebrate, and helps Pluto from fading into the obscurity that the other Kuiper Belt objects currently have to deal with.

So good for Illinois, good for Pluto, and good for you for not forgetting it. Someday, I would love for Pluto to be given a real, useful designation by the IAU, like “King of the Ice Dwarfs.”

Perhaps we’ll discover some wonderful things about it when New Horizons gets there; April of 2015, folks, and don’t forget to celebrate Pluto day on March 13th!

Everything Looks Perfect From Far Away Wed, 04 Mar 2009 20:24:33 +0000 ethan This morning, I was driving to work and the song Such Great Heights by Iron and Wine came on my iPod; click the play button below to listen while you read.

But I was listening to the lyrics, and one line from the chorus has been haunting me all day:

They will see us waving from such great heights
“Come down now,” they’ll say
But everything looks perfect from far away
“Come down now,” but we’ll stay

Why would this haunt me? Because it’s so simple and so true, and really encapsulates in just six words what’s so beautiful about astronomy. Take a look at the Moon up close, with all of its craters, gullies, mountains, and imperfections:

Versus how perfect it looks from far away:

And the same is true for any star, such as the Sun, which has a flaring, intricate, spotted surface when you view it up close, but looks like a perfect point (or sphere) of light from far away:

And this is true for even galaxies, which look like an intricate mess up close, full of stars everywhere, dust lanes, voids, star-forming regions, etc. (just look at our own):

But, again, look so simple and perfect from far away:

And this is even true for the large-scale structure of the Universe. You look at where things are up close, and you find regions where there are many galaxies, and then vast regions where there are none, and it looks like a complicated mess:

But if you stood outside of our Universe and looked in on it, it looks almost smooth and perfect, like an ultra-fine web of light and matter, from far away:

Maybe I just like standing back and enjoying the view. Maybe I needed some lighter fare after yesterday’s article about ethics. But this has been profound enough that it’s been in my head all day, that everything looks perfect from far away. Enjoy the view.

Random Ethics Question: Project Paperclip Tue, 03 Mar 2009 20:13:19 +0000 ethan Sometimes, when I come home from a long day at work and need to unwind, I start reading BBC News, and occasionally the news archives are more interesting than the actual present news.

Last night, I came across a really interesting read from 2005, about Project Paperclip, which was the US Government’s plan to bring the former Nazi scientists to America and use their knowledge and expertise to further the scientific and military enterprises we had going on here, and also to deny the Soviets that knowledge.

But this required giving amnesty to Nazis (and sometimes even former SS officers) like Werner Von Braun, Arthur Rudolph, Kurt Debus, and Hubertus Strughold. The atrocities that these men and their subordinates were responsible for are well-documented, and include the death of 20,000 slave laborers producing Von Braun’s V-2 missiles, freezing inmates and putting them into low-pressure chambers, and performing human experiments at Dachau and Auschwitz. Truly, these are some of the most despicable things that human beings have ever done to one another.

And yet, the Germans had an incredible amount of scientific, aeronautic/aerospace and military knowledge that we did not. Some examples? Supersonic rockets, nerve gas, jet aircraft, guided missiles, stealth technology, and hardened armor. All in 1945. So what was the ethical thing to do? What was the smart thing to do? And overall, what was the right thing to do? US Air Force Major-General Hugh Knerr’s opinion was the prevailing one:

If we do not take the opportunity to seize the apparatus and the brains that developed it and put the combination back to work promptly, we will remain several years behind while we attempt to cover a field already exploited.

So, for better or worse, about 700 Nazi scientists were put to work for the United States.

The technological advancements and scientific breakthroughs were numerous and swift, and over the next 25 years, Werner Von Braun had masterminded the Apollo program and the Moon landings, Arthur Rudolph had designed the Saturn V rocket, Hubertus Strughold designed NASA’s life support systems, and countless other technologies such as Cruise Missiles, the B-2 Stealth Bomber, and scramjets came as outgrowths of Nazi research. So as a counterpoint to one of the worst atrocities in human history, we also have one of the greatest achievements in human history:

That’s the story of what we did and one of the steps we took to get there. Did we do the right thing? Did we do the just thing? Did we do the smart thing? For me, the answers are yes, no, and yes. And I, for one, am glad I didn’t have to be the one to make that decision. What do you think?

And this last week, so much happened in space news that we not only have a carnival, we have a carnival sideshow as well; enjoy!

Hunting The Great Red Spot Fri, 27 Feb 2009 23:47:16 +0000 ethan I’ve been interested in the planet Jupiter ever since I was told, as a little boy, that it was the place I needed to go. Although, come to think of it, I was lumped in with all boys, and told that we “go to Jupiter to get more stupider,” which isn’t a very good reason, in hindsight. Jupiter is one of the brightest objects in the entire night sky, beaten only by the Moon and Venus, and occasionally by Mars.

The above photo is of the Moon, Venus, and Jupiter on a slightly cloudy night. Sure, the Moon and Venus are brighter. But they’re also way closer than Jupiter is. It’s sheer coincidence that it’s named after the ruler of the Roman Pantheon and also happens to be the largest planet in our Solar System. But it is huge; if you wanted to compare Jupiter to Earth, Jupiter doesn’t just have us beat, it makes us look like an insignificant moon in comparison:

And once you get past its giant size, you start noticing other things about Jupiter, such as the different bands of moving air at different latitudes. But what I’ve always been fascinated by is that huge birthmark on its face: the great red spot, bigger than Earth in its own right.

And even though this animation that I’ve placed below is 30 years old, it’s still the best thing I know of to illustrate what Jupiter actually does; take a good look!

Unlike Earth, where the weather is transient, and even the greatest storms only last a few weeks, Jupiter is practically all weather. That great red spot is totally obvious, even without the red. This counterclockwise hurricane is about 2-3 times the size of Earth, and has been a fixture on Jupiter’s surface for as long as we’ve been able to see Jupiter’s surface features: more than 300 years!

So how impressive is this great red spot? Some impressive facts:

  • Width — anywhere from 24,000 km to 40,000 km
  • Height — anywhere from 12,000 km to 14,000 km
  • Time to rotate — 6 Earth days
  • Maximum Wind Speed — 250 miles per hour (400 km / hour)

But what’s the big news? See that big variety in width above? It turns out that Jupiter’s great red spot is shrinking! In fact, the great red spot, right now, is smaller than it’s ever been. We don’t think it’s going to disappear or dissipate, but it’s definitely shrinking right now. It’s possible that this is just a fluctuation, and the system is stable, like this animation of Hurricane Isabel on Earth.

But it’s also possible that if you’d like to see the great red spot, you’re best off doing it sooner rather than later! Remember, Jupiter rotates quickly (in under 10 hours), so you have to get lucky to have the great red spot facing you. You can see great details on Jupiter with just a 10″ telescope, but if you don’t plan ahead, the great red spot will be on the other side of the planet:

My advice? Use this handy calculator. And the great red spot isn’t just for professionals; here are six pictures of Jupiter, with the great red spot visible, taken by the amateurs Damian Peach, Christopher Go, and Anthony Wesley:

Remember, Jupiter has phases (as seen from Earth) too, so try to catch it when Jupiter is “full”! So, hopefully this shrinkage of the great red spot is just temporary, but my advice is to get out and have a look while you still can! Happy hunting!

How does Hydrogen dust block light? Wed, 25 Feb 2009 22:21:51 +0000 ethan I wrote an article last week where I talked about our Universe when it was younger, and discussed that you can’t see too far back because it’s too dusty. In the same way that a fog obscures distant objects, I said, this neutral hydrogen will obscure distant galaxies, and so we have a very hard time figuring out how to see astronomical objects beyond a certain distance.

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!