Sunday, May 12, 2013

Touring Orion: Astronomy and the Depth of Wonder

When I heard the learn’d astronomer,
When the proofs, the figures, were ranged in columns before me,
When I was shown the charts and diagrams, to add, divide, and measure them,
When I sitting heard the astronomer where he lectured with much applause in the lecture-room,
How soon unaccountable I became tired and sick,
Till rising and gliding out I wander’d off by myself,
In the mystical moist night-air, and from time to time,
Look’d up in perfect silence at the stars.
- Walt Whitman, When I Heard the Learn'd Astronomer

People look at you funny when you talk about outer space. The artists and poets think you're too cold and analytical, and others think you're a nerdy space cadet who should come down to earth. People think pondering the depths of space just isn't cool. But, then, people can be pretty silly. And as much as I love Walt Whitman, he was being a little silly, too. His mistake in the poem above was thinking we have to choose between science and the experience of gazing in awe at the night sky. It's a false dichotomy. You can have both at the same time, if you just decide your mind is big enough to hold them both. Besides, science can make the experience of looking up at a night sky all the more awe-inspiring, by telling you just how ancient and far away those stars really are.

So, if you're one of those people who think space is either too square or too dorky, I'd like to try to convince you otherwise.

First, let's just walk out and take a look at the night sky, like Mr. Whitman did. Imagine that we're walking out in the early spring, to see something like the image below. There's the Milky Way arched across the sky, and Orion the hunter with his bow outstretched. The little cluster of stars on the right is the Pleiades, and the bright star on the left is Sirius, the Dog Star.  Between Orion and the Pleiades is Jupiter, which is not a star at all, but the biggest planet in the solar system. If you had a pair of binoculars, you could look up and see four of it's moons. It's the first warm night of the year, and the frogs are singing off in the distance. Altogether, it's a glorious, overwhelming experience.

But what if we could look deeper? What if we could take a closer look at those distant worlds, and see them as more than just points of light? Well, we can, at least in our imaginations. And the reason we can is all those learn'd astronomers with their charts and figures, which helped them figure out so much about what's really going on out there.

So let's take an imaginary trip, straight into Orion and beyond, to see how much deeper the sky is than we can see with our naked eyes. First, let's consult a map to see where we're going. Here's our itinerary:

The trip seems to jump around so much because we're starting with the closest objects, and making our way toward the most distant ones. We're going to start with Jupiter, and then go deeper and deeper into space, in the general direction of Orion. I'm not going to dwell on the planets much, because most people learned those in school pretty well. What most people don't know is how things are arranged out beyond the solar system. That's where we're headed, but we'll stop at Jupiter, since it's on the way.

Jupiter is the first and largest of the gas giant planets out beyond Mars and the asteroid belt. It's not a solid object, except at its rocky core. Its famous bands are formed by rising and falling clouds of ammonia ice and ammonium hydrosulfide. Jupiter is the big boy of the solar system--over 1400 Earths would fit inside it, and it's twice as massive as all the other planets combined.

But Jupiter is a pipsqueak compared to the sun, which is over 1000 times as massive. Really, the planets are just bits of detritus around the sun, and even the sun isn't that impressive by astronomical standards. This becomes obvious if we look backward and remove the constellation lines, to see the inner solar system as it would really look from Jupiter. The sun just looks like an unusually bright yellow star from here, and we wouldn't even recognize the inner planets--including our own--if they weren't labeled. But then, we've come a long way, by our normal standards. To put the distance to Jupiter in perspective, if you could point your car there and start driving at a steady 75 miles per hour, it would take several hundred years to get there. You would need to bring a lot of music.

But we've barely gotten started. Our next stop is Sirius, which is 8.6 light years away--a whole different level of far away. A beam of light, which can goes fast enough to circle the world over 7 times a second, would take 8.6 years to get to Sirius. And Sirius is one of the closest stars to Earth. If the Voyager 1 spacecraft, which has just reached the edge our solar system after 35 years, were heading for Sirius, it would take about 180,000 years to arrive. Travel to the stars is still pure science fiction.

Luckily, we're taking an imaginary trip, so we can get to Sirius in no time at all. As we approach, we see the single point of blue-white light resolve into two stars. It turns out Sirius is a binary star system, with two stars orbiting a common center of mass. The bigger one, Sirius A, is about twice as massive as our sun, but it's much hotter, and thus 25 times as bright. Sirius B is an entirely different story. It's a white dwarf star, slightly smaller than the earth, but more massive than the sun. In other words, it's astoundingly dense--a piece of it the size of a sugar cube would weigh about a ton. Try dropping that in your coffee.

White dwarfs like Sirius B are the shrunken cores of larger, deceased stars. Around 120 million years ago, Sirius B resembled Sirius A, but it became unstable as it used up all its fuel. This caused it to swell up into a huge red giant star, and eventually puff its outer layers of gas into space, creating a luminous nebula which has long since dispersed. All that remained was the hot, tiny core, which will burn with stored heat for billions of years to come, until it finally cools down into cold, dark object called a black dwarf.

The next leg of the trip is a big one. We're going to the Pleiades, or Seven Sisters, a group of stars at least 390 light years from Earth. That means the light we now see on earth when we look at them has been traveling through space since around the time the Mayflower landed. Unlike most constellations, the Pleiades are an actual group of stars, called an open cluster. But I like to think of them as a litter, because they are siblings--born together in a giant cloud of collapsing gas. The bright blue stars we see are blue giants, which are bigger, hotter, and far brighter than most of the other 1000 or so stars in the cluster. Blue giants are like rock stars, living hard and dying young, burning through their fuel at a furious pace. Some of the ones in the Pleiades are already showing their age, and they're a mere 100 million years old. The smaller yellow and red stars take the slow and steady approach. They will live on for billions of years, some of them for many times the current age of the universe.

Rogelio Bernal Andreo (Creative Commons Att. Share Alike)
Now it's time to head for Orion itself. First, let's take another, closer look at Orion as it appears from earth.  The amazing photo to the right is a long-exposure mosaic showing the clouds of gas and dust that fill the constellation. The red clouds, called emission nebula, glow because they're heated up by the stars forming in and around them. If you look closely, you can see darker clouds too, which show themselves by blocking the stars behind them. The Orion region is one of the most active regions of star formation known. In fact, some of the stars visible around Orion's belt and sword were born together out of those clouds. It's unusual for the stars in a constellation to be related, but many of the stars in Orion are, possibly including the next two we're going to meet.

Our next stop is Betelgeuse, the enormous red supergiant that defines Orion's left shoulder. When I say Betelgeuse is enormous, I mean it's really just stupendously huge. If the center of Betelgeuse were where our sun is, it would swallow all the planets up through Mars, and come nearly to the orbit of Jupiter. But Betelgeuse is nearing the end of its life, and it's really starting to fall apart. It roils and pulsates, belching plumes of gas as large as our solar system. In about a million years, it will collapse and then explode as a supernova. Anyone still around on earth will see it shine as bright as the moon for a few weeks, even though it's 640 light years away.

The brilliant blue star that forms Orion's right foot is called Rigel. It's a young blue supergiant star, around 8 million years old. It's no small fry itself. If the sun were the size of a BB, Rigel would be about the width of a beach ball. It's not nearly as big as Betelgeuse, but it's big. It's also tremendously bright--at least 117,000 times as bright as the sun. Its brilliance caused partly by its size, but mostly by its intense heat (with stars, size is not nearly as important as heat). Rigel's brilliance is the reason we see it so clearly, even though it's about 860 light years away, and the light we see left it around the time Genghis Khan was born.

If you walk out on a clear night and look up at Orion, the middle of his sword is actually not a star at all, but a glowing red cloud called the Orion Nebula. It's one of the most spectacular star formation sites known, so let's go take a closer look. As we approach the nebula, about 1,344 light years from earth, we see that it's really a bright cavity in a more extensive cloud. It's like an amphitheater full of thousands of stars. The brightest, as usual, are blue giants and supergiants, but there are stars of all other sizes and temperatures being born too. Some are still wrapped in discs of dusty debris which will one day aggregate into planets. Others are in the so-called bipolar outflow stage, with great jets of gas shooting out from each pole. Astronomers have even seen brown dwarfs, balls of gas too small for fusion reactions to ignite, so they never quite turn into stars. If we could look into the cloud with infrared vision (and astronomers can do just that) we would see even younger stars forming from the dense gas. After they form, stellar winds of radiation will push back and illuminate the gas, deepening the cavity of the Orion Nebula. This, in turn, will cause gas further back in the cloud to collapse toward stardom. Starbirth propagates itself like spreading wildfire, so that as we move deeper and deeper into Orion, we find younger stars. Looking back toward Earth, stars like Rigel and Betelgeuse may be older progeny of the same great cloud across Orion, born in clusters like the stars of the Orion Nebula, but drifting away from their siblings over millions of years.

We've gone straight through the heart of Orion, and now we're on the other side. Let's keep going, to see what we can see. Our next stop, the Crab Nebula, is many times farther away than anything we've seen so far--6,500 light years. That means we see it as it was about a thousand years before the Sumerians built the world's first cities. The Crab Nebula is a completely different animal than the Orion Nebula. It's what's left over from a supernova explosions in 1054 AD. Chinese astronomers at the time recorded a "guest star", which appeared all the sudden, bright enough to be seen in the daytime. What we see today is a cloud of glowing gas 11 light years across, and still expanding at about 1,500 kilometers per second. Yes, per second. At the center of the cloud is the leftover core of the old star. When the core collapsed and then rebounded, the pressure was so great that it collapsed protons and electrons into neutrons, forming a ball of neutrons as dense as an atomic nucleus, but as large as a city. It's still spinning about 30 times a second, pouring radiation out from each magnetic pole. We see this radiation as rapid-fire pulses, so this kind of neutron star is also known as a pulsar. It's hard to believe anything this extreme really exist out there, but nature is full of surprises.

In the long history of the Milky Way galaxy, there have been countless supernova explosions like the one that created the Crab Nebula. In fact, we owe our existence to them. When the universe was born in the Big Bang, the only elements that existed were hydrogen, helium, and traces of lithium. Then, when the first stars lit up, they burned by fusing hydrogen and helium into heavier elements, creating the rest of the periodic table--the atomic alphabet that makes life possible. All stars create a few heavier elements, but some of the most crucial elements for life, including sulfur, sodium, and potassium, are created in supernova explosions. We are, quite literally, made of stardust--stardust blasted into space in some of the most violent explosions in the universe. It's a pretty amazing heritage, and we share it with everything in the solar system.

Maybe we've come far enough for now. We're over 6,500 light years from home, farther than light could have traveled in all of written history. But just how far is that, in the grand scheme of things? Not very. In the picture below, the yellow line shows roughly where we have been so far. Except for the Crab Nebula, which is in the Perseus Arm of the Milky Way, everything we saw on our tour was in the little sub-arm of the galaxy known as the Orion Spur.

The galaxy as a whole is over 100 thousand light years across, and it contains at least 200 billion stars--more than you could count in several lifetimes. We've only seen a tiny section of it, and then only in our imaginations. A real trip like this is still completely beyond our grasp, and will be for the foreseeable future. And the Milky Way is just one galaxy; part of a small group of galaxies on the edge of a giant cluster that contains thousands of others, in a universe that contains untold numbers of such clusters. We could keep on traveling outward, to see where our galaxy fits in with others, but we've surely gone far enough for now. 

Now, here's the question. If you walk out one night and look up at Orion, does the tour we just took make  looking up at a night sky somehow less amazing? I don't see how it could. For me, it just makes a great thing that much greater, showing us wonders we never could have imagined if learn'd astronomers hadn't made all those charts and figures.

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