Tuesday, May 14, 2013

A Journey Through the Universe

We shall not cease from exploration. And the end of all our exploring will be to arrive where we started and know the place for the first time. - T.S. Eliot
Have you ever looked up at the night sky and wondered, "Where ARE we in all that?" Astronomers have found some amazing answers to that question. That was the topic of a program we had here at the Main Library last week, called A Journey Through the Universe. If you missed it, we took an imaginary journey through space, stopping to look at some of the astounding things astronomers have found out there. Since it's an imaginary journey, we can do it just as well in a blog post as in a talk. So let's take a quick little jaunt through the galaxy.

First, let's look at where we're going. 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 at the top, and Orion the hunter in the middle. 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 its moons. The sun, the moon, and Jupiter all seem to move along that red line, called the ecliptic, because all the moons and planets are in the same plane, so from Earth they appear to move along a single line. The stars and the Milky Way stay put relative to each other, but they all seem to spin around the north star. That's because the earth spins, and its axis just happens to point at the north star. That's lucky for people in the northern hemisphere--there is no "South Star" to show the people in Australia which way is south.

Our trip is going to take us deep into space, in the direction of Orion, starting with number 1, Jupiter, and ending with number 7, the Crab Nebula. The reason we're going to jump around the map so much, instead of going from left to right, is that we're starting with the closest objects, and making our way toward the most distant ones. Jupiter and Betelgeuse look pretty similar to the naked eye, but Jupiter is much, much closer (and smaller). I'm not going to dwell on the planets much, because most people learned those in school pretty well (although poor Pluto has been demoted to a dwarf planet since some of us were in school). 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--you would need to bring oxygen on a real visit. 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. The planets are really just bits of debris 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. From here, the sun just looks like an unusually bright yellow star, and we wouldn't even recognize the inner planets--including our own--if they weren't labeled. You could get lost pretty easily around here. 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 before you arrived. 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. That means a beam of light, which 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. If aliens from other star systems had ever visited Earth, they would need technology almost beyond our imagining. Either that or long lives and a whole lot of patience.

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 that Sirius is a binary star system, with two stars orbiting a common center of gravity. 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 an even bigger jump. We're going to the Pleiades, or Seven Sisters (you may also recognize them from the Subaru logo). The Pleiades are a group of stars at least 390 light years away. 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're 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 thousand or so stars in the cluster. Blue giants burn bright and die young, blazing 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.

NASA, ESA, AURA/Caltech, Palomar Observatory

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 star that defines Orion's left shoulder. When I say Betelgeuse is enormous, I mean it's really just stupendously sizeable. A common comparison is that if our sun were replaced with Betelgeuse, it would swallow Mercury, Venus, Earth, and Mars, and come nearly to the orbit of Jupiter. The giant star is nearing the end of its life, though, 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 width of a dime, Rigel would be about the width of a hula hoop. 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 is caused partly by its size, but mostly by its intense heat. Rigel's brilliance is the reason we see it so clearly, even though it's about 860 light years away, which means the light we see left it around the time Genghis Khan was born.

Now let's take a closer look at a place where stars are born (no, not Hollywood). 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 1,340 light years from earth, we see that it's really a bright cavity in a more extensive cloud. It's like an amphitheater packed with 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.

NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team
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 explosion in 1054 AD. Back then, Chinese astronomers 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--a neutron star. 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 exists out there, but if science has taught us anything, it's that nature is full of surprises.

X-Ray: NASA/CXC/J.Hester (ASU); Optical: NASA/ESA/J.Hester & A.Loll (ASU); Infrared: NASA/JPL-Caltech/R.Gehrz (Univ. Minn.)
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 couldn't exist without them. Scientists think when the universe was young, 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.

Let's stop and get our bearings. 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 far. In the picture below, the yellow line shows roughly where we have been on our tour. 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.

NASA/JPL-Caltech/R. Hurt
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. In the next image, we see that our galaxy is part of a group of several galaxies, called the Local Group ("local" is a relative term, obviously). The Milky Way and another large spiral galaxy, the Andromeda Galaxy, are the biggest galaxies in the system, but there are also over 50 smaller galaxies orbiting around the larger ones. The Andromeda Galaxy is visible to the naked eye on clear nights, in the constellation Andromeda. Of course, since it's 2.5 million light years away, we see it as it was 2.5 million years ago. The deeper we look into space, the farther back in time we see.

Andrew Z. Colvin, CC Attribution, Share-Alike License
The Local Group is a fairly small association of galaxies. There are much larger associations, called clusters, which can hold over 1000 galaxies. Groups and clusters, in turn, combine together to form superclusters of galaxies. In the next image, we see that the Local Group is linked to several clusters of galaxies making up the Virgo Supercluster. 

Andrew Z. Colvin, CC Attribution, Share-Alike License
If we keep zooming out, we'll find that the observable universe is made up of countless superclusters and clusters of galaxies, strung together in walls and filaments. In between these are gigantic regions of empty space known as voids. The pattern is a little like a foam of soap bubbles, where the galaxies form the walls of the bubbles, and the voids are the space inside the bubbles. This is the largest level of structure we know about. At the largest scale, the universe seems to be a frothy ocean of galactic clusters and superclusters, extending for billions of light years in every direction. It's a pretty big place. 

Andrew Z. Colvin, CC Share-Alike License

The first images were created with two free programs. Stellarium is a free planetarium application that lets you see what stars will be out at night wherever you live. Celestia is a "space simulation" application which allows you to virtually fly around space.

Good Books and Websites on Astronomy:

Astronomy : a beginner's guide to the universe / Eric Chaisson, Steve McMillan.

Space odyssey : voyaging through the cosmos / William Harwood.

Hayden Planetarium: American Museum of Natural History

HubbleSite.org: Amazing images and discoveries from the Hubble Telescope.