The above picture from the Australian National University is an artist rendition of a process called Accretion. Understanding how accretion works is fundamental in understanding not only the history of the Universe, but also its potential futures. Finding a good analogy for accretion on Earth is pretty simple; you need only visit a pizzeria. When a chef takes a lump of dough and spins it in the air, the centrifugal force will flatten the dough into a disk shape. The process is more complex when it happens in space, but for now, you should familiarize yourself with the circular shape seen above. In order to understand accretion, we will take a look at our own solar system and how it formed over 4 billion years ago. We will begin our journey all those years ago when the Earth, the Sun, and all of the other planets were still just floating clouds of gas circling the center of our Galaxy.
What is a Nebula?
Throughout the history of the Universe, gas and plasma have been much more common than solids and liquids. A Nebula is just an enormous cloud of gas and dust floating in space. The picture on the right is the Eagle Nebula in the constellation Serpens. The three dominant structures in the photo (yes this is a photo, not an artist rendition) are affectionately called The Pillars of Creation. In the photo, the darker regions are thicker and more densely filled with gas than the surrounding areas. You can think of the darker areas like the gray belly of a rain-cloud, where the heavy moisture is pulled lower than the lighter air and eventually falls to the ground as rain. Nebulae can have different chemical compositions, but the main ingredient in many nebulae is Hydrogen.
Now that you've seen a picture of a nebula, look back up at the picture of the accretion disk above. If you could see inside the darker parts of the Pillars of Creation, you would be able to see dense little pockets that have begun the accretion process. In fact, every solar system in the entire universe was born from a nebula. The density and composition of gas inside the nebula defines everything from what kind of star to what kind of planets will form. Soon, we'll talk about how nebulae like the pillars of creation eventually form solar systems. But, before we move on, its important that you get a sense of the size of the nebula in this picture.
The largest pillar in the picture is about 4 light years tall. In other words, that enormous pillar of gas would stretch from our Sun to Proxima Centuri. It's big. From the last section on distance, we already know how many miles there are in 4 light years, but it is worth repeating. You are looking at a picture of a cloud of gas that is about 25,000,000,000,000 (twenty five trillion) miles tall! And that's just one of the pillars! And the pillars are only a small part of the entire nebula! The entire Eagle Nebula is hundreds of light years in size.
Now that we have an idea of the enormity of nebulae, we're going to look at what happens in smaller regions inside the clouds, regions perhaps just a few light years across, when the gas gets so dense that it begins to swirl around and form a disk.
Now that you've seen a picture of a nebula, look back up at the picture of the accretion disk above. If you could see inside the darker parts of the Pillars of Creation, you would be able to see dense little pockets that have begun the accretion process. In fact, every solar system in the entire universe was born from a nebula. The density and composition of gas inside the nebula defines everything from what kind of star to what kind of planets will form. Soon, we'll talk about how nebulae like the pillars of creation eventually form solar systems. But, before we move on, its important that you get a sense of the size of the nebula in this picture.
The largest pillar in the picture is about 4 light years tall. In other words, that enormous pillar of gas would stretch from our Sun to Proxima Centuri. It's big. From the last section on distance, we already know how many miles there are in 4 light years, but it is worth repeating. You are looking at a picture of a cloud of gas that is about 25,000,000,000,000 (twenty five trillion) miles tall! And that's just one of the pillars! And the pillars are only a small part of the entire nebula! The entire Eagle Nebula is hundreds of light years in size.
Now that we have an idea of the enormity of nebulae, we're going to look at what happens in smaller regions inside the clouds, regions perhaps just a few light years across, when the gas gets so dense that it begins to swirl around and form a disk.
Inside the Clouds.
On Earth, gravity pulls everything down. In a nebula, the process is more complex because the gas and dust doesn't have a well-defined center of gravity, at least, not at first. However, one of the most important mechanisms in the Universe is also one of the easiest to understand: stuff tends to clump together. If the gas in a nebula were uniform (if it didn't clump up), enormous structures like the Pillars of Creation would never form, and the entire Eagle Nebula would look more like a featureless fog than the dynamic clouds pictured above. But, thankfully, in our Universe, stuff clumps together. And furthermore, the thicker the clump of gas and dust the more gravity it has.
Thus, accretion begins when a clump of matter (in this case the gas in a nebula) gets bigger, and becomes denser than the surrounding area. This lump of gas creates more gravity, which then sucks up more of the surrounding gas and dust, which then creates more gravity, which then... well you get the picture. When matter (any kind of matter including solids, liquids, gases, and plasma) gets closer together, it creates more gravity. In space where huge lumps of gas float freely, the gas can condense, get thicker, pull in more gas, and the overall effect of all that gas falling in toward the thicker pocket of gas is that the entire structure begins to spin - in accordance with the Law of Angular Momentum. The Law of Angular Momentum states that the closer something gets to its Axis of Rotation, the faster it will spin. You can visualize this easily if you imagine a Figure Skater spinning on the ice. If she extends her arms outward, she will spin more slowly. If she pulls her arms closer to her body (and closer to the Axis of Rotation that is an invisible line that extends upwards from the point where her skate touches the ice to the top of her head) she will spin faster. The same thing happens in accretion disks. As the gas begins to collapse, it spins faster.
Right now you may be thinking, "Well, if thick pockets of gas have all this gravity, why don't they just suck up everything around them and turn into a big structure like a star?" It's a good question to ask, and the answer is that they do! In fact, this is exactly how stars are born. If you look at the top picture once more, you'll see that when matter goes through the accretion process, it forms a spinning disk with a bulging center. This is because the center of the disk has the most gravity, and thus it is sucking in the most gas.
Stars are born when enough gas from a nebula gets packed into a small space through the process of accretion. Much like the chef and his pizza dough, the accretion disk spins rapidly around the center. The spinning generates a stream of accelerated particles, some orbiting the center of mass, some falling in, and the collisions between these particles creates friction within the disk. This friction creates heat - a lot of heat.
Thus, accretion begins when a clump of matter (in this case the gas in a nebula) gets bigger, and becomes denser than the surrounding area. This lump of gas creates more gravity, which then sucks up more of the surrounding gas and dust, which then creates more gravity, which then... well you get the picture. When matter (any kind of matter including solids, liquids, gases, and plasma) gets closer together, it creates more gravity. In space where huge lumps of gas float freely, the gas can condense, get thicker, pull in more gas, and the overall effect of all that gas falling in toward the thicker pocket of gas is that the entire structure begins to spin - in accordance with the Law of Angular Momentum. The Law of Angular Momentum states that the closer something gets to its Axis of Rotation, the faster it will spin. You can visualize this easily if you imagine a Figure Skater spinning on the ice. If she extends her arms outward, she will spin more slowly. If she pulls her arms closer to her body (and closer to the Axis of Rotation that is an invisible line that extends upwards from the point where her skate touches the ice to the top of her head) she will spin faster. The same thing happens in accretion disks. As the gas begins to collapse, it spins faster.
Right now you may be thinking, "Well, if thick pockets of gas have all this gravity, why don't they just suck up everything around them and turn into a big structure like a star?" It's a good question to ask, and the answer is that they do! In fact, this is exactly how stars are born. If you look at the top picture once more, you'll see that when matter goes through the accretion process, it forms a spinning disk with a bulging center. This is because the center of the disk has the most gravity, and thus it is sucking in the most gas.
Stars are born when enough gas from a nebula gets packed into a small space through the process of accretion. Much like the chef and his pizza dough, the accretion disk spins rapidly around the center. The spinning generates a stream of accelerated particles, some orbiting the center of mass, some falling in, and the collisions between these particles creates friction within the disk. This friction creates heat - a lot of heat.
Up Close and Personal:
Imagine being inside a nebula in a space suit, just floating through the gas. The first thing you'll notice is that there is very little gravity pulling you in any direction. If there are no stars or other large objects near you, you will float with the nebula in a very large circle around the center of our galaxy. As long as the nebula isn't disrupted by the gravity of another object, very little will seem to change. Because there are no stars near, it's very cold, but the light from distant stars may light up the nebula around you in fantastic colors. Let's speed up time.
Over the course of thousands, or perhaps millions of years, you'll see the gas around you starting to form clumps -- clouds. When a cloud around you get's dense enough, it will start to get darker as light has trouble passing through it. When it gets big enough, you will find yourself falling gently toward it. As you fall, you'll notice that the dark cloud is starting to rotate. Closer now, you'll see that the cloud is starting to form a disk shape. You probably won't fall in a straight line toward the center of the disk. Instead, you will find yourself being slung in circles around it, like water falling into a drain. As you move closer, you'll notice that you are being flung faster and faster around the center of the disk. As more gas falls in on top of you, you'll also notice that the temperature has begun to rise, and the center of the disk will begin to glow. Around you the calm of the nebula is gone, and the gas is churning violently. The friction between the particles of gas and dust as it grinds together creates the heat that you feel. Finally, when the temperature in the glowing center of the disk reaches about 15 million degrees, a star will ignite.
You may still be getting slung in a circle around the star, or you may have fallen into it. Either way, you've just experienced the birth of a solar system!
Over the course of thousands, or perhaps millions of years, you'll see the gas around you starting to form clumps -- clouds. When a cloud around you get's dense enough, it will start to get darker as light has trouble passing through it. When it gets big enough, you will find yourself falling gently toward it. As you fall, you'll notice that the dark cloud is starting to rotate. Closer now, you'll see that the cloud is starting to form a disk shape. You probably won't fall in a straight line toward the center of the disk. Instead, you will find yourself being slung in circles around it, like water falling into a drain. As you move closer, you'll notice that you are being flung faster and faster around the center of the disk. As more gas falls in on top of you, you'll also notice that the temperature has begun to rise, and the center of the disk will begin to glow. Around you the calm of the nebula is gone, and the gas is churning violently. The friction between the particles of gas and dust as it grinds together creates the heat that you feel. Finally, when the temperature in the glowing center of the disk reaches about 15 million degrees, a star will ignite.
You may still be getting slung in a circle around the star, or you may have fallen into it. Either way, you've just experienced the birth of a solar system!
Starbirth.
The picture on the right from nasa.gov is an artist rendition of the birth of a star. By now, you should be able to label most parts of the picture. On the boarders of the picture, we see the parent nebula. Gas is now swirling into the very center of the picture - the birthplace of a star. Notice that the disk of gas that formed hasn't been completely sucked into the young star. When the star reaches the right temperature and activates, all of the light and heat energy it emits pushes the gas away even while the gravity of the star pulls at it. Thus, gas left over from the formation of the star may continue to orbit it. The left-over dust and gas, now in orbit around the star, will then begin to form smaller accretion disks which will eventually form planets and moons.
In our solar system, all of the planets (Pluto not included) orbit the Sun along roughly the same plane. There is a small amount of variance, but all of the planets circle around the Sun near it's equatorial plane. The reason the planets in our solar system line up so nicely is because, long ago, accretion stretched out the remaining gas into a relatively flat disk from which the planets, Earth included, formed.
You may also notice that in this picture the newborn star seems to have two yellow jets shooting out of its north and south pole. We'll talk more about those jets and why they form later, but for now just know that some of the gas being sucked in by the star gets burped up in twin jets. That gas is shot through space and back into the nebula. This process helps to rid the solar system of debris, carving a hollow space in the clouds of gas. Now, you may also realize that we don't see those jets shooting out of our own Sun right now. That's because they do not last forever. Long ago, before our solar system calmed down, the Sun did spit out two jets of super heated gas from its poles. However, at that time, Earth didn't exist yet. If the jets look familiar to you, you might think you were looking at a quasar. However, this is not a quasar, though the mechanisms that produce both phenomenon are similar.
So now we have a newborn star emitting light and heat and gravity; we have a spinning cloud of gas orbiting the new star; and, we have a lot of space emptied out surrounding these two things. What happens next?
In our solar system, all of the planets (Pluto not included) orbit the Sun along roughly the same plane. There is a small amount of variance, but all of the planets circle around the Sun near it's equatorial plane. The reason the planets in our solar system line up so nicely is because, long ago, accretion stretched out the remaining gas into a relatively flat disk from which the planets, Earth included, formed.
You may also notice that in this picture the newborn star seems to have two yellow jets shooting out of its north and south pole. We'll talk more about those jets and why they form later, but for now just know that some of the gas being sucked in by the star gets burped up in twin jets. That gas is shot through space and back into the nebula. This process helps to rid the solar system of debris, carving a hollow space in the clouds of gas. Now, you may also realize that we don't see those jets shooting out of our own Sun right now. That's because they do not last forever. Long ago, before our solar system calmed down, the Sun did spit out two jets of super heated gas from its poles. However, at that time, Earth didn't exist yet. If the jets look familiar to you, you might think you were looking at a quasar. However, this is not a quasar, though the mechanisms that produce both phenomenon are similar.
So now we have a newborn star emitting light and heat and gravity; we have a spinning cloud of gas orbiting the new star; and, we have a lot of space emptied out surrounding these two things. What happens next?
The Formation of a Solar System.
The picture below shows the entire process of accretion from beginning to end. You should already be familiar with the first two stages.
In Section A: a nebulous cloud of gas begins to collapse and undergo accretion, forming a disk with a bulging center.
In Section B: we see that the center of the collapsing cloud of gas has gotten hot enough to ignite a star.
In Section C: we see the star is putting outward pressure via heat and energy, and things are starting to become more organized.
In Section D: the swirling disk of gas has become even more orderly, and bigger objects are starting to form called Planetesimals. There are a lot of forces at work in the formation of planetesimals in the remaining disk of gas surrounding a young star, but the main cause is the same as what causes stars to be born: stuff clumps together. Planetesimals form when stuff clumps together in the swirling disk of gas and dust around a new star. You can think of them as miniature planets. These denser objects have more gravity than the surrounding gas, so as they orbit the star, they suck up all the gas and dust that is in their orbital path. That is why in both C and D we see that the accretion disk is forming clear concentric circles around the star. In essence, young planets have cleaned out all the gas in their path.
In Section A: a nebulous cloud of gas begins to collapse and undergo accretion, forming a disk with a bulging center.
In Section B: we see that the center of the collapsing cloud of gas has gotten hot enough to ignite a star.
In Section C: we see the star is putting outward pressure via heat and energy, and things are starting to become more organized.
In Section D: the swirling disk of gas has become even more orderly, and bigger objects are starting to form called Planetesimals. There are a lot of forces at work in the formation of planetesimals in the remaining disk of gas surrounding a young star, but the main cause is the same as what causes stars to be born: stuff clumps together. Planetesimals form when stuff clumps together in the swirling disk of gas and dust around a new star. You can think of them as miniature planets. These denser objects have more gravity than the surrounding gas, so as they orbit the star, they suck up all the gas and dust that is in their orbital path. That is why in both C and D we see that the accretion disk is forming clear concentric circles around the star. In essence, young planets have cleaned out all the gas in their path.
Planar Geometry: Equators, Spins, Poles.
Before we move on, lets talk a little bit about some of the terminology used to describe space. Let's use the Sun as an example, since we're already pretty familiar with these concepts on Earth. Gas that is falling into the center of an accretion disk doesn't fall exactly in a straight line toward the center. In fact, most of the gas falls along a curbed path similar to a the path of a plane landing on Earth, or a meteorite entering our atmosphere. As a reminder, the result of this inherent property of gravity is that when objects are formed in space, they spin.
Our Sun has its own spin, and the nature of that spin can be intuited by the way the planets orbit it. If you took a ping pong ball and spun it with your fingers, the bottom of the spinning ball would be its south pole, and the top it's north pole. If you stuck a needle through the ball that aligned with both the north and south pole, the needle would define the Axis of Rotation, or just the axis. If you could draw a line around the the ball directly between the north and south pole, you would have drawn it's equator. If you could hypothetically stick a flat piece of paper through the equator, you would have defined the Equatorial Plane.
As you can imagine now that you know the shape of accretion disks, the equatorial plane of our Sun was roughly where the disk itself spun around it. The planets that formed from the disk, then, had a tendency to orbit the Sun along its equatorial plane, as you can see in the picture above. We can tell where the north and south poles of the Sun are just by looking at the orbits of the planets. The planets themselves formed from the leftover dust and gas in the same way as the Sun did, so they have a tendency to have poles that line up with the Sun's - that is, they tend to spin in the same direction, along the same axis, and orbit along the Sun's equatorial plane. You can see from the picture, however, that in reality everything doesn't line up perfectly, even in our own solar system. In space, there are other factors that need to be taken into account.
Our Sun has its own spin, and the nature of that spin can be intuited by the way the planets orbit it. If you took a ping pong ball and spun it with your fingers, the bottom of the spinning ball would be its south pole, and the top it's north pole. If you stuck a needle through the ball that aligned with both the north and south pole, the needle would define the Axis of Rotation, or just the axis. If you could draw a line around the the ball directly between the north and south pole, you would have drawn it's equator. If you could hypothetically stick a flat piece of paper through the equator, you would have defined the Equatorial Plane.
As you can imagine now that you know the shape of accretion disks, the equatorial plane of our Sun was roughly where the disk itself spun around it. The planets that formed from the disk, then, had a tendency to orbit the Sun along its equatorial plane, as you can see in the picture above. We can tell where the north and south poles of the Sun are just by looking at the orbits of the planets. The planets themselves formed from the leftover dust and gas in the same way as the Sun did, so they have a tendency to have poles that line up with the Sun's - that is, they tend to spin in the same direction, along the same axis, and orbit along the Sun's equatorial plane. You can see from the picture, however, that in reality everything doesn't line up perfectly, even in our own solar system. In space, there are other factors that need to be taken into account.
Earth's Position in the Solar System.
To bring these points home, we're going to take a quick look at the Earth and how it orbits the Sun. In order to make this picture easier to understand, you should note that the black line that says "orbit direction" denotes the plane at which the Earth orbits around the Sun, and it lines up neatly with the Sun's Equatorial Plane. The vertical black line that is labeled "Perpendicular to orbit" is similar to where the Sun's north and South poles would be. The degree of variance between the "Perpendicular to orbit" line and the "Rotation Axis" line defines the tilt of the Earth.
This tells us a few things. It tell us that the Earth's equator does not line up with the Earth's orbit around the Sun, which tell's us that the Earth's equator does not line up with the Sun's equator. Incidentally, you can blame this nuance of Earth for the seasons we experience here. The "Ecliptic" line shows us Earth's orbit, while the "Celestial Equator" line shows us Earth's equator. If you measure the angle between those two lines, you'd see a difference of about 32 degrees. If you look at the poles of Earth, you can also see that circular arrows are drawn around the polar line. This tells us that Earth wobbles a bit.
But why does Earth not line up perfectly with the Sun if they both formed from the same accretion disk? How can the equator of Earth not line up perfectly with its orbital plane? Why does it wobble? These are exactly the questions you should be asking, and the questions astronomers asked. Many of you may be able to easily intuit some possible answers, but let's go ahead and talk about more about the planets, how they form, and why they don't always follow the rules.
This tells us a few things. It tell us that the Earth's equator does not line up with the Earth's orbit around the Sun, which tell's us that the Earth's equator does not line up with the Sun's equator. Incidentally, you can blame this nuance of Earth for the seasons we experience here. The "Ecliptic" line shows us Earth's orbit, while the "Celestial Equator" line shows us Earth's equator. If you measure the angle between those two lines, you'd see a difference of about 32 degrees. If you look at the poles of Earth, you can also see that circular arrows are drawn around the polar line. This tells us that Earth wobbles a bit.
But why does Earth not line up perfectly with the Sun if they both formed from the same accretion disk? How can the equator of Earth not line up perfectly with its orbital plane? Why does it wobble? These are exactly the questions you should be asking, and the questions astronomers asked. Many of you may be able to easily intuit some possible answers, but let's go ahead and talk about more about the planets, how they form, and why they don't always follow the rules.
Violence: Planetesimals and Planets.
Lets return to the swirling disk of gas and dust surrounding a newborn star. What happens inside that disk is accretion but on a smaller scale. The small planet like objects suck more and more gas and dust which either gets pulled into the object by gravity (making the planetesimal bigger), or begins to orbit around it (possibly to coalesce into a moon). As we already read, planets form from accretion just like stars do, so both planets and stars begin their lives rotating - spinning along an axis. However, because of the violence of the early solar system, most planetesimals will suffer countless impacts with other objects. These impacts, and the somewhat chaotic gravitational forces between objects play a part in defining the ultimate fate of a planet's rotation speed, axis, and orbit. A good example of this in our own solar system is Uranus. From the perspective of the Sun and the rest of the planets, Uranus is actually rolled over on its side, possibly due to a large impact.
Thus the entire process of accretion and solar system formation is extremely violent. Our own solar system probably formed more than a hundred planetesimals which were then constantly disrupted in their orbits, some crashing into each other, some falling into the Sun, and some even shooting out into space. But, all of that violence is creative rather than destructive. Eventually, after countless impacts, the solar system calmed down, and huge objects called Planets remained. Planets have a steady orbit around the star -- usually at or near the equatorial plane of the star -- but there are exceptions. In Space, there are exceptions to everything.
Which finally brings us to Section E in our picture of accretion: a relatively calm and gentle solar system of planets orbiting a star. A solar system just like the one we call home! Now that you have an idea about how accretion works, lets move on to the next section where will talk about the force that makes it all possible: Gravity.
Thus the entire process of accretion and solar system formation is extremely violent. Our own solar system probably formed more than a hundred planetesimals which were then constantly disrupted in their orbits, some crashing into each other, some falling into the Sun, and some even shooting out into space. But, all of that violence is creative rather than destructive. Eventually, after countless impacts, the solar system calmed down, and huge objects called Planets remained. Planets have a steady orbit around the star -- usually at or near the equatorial plane of the star -- but there are exceptions. In Space, there are exceptions to everything.
Which finally brings us to Section E in our picture of accretion: a relatively calm and gentle solar system of planets orbiting a star. A solar system just like the one we call home! Now that you have an idea about how accretion works, lets move on to the next section where will talk about the force that makes it all possible: Gravity.