We begin our discussion of Stars with this photo of a total eclipse. Though the Sun is millions of times the size of our moon, they both appear to be a similar size in our sky when viewed from Earth. In other words, the Sun and Moon are each the right distance away from Earth so that they take up a similar space in the sky. We don't know how common this is, but when you consider that the Moon hasn't always been the same distance away from the Earth, we live in just the place at just the right time to see an almost perfect solar eclipse. Eclipses also played a historical role in modernizing our understanding of Stars. For a long time people assumed, because of how it appears from Earth, that the Sun was a flawless pearl of fire that would burn eternally. This eternal Sun agreed with many religious notions about how our Universe came to be, and how it never changes. However, when you look at the photograph of our Sun during an eclipse you can immediately tell that the surface of the Sun is probably not smooth. The light coming off of its surface is not uniform, and if you're really observant, you might notice that the light seems to define poles. As you know from the section on gravity, north and south poles only exist on objects that spin.
I Love to summarize.
So let's recap what we've already learned about stars. Firstly, we know that even small stars are enormous relative to the Earth. We know that stars form when a portion of a nebula collapses and spins, forming an accretion disk. We know that accretion disks have the most mass located in the center, and that the center of the disk has the most gravity. We've learned that when the center of the swirling disk heats up due to friction a star ignites because the extreme temperatures and pressures cause thermonuclear fusion to occur. We know that fusion changes the chemical composition of a star over time, and that it also releases a lot of light and heat, which is why stars shine. We know that hydrogen is the most common fuel for stars.
As you can see, at this point, we already know a lot about stars. Well, we know a lot about what all stars have in common. In these next few sections were going to briefly cover how stars are classified, and we're even going to look inside of one! However, for anyone looking for just a brief overview, the following sections may be extemporaneous. If you aren't interested in even briefly dissecting a star, feel free to skip down to the Smaller Than the Smallest Stars: Brown Dwarfs section below, where we talk about the size of the smallest stars. However, we are going to talk a lot about how stars die in the next section on Supernova, and in order to have even a layman understanding of those processes, you need to understand the processes going on at the core of stars. Thus, I recommend you at least read the section The Core, because a star's death begins there.
As you can see, at this point, we already know a lot about stars. Well, we know a lot about what all stars have in common. In these next few sections were going to briefly cover how stars are classified, and we're even going to look inside of one! However, for anyone looking for just a brief overview, the following sections may be extemporaneous. If you aren't interested in even briefly dissecting a star, feel free to skip down to the Smaller Than the Smallest Stars: Brown Dwarfs section below, where we talk about the size of the smallest stars. However, we are going to talk a lot about how stars die in the next section on Supernova, and in order to have even a layman understanding of those processes, you need to understand the processes going on at the core of stars. Thus, I recommend you at least read the section The Core, because a star's death begins there.
The Classification of stars
Looking at this diagram, can you guess that our Sun is a G class star? You might also notice that this diagram seems color coordinated. Do you see ROYGBIV in this picture? That is because the hottest stars are usually the biggest, and the biggest stars usually burn closer to the violet side of the spectrum, leaving smaller stars to burn towards the red. The color of a star is determined by the temperature of its surface. Yes, stars do have a surface, in a manner of speaking. Knowing this, lets go ahead and dissect a star so we can see its different layers!
Stars: Outter Composition.
Corona.
If you scroll up to the introductory picture of this section, the picture of a solar eclipse, the light that you see is actually part of the Sun's corona. The machinations of a stars corona are extremely complicated, and many are still under debate to this day. For now, you should know that a star's corona is the outer-most layer of a star's atmosphere. The world corona comes from the Latin word for "crown", but in reality a star's corona shifts around, sometimes gathering at the equatorial line, sometimes stretching out to the poles. Counter-intuitively, a stars Corona can be millions of degrees hotter than the actual surface of the star - the photosphere.
Chromosphere.
Before we descend through the extremely hot Corona down onto the surface of the star, we should stop and take note of the chromosphere. Without getting into great detail, the chromosphere of a star is merely a wide pocket of low density gas between the Corona and the photosphere. Like the corona, the chromosphere of our Sun can only be seen from Earth during an eclipse. If we look down from here, we'll see a breathtaking sight. The opaque surface below us does not look like lava flowing out of a volcano. If we could somehow however inside our Sun's chromosphere and look down, we would see enormous pillars of super heated gas jetting up from the surface of the star. We may think we're looking onto a field of fiery grass!
Photosphere: Star Surface.
When we talk about the Photosphere, we're talking about the apparent "surface" of a star. However, the photosphere is not solid, or liquid, but super-heated gas. Think of it like this: we can see through the Sun's corona, though it is extremely bright, and we can see through the Chromosphere though it is actually very colorful, but when we hit the region of the star where we can't see any deeper, that region is the photosphere and we call that region the "surface" of a star. The photoshpere of a star is anything but flat and calm. Heat rising from deeper in the star churns the surface of the star creating tidal waves of hot gas, and pillars of fire! And you should recall that it is the temperature of the photosphere which gives stars their apparent colors.
If you scroll up to the introductory picture of this section, the picture of a solar eclipse, the light that you see is actually part of the Sun's corona. The machinations of a stars corona are extremely complicated, and many are still under debate to this day. For now, you should know that a star's corona is the outer-most layer of a star's atmosphere. The world corona comes from the Latin word for "crown", but in reality a star's corona shifts around, sometimes gathering at the equatorial line, sometimes stretching out to the poles. Counter-intuitively, a stars Corona can be millions of degrees hotter than the actual surface of the star - the photosphere.
Chromosphere.
Before we descend through the extremely hot Corona down onto the surface of the star, we should stop and take note of the chromosphere. Without getting into great detail, the chromosphere of a star is merely a wide pocket of low density gas between the Corona and the photosphere. Like the corona, the chromosphere of our Sun can only be seen from Earth during an eclipse. If we look down from here, we'll see a breathtaking sight. The opaque surface below us does not look like lava flowing out of a volcano. If we could somehow however inside our Sun's chromosphere and look down, we would see enormous pillars of super heated gas jetting up from the surface of the star. We may think we're looking onto a field of fiery grass!
Photosphere: Star Surface.
When we talk about the Photosphere, we're talking about the apparent "surface" of a star. However, the photosphere is not solid, or liquid, but super-heated gas. Think of it like this: we can see through the Sun's corona, though it is extremely bright, and we can see through the Chromosphere though it is actually very colorful, but when we hit the region of the star where we can't see any deeper, that region is the photosphere and we call that region the "surface" of a star. The photoshpere of a star is anything but flat and calm. Heat rising from deeper in the star churns the surface of the star creating tidal waves of hot gas, and pillars of fire! And you should recall that it is the temperature of the photosphere which gives stars their apparent colors.
Stars: Inner Composition.
The Convection Zone.
As we descend further into the star, the pressure of all of that gas above us is enormous, crushing us down. We reach a point where we can actually sort of recognize this weather pattern inside the convection zone. The convection zone operates sort of like hot and cold air do on Earth. On Earth, hot air rises into the atmosphere, where it cools, and then falls back down toward the Earth. In a star's convection zone this same pattern is at work, only its happening to fire, not air. In a stars convection zone, plasma that is heated by the energy deeper in the core rises toward the surface of the star, where it cools and then begins to descend back toward the core. It is called the convection zone because energy is transferred via convection, or the transfer of heat by the flowing of a liquid, gas, or plasma.
As we descend further into the star, the pressure of all of that gas above us is enormous, crushing us down. We reach a point where we can actually sort of recognize this weather pattern inside the convection zone. The convection zone operates sort of like hot and cold air do on Earth. On Earth, hot air rises into the atmosphere, where it cools, and then falls back down toward the Earth. In a star's convection zone this same pattern is at work, only its happening to fire, not air. In a stars convection zone, plasma that is heated by the energy deeper in the core rises toward the surface of the star, where it cools and then begins to descend back toward the core. It is called the convection zone because energy is transferred via convection, or the transfer of heat by the flowing of a liquid, gas, or plasma.
The Radiative Zone.
Remember that the deeper we go into the star, the more pressure we feel from all of that gas crushing down on top of us. When you put so much pressure on matter, it can begin to act very strangely. The radiative zone is packed so tightly that light itself has trouble getting through it. In the picture on the right, we track the path of one photon of light as it leaves the core and moves through the radiative zone. As you can see, the light can not easily escape. The photon of light travels a very long and arduous journey through the thickly packed matter of the radiative zone, carrying heat and energy with it. That is why we call this portion of the core of a star the radiative zone, because unlike the convection zone which relies on the movement of gases to distribute heat, this portion of the star transfers its heat primarily through radiation. Light produced by nuclear fusion in the core of our Sun can bounce around in the radiative zone for 200,000 years! In other words, the light that shines on us today was born in the core of our Sun at around the same time that the very first humans walked the Earth!
Remember that the deeper we go into the star, the more pressure we feel from all of that gas crushing down on top of us. When you put so much pressure on matter, it can begin to act very strangely. The radiative zone is packed so tightly that light itself has trouble getting through it. In the picture on the right, we track the path of one photon of light as it leaves the core and moves through the radiative zone. As you can see, the light can not easily escape. The photon of light travels a very long and arduous journey through the thickly packed matter of the radiative zone, carrying heat and energy with it. That is why we call this portion of the core of a star the radiative zone, because unlike the convection zone which relies on the movement of gases to distribute heat, this portion of the star transfers its heat primarily through radiation. Light produced by nuclear fusion in the core of our Sun can bounce around in the radiative zone for 200,000 years! In other words, the light that shines on us today was born in the core of our Sun at around the same time that the very first humans walked the Earth!
The Core.
You might be wondering what's up with this picture, and how it relates to the core of a star. First, lets make some things clear. Every other section of a star that we've covered from the Corona to the radiative zone is hot. However, the only place in a star that is hot enough to sustain nuclear fusion - at least 15 million degrees Kelvin - is the core. It might seem like the entire star is burning; it is not. Only the core is "burning" in the sense that only the core is fusing elements together to release energy. The core of a star is roughly 1/4 the total size of the star. In the case of our sun, the core is about 200,000 miles across.
Because we know a little bit about nuclear fusion, the picture on the left may make immediate sense to us. If it doesn't, you can pull out a periodic table and follow the logical progression of elements seen in the picture. We begin with Hydrogen, the lightest element, which is at the top. When hydrogen fuses, it becomes helium, which is a little heavier, and thus falls closer to the center of the core due to gravity. Helium then fuses into something heavier, like carbon, and we work our way down through heavier and heavier elements until we reach Iron. In the core of a star, heavier elements begin to form "layers", with the heaviest being pulled down to the very center, as seen in the photo. Like hydrogen, Iron is a very important element in determining the way our universe has formed. Iron is the heaviest element a star can produce through thermonuclear fusion alone. Why is that important? Well, that means that if a star successfully produces Iron, it is doomed.
Because we know a little bit about nuclear fusion, the picture on the left may make immediate sense to us. If it doesn't, you can pull out a periodic table and follow the logical progression of elements seen in the picture. We begin with Hydrogen, the lightest element, which is at the top. When hydrogen fuses, it becomes helium, which is a little heavier, and thus falls closer to the center of the core due to gravity. Helium then fuses into something heavier, like carbon, and we work our way down through heavier and heavier elements until we reach Iron. In the core of a star, heavier elements begin to form "layers", with the heaviest being pulled down to the very center, as seen in the photo. Like hydrogen, Iron is a very important element in determining the way our universe has formed. Iron is the heaviest element a star can produce through thermonuclear fusion alone. Why is that important? Well, that means that if a star successfully produces Iron, it is doomed.
Iron: The Starkiller.
"To be clear, you're talking about the stuff that my cast iron skillet is made of?" Yes! The only place in the entire universe where the iron in your cookware, your car, and your blood can form is in the core of a star! And, not just any stars, but big stars. Giant stars. "Alright, I get it. That's what Carl Segan meant when he said we're made of star stuff. Without stars fusing elements into heavier elements, the Universe would still be almost completely made of Hydrogen and many of the elements we take for granted wouldn't exist. That still doesn't explain how iron kills stars!" Fair point, but just once more lets think about the cast iron skillet in your kitchen. When you touch it, you are touching something that we know has been in the deepest, hottest, most hellish part of a star. It's been in a place that in all likelihood human beings will never see, much less be able to visit. Its amazing to think about.
Now, iron kills stars because even the biggest stars cannot fuse two iron atoms together. Fusing lighter elements like Hydrogen releases energy, while fusing two Iron atoms requires energy. A lot of energy. Thus, once a star starts fusing elements into Iron, it is dying. Stars are inflated by the outward pressure of the energy released by nuclear fusion, and if fusion should stop, a star's own gravity will begin to crush it. Once a star begins producing iron, it has reached the end of its rope. It can't release enough energy to hold itself up against its own weight, and it begins to collapse. This is the beginning of one of the processes that will set off a Supernova.
The entire next section of this site covers supernovae, which is a form of star death. A supernova requires that iron be produced in the core of a star. But not all stars can produce Iron. In fact, stars that can produce iron are getting more rare in our universe as time goes on. Our Sun for instance, will never fuse anything into iron. Our Sun only has enough energy to fuse things into Helium up to Oxygen, and therefore our Sun will not go supernova. It takes enormous, massive stars to generate enough energy to fuse elements into Iron, which is why the size of stars directly correlates with how they die. It works like this: smaller stars will burn up their hydrogen transforming it into helium. It takes more heat and pressure (more energy) to fuse helium into something bigger. The size of the star will dictate how hot it burns, and if the star is too small, it may not have enough energy to fuse anything heavier than helium or carbon or Oxygen. A star that cannot fuse Iron will not go supernova. Instead, they will become a Red Giant.
Now, iron kills stars because even the biggest stars cannot fuse two iron atoms together. Fusing lighter elements like Hydrogen releases energy, while fusing two Iron atoms requires energy. A lot of energy. Thus, once a star starts fusing elements into Iron, it is dying. Stars are inflated by the outward pressure of the energy released by nuclear fusion, and if fusion should stop, a star's own gravity will begin to crush it. Once a star begins producing iron, it has reached the end of its rope. It can't release enough energy to hold itself up against its own weight, and it begins to collapse. This is the beginning of one of the processes that will set off a Supernova.
The entire next section of this site covers supernovae, which is a form of star death. A supernova requires that iron be produced in the core of a star. But not all stars can produce Iron. In fact, stars that can produce iron are getting more rare in our universe as time goes on. Our Sun for instance, will never fuse anything into iron. Our Sun only has enough energy to fuse things into Helium up to Oxygen, and therefore our Sun will not go supernova. It takes enormous, massive stars to generate enough energy to fuse elements into Iron, which is why the size of stars directly correlates with how they die. It works like this: smaller stars will burn up their hydrogen transforming it into helium. It takes more heat and pressure (more energy) to fuse helium into something bigger. The size of the star will dictate how hot it burns, and if the star is too small, it may not have enough energy to fuse anything heavier than helium or carbon or Oxygen. A star that cannot fuse Iron will not go supernova. Instead, they will become a Red Giant.
Iron-Less Stars: The Red Giants
When all of the Hydrogen burns out of a star that is .3 to 8 Solar Masses (Stars a mere fraction the size of our Sun all the way up to Stars that are the size of 8 Suns) the outward pressure begins to wane and the core (which is now mostly helium) begins to collapse. This raises the temperature and also allows some of the Hydrogen from the outer parts of the core to fall deeper in, and this new Hydrogen ignites, fusing into more helium, only it is burning much hotter and putting out a lot more pressure. The energy and outward pressure from this death throw expands the star enormously, forming a Red Giant. The Red Giant is less dense than the original star, and its outer layers are much cooler as you can see above, but because the Red Giant has swelled, and is now... well, giant, the star gets brighter than it ever was before. The outer layers of gas, however, begin to disperse and are able to break free of the star's gravity. Eventually huge puffs of gas begin to be expelled from the star, like enormous cosmic breaths, where they rapidly cool and float out into space. The core of the star is still pretty hot, and shining, and that light reflects off the escaping gas and forms a Planetary Nebula. Planetary nebulas are much smaller than the nebulas we talked about before because they only contain materials that were already present in the star. Also the term planetary nebula is misleading, as they have nothing to do with planets. Another name for a planetary nebula is a Stellar-Remnant Nebula, which is a little more accurate. Stellar-Remnant nebulas are just floating clouds of gas that have been ejected from a dying star, and so they differ in size and chemical composition (they have more helium and heavier elements) than the typical regular nebulas we talked about in the earlier section. However, that doesn't stop them from being just as beautiful.
When a stellar-remnant nebula forms from a red giant, the core of the original star remains. It is highly compressed because once fusion stopped all the weight of the material collapsed under gravity, and with no new fusion to put out pressure, the core can shrink to around the size of the Earth, but still have the mass of something as big as the Sun. It is incredibly dense. This shining core which lights up the forming nebula, is called a White Dwarf, and though it is no longer fusing elements to produce energy, a white drawf will still shine for billions, maybe even trillions of years.
So to recap:
-Stars more than 8 times the size of our Sun, burns toward the blue side of the spectrum, can fuse elements into Iron, and will die in a supernova explosion
-Stars less than 8 times the size of our Sun burns toward the red side of the spectrum, cannot fuse elements into Iron, become red giants at the end of their life, and will leave behind a shining core called a white dwarf.
When a stellar-remnant nebula forms from a red giant, the core of the original star remains. It is highly compressed because once fusion stopped all the weight of the material collapsed under gravity, and with no new fusion to put out pressure, the core can shrink to around the size of the Earth, but still have the mass of something as big as the Sun. It is incredibly dense. This shining core which lights up the forming nebula, is called a White Dwarf, and though it is no longer fusing elements to produce energy, a white drawf will still shine for billions, maybe even trillions of years.
So to recap:
-Stars more than 8 times the size of our Sun, burns toward the blue side of the spectrum, can fuse elements into Iron, and will die in a supernova explosion
-Stars less than 8 times the size of our Sun burns toward the red side of the spectrum, cannot fuse elements into Iron, become red giants at the end of their life, and will leave behind a shining core called a white dwarf.
Size Matters: Smaller than the smallest stars.
When we talk about the smallest possible stars, we're talking about objects that can be very small, a fraction of the size of our Sun. We're not sure exactly how big the smallest stars have to be to ignite, because mass is not the only factor in star formation. Another factor is density. If you pack enough matter into a small space, a star can ignite. When we talk about the smallest possible stars, we're getting into murky territory that includes failed stars called Brown Dwarfs. A brown dwarf is an object that is at least twice the mass of Jupiter, but not quite big enough to ignite fusion within its core. Therefore, it is not, technically, a star. In this sense, Jupiter itself can be viewed as a failed star. Jupiter has an enormous amount of mass, but not nearly enough to cause nuclear fusion to occur.
In the picture on the right we see our Sun. Under the image of the Sun we see a surface temperature, a description, and under that we see the description "1 MS". In this picture, "1 MS" means "1 Solar Mass". So we're going to use the size of the Sun to measure other objects in this picture. For instance, Gliese 229a, the next star over, is about "1/2 Solar Mass" or half the size of our Sun. The next object, Teide 1, is not a star. It is a failed star, a brown dwarf that is between 55-65 Jupiter Masses. If "1 MS" is one solar mass, you can probably guess that "1 MJ" is equal to "1 Jupiter Mass" or the simply the mass of Jupiter. Teide 1 weighs about as much as 60 Jupiters, give or take a few Jupiters! Thus, we can't give an exact answer to the question of what are the smallest stars, but now we know that the smallest stars probably weigh between 55-65 Jupiter Masses, and 1/2 Solar Mass.
We'll talk more about Brown Dwarfs and their compositions later! For now, use this picture to get a working idea of the size of the smallest stars. For perspective in the above picture, just remember that Jupiter could fit over 1000 Earths inside it. You may be wondering how Teide 1 can be the same rough size as Jupiter, yet still be 60 times more massive? Well, nuclear fusion in a star's core creates a very intense outward pressure in all directions, acting against gravity and inflating the star. To put this into perspective, if the Sun stopped nuclear fusion right now, the outward pressure from the reaction would stop, and the enormous weight of all that gas would crush the entire object down to about the size of the Earth! That's right. Without the outward pressure created by fusion, the Sun would collapse into an object about the size of the Earth. The more mass an object has the more gravity it creates, and thus, in nature we find that heavier does not always mean bigger. Because objects like Teide 1 do not fuse elements at their cores, there is no outward pressure to counteract gravity, and so all of that mass crushes itself into a very dense ball. As a result of extreme density, brown dwarfs are extremely hot and have some very exotic weather. If you could drive a spaceship into the atmosphere of a brown dwarf, you could find yourself under a storm of liquid Iron raining from the skies!
In the picture on the right we see our Sun. Under the image of the Sun we see a surface temperature, a description, and under that we see the description "1 MS". In this picture, "1 MS" means "1 Solar Mass". So we're going to use the size of the Sun to measure other objects in this picture. For instance, Gliese 229a, the next star over, is about "1/2 Solar Mass" or half the size of our Sun. The next object, Teide 1, is not a star. It is a failed star, a brown dwarf that is between 55-65 Jupiter Masses. If "1 MS" is one solar mass, you can probably guess that "1 MJ" is equal to "1 Jupiter Mass" or the simply the mass of Jupiter. Teide 1 weighs about as much as 60 Jupiters, give or take a few Jupiters! Thus, we can't give an exact answer to the question of what are the smallest stars, but now we know that the smallest stars probably weigh between 55-65 Jupiter Masses, and 1/2 Solar Mass.
We'll talk more about Brown Dwarfs and their compositions later! For now, use this picture to get a working idea of the size of the smallest stars. For perspective in the above picture, just remember that Jupiter could fit over 1000 Earths inside it. You may be wondering how Teide 1 can be the same rough size as Jupiter, yet still be 60 times more massive? Well, nuclear fusion in a star's core creates a very intense outward pressure in all directions, acting against gravity and inflating the star. To put this into perspective, if the Sun stopped nuclear fusion right now, the outward pressure from the reaction would stop, and the enormous weight of all that gas would crush the entire object down to about the size of the Earth! That's right. Without the outward pressure created by fusion, the Sun would collapse into an object about the size of the Earth. The more mass an object has the more gravity it creates, and thus, in nature we find that heavier does not always mean bigger. Because objects like Teide 1 do not fuse elements at their cores, there is no outward pressure to counteract gravity, and so all of that mass crushes itself into a very dense ball. As a result of extreme density, brown dwarfs are extremely hot and have some very exotic weather. If you could drive a spaceship into the atmosphere of a brown dwarf, you could find yourself under a storm of liquid Iron raining from the skies!