The Life Of A Star

This is a long one, but it's worth it.

That's what she said!

Before we get into the nuts and bolts of a black hole, let's go through the life of a star.

A star can take two basic paths in this life: the quiet suburban life or the life of a rock star––live fast, die young and go out in a blaze of glory, leaving a big hole behind (in the hearts of your fans).

It all starts in things called molecular clouds, which are very dense nebulae (which are just a bunch of dust and gas and crap like that). The gases in the cloud, mostly hydrogen, are attracted to other gas molecules because they have mass, and if you have mass, you have gravity.  As more and more gas comes together, their collective gravity increases, attracting even more gas to join this gravity party. Now if you've ever been to a really crowded party, you might notice that the middle of the dance floor is a pretty hot place to be: bodies are rubbing together, and everyone is sweating. The same is true for the center of that globule of gas that is forming in the nebula, the atoms of gas are bumping into each other and creating heat. At a stellar birthday party, there is no doorman; the more atoms that are there, the more atoms that are let in, making the center of this gas glob a pretty dense place––our little gas party has become a protostar.

On our dance floor, the people in the middle are getting kinda squished so they star pushing back out, so they can have a little more room to wiggle and shake their groove thing. The pressure of the gas in a protostar does the same thing, it starts pushing outward.

At this point, two thing can happen, the party can just keep going but never reach the point where things get out of hand and our protostar becomes a brown dwarf, too big to be a planet and too small to be a star because its internal temperature and density never reach the point where nuclear fusion can occur.

However, if the party really gets hot and more people still try to force their way onto the dance floor, a fight might break out, or in the case of our gases, nuclear fusion will occur. And if you ever saw a fight break out in the middle of a crowd, you know how quickly it can spread. Am I the only one that went to these kind of parties?  The second this fusion occurs, the star is born and it is now considered a “main sequence” star––a star that is fusing hydrogen together into helium.

What type of star that molecular cloud will generate depends on how much mass the protostar had when it was forming. To continue with our party reference, if those entering the party lined up nicely and just kept walking into the club, you will end up with a star like our Sun which will burn it's supply of fuel slowly and have a very long life. But if the doors to the club are just flung wide open, and the party guests just flood in, the club's capacity can be exceeded before anyone knows what happened and you end up with a star that burns extremely bright and hot but has a very short life span because it uses up it's fuel so quickly.

To put it another way, a smaller star has less fuel to burn, but also less mass pushing in, so the star doesn't have to burn it's fuel quickly in order to maintain equilibrium (where the energy going out equals the gravity of the star's mass pushing in). The bigger the star, the greater the gravity, so that big huge star has to burn it's fuel at a much higher rate to keep itself going.

As a person gets older, sometimes he or she will have trouble supporting their own weight, the joints start to go a bit and the muscles start to lose some of their mass. The same is true of stars as they burn their fuel, they end up with more helium and less hydrogen. With less fuel, the outward pressure of the nuclear fusion is less, but you still have all that mass pushing in; this is a star that has reached old age. At this point a few things can happen and once again, it all depends upon a star's mass:

For a star with very low mass, scientists are not sure what happens because they have such long life spans that even the low mass stars that were formed right in the beginning of our universe are still chugging along, like my great Aunt Bessie, 103 years old and still going. After they use up their source of fuel in their core (hydrogen), they don't have enough mass to begin fusing the helium that all that hydrogen turned into, so it is thought that they will slowly burn out and become white dwarfs, which are very small, very dense dying stars and eventually a black dwarf––the corpse of a star.

For a star like our Sun, it is a bit different. As the hydrogen in the star's core gets used up and more and more helium is present in there, the equilibrium of the star is affected (as mentioned above), the star begins to contract, or get smaller until the pressure and heat in the stars core reaches the point where the helium undergoes nuclear fusion and becomes carbon.

Here's the really cool part, while there may not be any hydrogen left to burn in the star's core, there is still a bunch of it in the layers of the star surrounding that core and as the star contracts, the hydrogen in the star's outer layers can undergo nuclear fusion because of the pressure that builds there in the collapse. Since those outer layers are not under such massive constraints of gravity as the core, when they undergo the nuclear reaction, those outer layers of the star expand and make the star enormous. As the outer layers of the star pull away from the core, there is not nearly as much pressure on the hydrogen in them and so the outer layers cool down quickly, turning the star red––a red giant. When our Sun runs out of fuel and becomes a red giant, it's outer layers will be beyond the inner planets.

Cat's Eye Nebula
Image credit: NASA, J. P. Harrington (U. Maryland) and K. J. Borkowski (NCSU)

At this point, the star is not very stable, and the outer layers can eventually be blown away from the star's core, possibly to create a planetary nebula (which really has nothing to do with planets, oddly enough). Examples of planetary nebulae are the Cat's Eye Nebla and the Ring Nebula. And what is left behind, eventually becomes what those lower mass stars we talked about before become, a white dwarf.

Ring Nebula
Image Credit: Aura, STScI, NASA

Finally, we have the big boys!

The hydrogen in the cores of these stars gets used up very quickly and the stars have so much mass and therefore gravity, that the helium created from the hydrogen fusion begins to fuse into carbon without missing a beat once the hydrogen supply is exhausted. When the helium in a star undergoes nuclear fusion, it turns into carbon and when that supply of helium is exhausted…you guessed it, the Carbon undergoes nuclear fusion to create Neon, which burns up to create Oxygen, which burns up to create silicon which burns up to create nickel (which becomes iron). And that's when a star goes “boom.”

You see, the iron that is created in the core of this massive star actually takes in energy, it doesn't release it, so all of the sudden, this massive star experiences a massive collapse because the core is no longer pushing outwards, it's actually yanking stuff in. So you have all of the mass of this huge star suddenly collapsing in, everything gets real crowded real fast and a supernova occurs. A supernova, for a brief period of time can outshine the galaxy that houses it. Material is thrown off and can create a nebula, like the Crab Nebula. What's left behind is a neutron star, or if there is enough mass, a black hole.

So you have a whole lot of mass in a very small space. Now remember, size doesn't matter when you are talking about gravity, mass does. If the mass of this remnant of the star is less than 20 suns, it will be a neutron star, but if it is over that mass, it will be a black hole.

Random side note!

In theory, there is no upper limit to the size of a black hole and the lower limit is so small that for our purposes, we can say there is no lower limit for the size of a black hole either, what matters is the mass and whether that mass is sufficient to create enough gravity so that light could not escape it. When discussing a black hole's size, we are not talking about a surface like the Earth has, we are talking about the black hole's “event horizon” or the point at which nothing, not even light can escape the pull of it's gravity.

In the center of that event horizon is a singularity. A singularity is a point that has infinite density and no volume. So, falling into a black hole is really degrading. Not only do you die, but you lose your volume as well and your mass just adds to the black hole's strength.

So when we are talking about a massive black hole, what we are actually talking about is that black hole's sphere of influence or it's event horizon. The more mass that is added to the singularity, the larger an area that event horizon can grow to encompass.

To escape the Earth's gravity, you have to travel over 25,000 miles per hour. To break free of the moon's gravity, you only have to travel at 5,360 miles per hour; this is because the moon has less mass than the Earth and therefore less gravity, so it is easier to break free from it. To escape the gravitational pull of a black hole, you would have to travel faster than the speed of light (186,262 miles per SECOND) which is why we say nothing can escape a black hole, not even light.

But that doesn't mean that black holes are sucking in the universe and we are all doomed. Say a black hole is formed from a huge star, the mass is still the same, it is just more concentrated. So anything that enters the event horizon will be sucked in, but anything outside of it will not. Let's put it this way: if you could remove the Sun from our solar system and replace it with a black hole with the same mass, the planets would still orbit around that black hole the same way they orbited around the Sun. 

 I hope that makes you feel better.

I won't wish you luck in locating black holes.  You can't see them.  But I will wish you a good day, and happy stargazing!