Wednesday, May 2, 2012

How to make a black hole

Black holes are, well, odd.  They are the place where common sense goes to die.  Physics takes such a beating when it comes to black holes that when it comes to the central point of a black hole, its singularity, physicists are often left smiling and nodding as their equations are chewed up and turned into useless infinities.

But don't let this make you think that we know nothing about these cosmic anomalies.  We have gone from the days of black holes being nothing more than mathematical oddities to one of the most intriguing areas of studies in modern cosmology.

The idea of an object so dense as to have a mass greater than the escape velocity of light was first proposed in 1783 by John Michell, but it wasn't until 1916 that mathematical evidence for such an object was proposed.  Using Einstein's Theory of General Relativity (which was published a year earlier), Karl Schwarzchild was able to show that black holes were, at the very least, mathematically consistent.  But for the longest time, the process that led to stellar mass black holes was a mystery.  It was believed to involve dying stars, but little else was known.

With an increasing array of tools that are constantly becoming more sensitive and precise, physicists have been able to assemble a basic recipe for a stellar mass black hole.  Before I go on, I should clarify what I mean by a stellar mass black hole.  There are multiple kinds of black holes, each sharing the same physics but with a difference of scale.  From the supermassive black holes lurking in the cores if nearly, if not all major galaxies to the hypothetical primordial black holes left over from the big bang, there is more variety than the uninitiated would expect.  The best studied form is the stellar black hole.  These are all made through a similar process in the core of dying stars.

But not any star can form a black hole.  In fact, the theoretical lower limit of a star large enough to produce one of these stellar black holes is around 20 times the mass of our own sun.  Anything smaller just doesn't have the mass required to form a black hole.  They will either cool down into a white dwarf like our sun or form another bizarre stellar remnant such as a neutron star.  However, there are a lot of stars in the universe.  In our galaxy, the Milky Way, there is predicted to be around 100 million stellar black holes.

Now that we have a star of the right size, how does it become a black hole?  The answer lies in the core of a star.  Stars work by fusing matter together using their enormous gravity.  Normally, the nuclei of atoms are kept apart thanks to the electromagnetic force.  But with enough energy, the electromagnetic force can be overcome allowing things such as the strong nuclear force to take over.  When this happens, the atomic nuclei are forced together into a new, more massive nuclei.  Thanks to a little equation known as E=MC2, some of that mass is converted to energy.  This left over energy is what powers a star.

Stars primarily are composed of hydrogen and, as such, fuse more hydrogen into helium than anything else.  Our sun alone fuses 700 million tons of hydrogen into 695 million tons of helium every second.  The remaining 5 million tons is the left over energy that powers our sun and helps to keep us from, you know, dying.  But not all stars fuse at the same rate, it all depends on mass.  A star twice as massive as our sun will fuse hydrogen at ten times the rate of our sun, while a star twenty times will fuse 36,000 times as quickly.  The larger the star, the shorter the life time.

But a star never uses up all its hydrogen.  It is only in its core that the environment is right for nuclear fusion.  As the star's life continues, eventually the available hydrogen in its core begins to dwindle.  Soon helium has to be fused into carbon, carbon into neon, and so on.  The denser material settling at the core with the easy to fuse hydrogen being pushed further out.  Each step takes less time then the last with every element forming a ring around its denser counterpart.  Eventually, if a star is massive enough, it will fuse atomic nuclei all the way up to iron, but this is as far as it can go. 

Unfortunately for such massive stars, iron resists fusion and outside extreme conditions such as supernovae and hypernovae, iron nuclei cannot be fused into heavier elements.  For such massive stars, this is the beginning of the end.  For lighter stars that cannot fuse up to iron, the process is quite similar, the only difference is that the masses required to fuse up to iron are lacking and a lighter element will become the dense core that cools or collapses into one of a variety of stellar remnants.

The iron core, as I mentioned, cannot be fused inside a star.  Stars are kept from collapsing by the energy released through the fusion of its mass.  Since the iron cannot fuse, the star begins to lose its internal support, placing even greater force on the core.  This force, coupled with the cores already massive size, begins to cause the electrons that have been stripped from their atoms due to the extreme temperature and forces to be crammed together when the iron nuclei.  Thanks to the Pauli exclusion principle, these particles cannot inhabit the same locations in time-space and begin to form a counter pressure known as degeneracy pressure.  This pressure helps to keep the star together, but only for a time.

Once this degenerate matter core reaches a mass of at least 1.4 solar masses, it can no longer hold back the massive pressures and the pull of gravity. The core then collapses in an extraordinary way.  In a thousandth of a second this core collapses at speeds of around 45,000 miles a second, shrinking a core thousands of miles across to just a few miles across.  The resulting vacuum causes the remaining shell of the star to collapse, adding further pressure, before rebounding.  The star would continue to collapse, rebound and shrink again if not for the affects of a ghost like particle known as the neutrino.

Neutrinos are very weakly interacting particles.  About 65 billion of these particles are passing through every square centimeter of the Earth every second and nearly all of these particles pass through it as if there was nothing there.  But in the conditions found inside a dying star, the 10 to the 58th power neutrinos released in a ten second burst from the core are enough to shred the outer star leading to a supernova.  It is only here, in the final moments of a dying star, that the core of a massive enough star collapses into a black hole.

First the degenerate core will see such extreme pressure and gravity to force the electrons and protons together into neutrons.  This new material, neutronium, is the component of neutron stars.  If there is enough mass, the core will continue to collapse all the way into a black hole.  The resulting black hole will then begin to feed on the remnants of its parent star with an accretion disk forming around it.  Some of this material may find a stable orbit around the black hole while the rest is heated up through friction to the point of emitting light across the electromagnetic spectrum.

With thanks to Phil Plait, Ph.D. and his book Death From The Skies.
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Thorn said...

Even though I love astronomy and physics, I find myself woefully lacking in knowledge in these areas (perhaps I don't like them enough lol). Nonetheless, I'm compelled to offer a somewhat simplistic explanation of these seeming anomalies that, if nothing else, should serve as a question that I hope to get an answer to: since they're created by stars imploding, causing the their fusion process to reverse, it wouldn't be out of the question to assume that this is the "big bang" mechanics on a much smaller level, since it was basically fusion reactions that created the universe (as we know it)...would it?

Cyc said...

Actually there are many distinct differences and the analogy quickly breaks down.

The star's fusion process is not reversed, but stopped. It is the process of fusion that keeps a star in equilibrium, without its pressure a star would collapse into a far denser body.

Also, the explosion of a supernovae or even a hypernovae work through very different processes than the big bang. There was matter and anti-matter creation from what is most likely a quantum event from the void, it did not have the same kind of build up that such an explosion creates. And for the big bang to be a fusion process, it would mean that two other bodies were forced together to make our universe, which probably is not the case.

Thorn said...

I just thought that the event that caused the explosion was due to a massive collection of nebulous gases and so naturally I figured some kind go fusion reaction was involved. I'm hardly a physicist though.

I think the theories (and yes, I actually know what the word means) of modern science are actually closer to the ancient Greek concept of the eternity of the universe than any other ancient idea, minus the theistic applications, of course.

Thorn said...

Say, cyc, I'm looking into astrology right now, see, I've always encountered people who tend to fit the profile of their sign, but I've met a few who don't, nonetheless, theres a preponderance of those who do, in my experience. I'm wondering if there are any rational ways of explaining this other than coincidence, like environmental and or weather factors on the pregnancy that happens to fall under a certain know of anything like that?

Cyc said...

The modern view on the universe actually doesn't have it being very eternal, at least not what is inside it. The idea of a steady state universe, or even close to it, is long gone, especially when introducing things like the red shift of the distant galaxies (dark energy).

As for astrology, it relies on vagueness of ideas and people willing to incorporate these into their personality. Every experiment done where they have been randomized shows that they have no basis in reality at all. The only thing going on here is cognitive dissonance and charlatans willing to take advantage of it (not always charlatans to be honest as most have been duped into it as well and honestly believe their 'magic')

Thorn said...

Well, what I mean is that there was always a form of subtle matter, and there always will be, can there be anything else? Also, is it possible to see antimatter, and if so, wouldn't that make it matter? Or does it not have enough mass to qualify?

I guess its just my own distorted perceptions in my experiences with people born under various signs. I tend to meet more people who identify closely with their sign than those who seem like the polar opposite. What I'm wondering is if it does not eclipse a more rational science of prediction that is simply chalked up to stars for the sake of religious or mystical allure. Like, perhaps astrologers are actually statisticians of a kind sort, who pose as something else...

Thorn said...

"Star-tiscians", if yu will, lol.

Cyc said...

Actually there hasn't always been a form of matter, nor has there always been a universe. Matter can be described as a kind of coalesced energy, not the best description, but is accurate thanks again to E=MC2.

Anti-matter can be seen, we just don't see it around us because it annihilates instantly when it comes into contact with normal matter. It has opposite charges and a few other properties from normal matter, but it still absorbs and re-emits photons like normal and would be seen just as normal matter would. But if you are seeing it, things could go very wrong very quickly as that would be a lot of anti-matter. Its mass is also the same as its matter counterpart (remember, other than the C/P violation during the big bang, matter and anti-matter are made in equal amounts).

As for astrology, you are making an error based on personal experience, which is always biased. Here you begin to conflate correlation with causation, which is not always true. If you could provide evidence that people fit the definitions put forth by their star signs you might have something to suggest changes in personality based on time of year they were born, but every part of astrology falls apart, including their predictions. They are intentionally vague enough to fit the largest possible audience, it is what has made it such a successful piece of stupid.

Thorn said...

Well, I've known a few scorpios who fit the bill, and as for myself, well, I guess it really is just me. There are a lot of things that don't fit with my sign as well. And those scorpios, two were diagnosed as bipolar, so there's a rational explanation (they're considered the 'bad boys' of astrology) and other thigns can account for it, not to mention I knew one who was totally stupid, which doesn't go with the sign at all. Still...I can't help but thnk that someone found a way, long ago, how to make "educated guesses" based on the conditions of a person's birth, and then correlated it to the stars, sice they had been worshiped for so long anyway. I know the stars themselves can't have any real effect on people, the way that the moon has on tides and so forth, since they're so far away.

Doesn't the theory of relativity equate matter with energy and vice versa anyway?

Grundy said...

Phil Plait is the man, and you're cool by association. Nice post.

Cyc said...


If it is true that someone, long ago, made an 'educated guess' about someone's birth that translated over to the joke that is astrology, then what is the difference between that and the 'educated guess' that is religion?

Both function on the same premises, using really bad correlation to explain something that otherwise ignorant humans had no idea or were somewhat confused about. There is no scientific evidence at all to support astrology, to hold on to it is nothing more than a personal belief that does not fit reality.


Thank you for stopping by.

I have often admired Phil Plait and certainly agree on his status as being 'the man'. It is good to know that his coolness by association works even with me.