Expert: Courtney Seligman - 3/29/2010

Question
QUESTION: Hi, Courtney, I hope you can help me understand a couple of empty area in my knowledge, about the death of stars, and what happens!

I have watched and learned a lot from Nova, PBS, etc. I understand when a star, 10 to 30 times the mass of our Sun, fuses all the hydrogen it can into helium, it starts fusing helium into carbon, then carbon into oxygen, neon, silicon, sulfur and finally to iron.
I was going to ask why it doesn't fuse iron into heavier elements, , but I learned on a NASA "Life of a Star" site that thre's no energy to be gained, as Iron is the most stable nuclear element. I also knew that if a star is 15-20 times the mass of sun, upon its burnout of nuclear fuel it can form a Neutron Star, possibly a Pulsar, with a spinning X-Ray and Gamma-Ray source. Which I always thought was an awesome idea, that a teaspoon of Neutron Star matter would weigh 20,000 tons, and if you could "lift" it, then drop it, it would bore all the way thru Earth's core, to China, pop up, go back down through the core, out on our side, on and on, until friction caused it to rest at the core of Earth. And if it's 20-30 times, the nuclear force of the Iron atoms couldn't withstand gravity, so it would collape into a Black Hole, a singularity of Infinite Density.  So that was one question-

The other question was this, I hope it's not too tough: (do you, or anyone here, know about Astro-Physics ?)
OK, here goes.  
WHY, when a star reaches the "iron core" stage, WHY does it "collapse under it's own mass and gravity"? WHY can't it's atomic forces "hold it up", at a certain size?

Is there some ratio, about a chunk of an element reaching a certain mass (gravity), that it's atomic forces, the electrons and protons can no longer "hold up"?  I mean, if a huge ball of Iron, that was fused by a star, can just "collape uder its own gravity", why doesn't a SMALLER star collapse?

For that matter, WHY doesn't one of those 10-Ton Iron "Wrecking Balls", swinging at the end of a cable on a crane to smash a building "just collapse under its own gravity"? What's the difference, between that and an exhausted star? Know what I mean?

Bonus Question, for Anyone: What was there BEFORE the Big Bang? And if you say "nothing", then what caused the Big Bang?

ANSWER: (1) Iron-core collapse:

The maximum density of a star (or a region in a star) is usually determined by "electron degeneracy". In normal gases, electrons run around freely, with lots of empty space between them, creating a portion of the "gas pressure" which holds up the gas/star by collisions with other particles. In an electron-degenerate gas, the electrons act like impenetrable balls, which cannot be pushed closer together than a certain distance, and if you try to push them closer together, they resist that with a force which is much larger than the normal gas pressure. It is the electron degeneracy of the gases inside them which holds up white dwarfs against the enormous weight they have because of their small size and relatively large mass.

The odd thing is that the more massive a star is, the smaller it has to get before it is stabilized by electron degeneracy. That's because the "size" of the electron "balls" depends upon their average energy. The higher the temperature of the gas, the smaller the space each electron occupies. In low mass stars, the compression of the star to white dwarf size doesn't produce much heat (relatively speaking), because their low mass gives them a low gravity (millions of Earth gravities, but low compared to more massive stars of the same size), which produces a relatively low temperature (millions or tens of millions of degrees for the lowest mass stars, hundreds of millions of degrees for stars like the Sun). This is a result of thermodynamics. The heat generated by the compression of the gas is proportional to the force compressing it. Double the compressional force, and a given reduction in size will produce twice as much heat.

More massive stars have more gravity than less massive stars of the same size, so as they try to compress to electron-degenerate status, they generate more heat (billions or trillions of degrees could be achieved), which makes the electrons "look" and more importantly, act like smaller "balls". As a result, even though they have more mass and more electrons, more massive stars are smaller than less massive stars, if they are electron degenerate. (Whereas normally, more massive stars would be larger.)

You can read far more about this in any astronomy textbook which covers white dwarfs, which nowadays should be all of them. Look up Chandrasekhar in the index, and it will take you to a discussion of how more massive white dwarfs are smaller than less massive ones. It might not explain why this is so, but it should at least mention that if you have a sufficiently massive star, it could shrink to zero size and still never become degenerate. The mass involved is called the Chandrasekhar limit, and depends upon the composition of the gas, but is 1.44 solar masses for pure hydrogen gases, and less than that for other compositions, because other atoms have fewer electrons for a given amount of mass.

What this means is, if a star has significantly less than 1.44 solar masses, it could be held up by its electron degeneracy, which is exactly what happens in white dwarfs. But if it has a mass close to that limit, electron degeneracy fails to support it, and it can collapse to a much smaller size.

In the case of very massive stars, as they build up iron deep in their cores, the iron just sits there, doing nothing of importance, as long as the total mass of iron is small. But as more iron is made, and the mass of the iron core increases, it will gradually become electron degenerate. Once it reaches a mass comparable to the Chandrasekhar limit for iron, it will try to collapse to zero size. That doesn't actually happen, most of the time -- various catastrophes overtake the core and the star surrounding it when it tries to collapse to zero size -- but that is the idea.

In other words, low mass stars can be stable electron degenerate "white dwarfs", because their mass is less than the Chandrasekhar limit. High mass stars can be stable "normal" stars, as long as they are held up by their gas pressure, or if they have a degenerate core, so long as its mass is less than its Chandrasekhar limit. But if a high mass star develops an iron core with a mass comparable to the Chandrasekhar limit, then the core will collapse, and the star will either collapse to a black hole (which is thought to happen for at least some extremely massive stars), or go supernova.

(2) (I tried to answer this separately, but it seems it has to be appended as a revision; so you may get two answers, one with only part 1, and this one.)

We of course can't know for certain what happened before the Big Bang, as it would have destroyed any evidence of what went "before", but the most popular idea, based on quantum physics, is that a point of essentially zero space-time size expanded to become our Universe, as a result of a "quantum fluctuation" in energy. For that to happen in a given point of space-time is essentially impossible. But if you have a lot of space-time, it is inevitable. So the idea is that in some prior Universe, vast amounts of otherwise empty space-time could be imagined as being divided into an essentially infinite number of individual space-time points. In some of those, the nearly zero chance of a quantum fluctuation expanding to form a new Universe would be offset by the nearly infinite number of such points, and a number of points in the space-time of the prior Universe would expand to create new Universes. In the instant of each new Universe's creation, it would detach from the original Universe -- that is, the space-time of the original Universe is completely separate from the space-time of the new Universe -- so there would be no knowledge of the original Universe in the new one, or of the new Universe in the old one (in fact, the inability to detect the quantum fluctuation in the old Universe is a fundamental part of the idea of such fluctuations).

This sounds very odd, but leads to predictions of the nature of our Universe which very closely match its actual nature, and obeys all the laws of nature as we know them, with one possible exception. Namely, in the formation of the new Universe (ours), there has to be a moment of "inflation", which is presumed to have certain properties, but is a strictly theoretical concept, without any basis in experimental fact. However, most books on astronomy discuss the Big Bang in some detail, and the reason for believing in inflation is thoroughly explained in most of them, because it is by far the best accepted explanation of the facts we can observe about our Universe.

Btw, there is a brief discussion of this on my website, at http://cseligman.com/text/prologue.htm, which you might find helpful. It doesn't discuss the physics much, but it does summarize the results reasonably well.

Finally, if you ask "if our Universe originated from a space-time point of nearly zero size in some prior Universe, where did that one come from?", the answer is presumably, from a similar event in a still prior Universe. And that one, from a still prior Universe, and so on. In fact, there is a term, "multiverse", which is used to describe the presumably infinite collection of Unvierses, going backwards and forwards in time, which would be created by each Universe spontaneously generating many others, through quantum fluctuations. (It reminds me of a story about an astronomer giving a talk about the nature of the Universe, back when all we knew about was the stars in the Milky Way. An old lady approaches the astronomer, and says that she believes that the Earth is supported on the back of a gigantic turtle, which is supported on the back of an even more gigantic turtle, and so on. The astronomer objects, saying if that were so, what holds up the bottom turtle, and the old lady chortles, "It's turtles, all the way down!")

---------- FOLLOW-UP ----------

QUESTION: WOW! By the way it's written, it sounds like it all just "came out of your head"! Pardon my saying, but you are ONE SMART LADY!
(I'm assuming Courtney is a woman's name.)

I was half-joking with the "What came before the Big Bang?"- but you answered it! I had a feeling that modern cosmology would lean toward a "previous universe", or "multiverse theory". It's almost like the old "Chicken or Egg?" question, huh?

It sounds like you have years of study involved in this stuff; I, however, am more of a hobbyist, but I soak up as much as I have time for. When I was a kid, my dad had a telescope and binoculars- I was hooked!
When I saw that first composite photo of the Hubble Deep Field, with all those thousands of beautiful galaxies, like jewels in a crown, then the page would Zoom IN, on a very small area of the photo, and that SMALL area that was only "dots", was ANOTHER field of thousands of galaxies, some 14 Billion years old, it made the hairs behind my neck stand up.

GOOD JOB! You made a High School Graduate (above average) understand!
Now, don't think I'm dense, but just to clear my head- I understand your explanation of "electron degeneracy", and the apparent size of electrons, and their counter-pressure to gravity.
But my question about "Why don't smaller depleted stars' Iron Cores collapse?, or Why doesn't a wrecking ball collapse?, the answer is the "Chandrasekhar limit", right?
When a mass of iron reaches the Chandrasekhar limit, then it has reached a mass where the atomic forces can no longer back its own gravity, or "hold it outward", and it collapses, right? So in a way, my idea of a "ratio" wasn't too far off?
(I guessed, haha.)

Next I'll ask you about magnetars, someday!

Answer
Actually, I'm a he, but the mistake is understandable. When I was born (in the middle of the last century), Courtney was a mostly British boy's name, and in this country, pretty rare (I don't think I ever heard of anyone else being called Courtney, until I was in my 30's). Sometime in the 70's, some actress named her girl Courtney, and it became mostly a girl's name for a while; but I have noticed a lot of men with the name, such as Courtney Vance. I presume the greater notice the name received once it became a girl's name led to more boys being named Courtney, as well. Still, there might be an upside to the confusion, as I write romantic fiction, which is mostly a female profession, and a "unisex" name makes it possible for me to use my actual name without worrying about whether readers would prefer a book written by a woman.

Everything did just come of out my head, as a result of 50 years studying and teaching astronomy (in fact, I give a lecture pretty much like what I wrote, but with diagrams and a slightly more detailed discussion of how electron degeneracy works; but I don't use notes for my lectures, just start talking and keep on, till I'm done or run out of time). I rarely have to look up an answer that I provide here, because if I don't already know the answer, it's usually something I'm not at all familiar with, and don't feel qualified to answer.

The Hubble Deep Field images are incredible. I have a fairly large version of them on my website, but it shows only a fraction of what can be seen on the huge original image files.

You're correct in that the mass of the object is the key. Smaller stars can't collapse, because they don't exceed the Chandrasekhar limit, and once they are small enough (as white dwarfs) for electron degeneracy to become important, they can't get any smaller without making the electrons look smaller, which would require them to get hotter, which they can't do, because they have to contract to get hotter, and their degeneracy prevents them from getting smaller. So they're just stuck.

Only massive stars can have an iron core collapse, because less massive stars can't contract enough to become hot enough to make iron. The Sun, for example, will never even turn carbon into heavier materials, because it will become electron degenerate, and unable to contract further, before reaching the temperature required to drive "carbon-burning". It will get hot enough (about 100 million Kelvins) to turn helium into carbon; but that will be the last nuclear reaction it can manage.

I should point out that in the case of a wrecking ball, its self-gravity is too small to be of any importance, so it doesn't collapse because the forces which control its atomic structure are adequate to hold things in place. You need objects hundreds of miles across to start worrying about self-gravity. From there, up to several tens of Jupiter masses, atomic structure continues to win the battle against gravity. At a few percent the mass of the Sun, the atoms break down into a plasma (a completely ionized gas), and from then on, either the heat of the object or electron degeneracy holds it up (heat for normal stars, electron degeneracy for white dwarfs). But once you reach the Chandrasekhar limit, nothing can overcome gravity, and the thing just collapses. But that's at a staggering mass, in comparison to anything we're used to, here on Earth. So it's only when you are dealing with stellar masses, that you have to worry about such things.