Astronomy/Astronomy - Suns
QUESTION: I saw on a show on the Science Channel that when a sun produces iron, it begins to die. Does the sun begin to die simply because of the presence of iron or because of the reaction that produces iron? Also, what about iron makes the sun die, and how quickly will it die after producing iron?
ANSWER: When very massive stars start to produce large quantities of iron they "begin to die", because iron cannot be used as a nuclear fuel (it takes more energy to "burn" it than is released by burning it). Once the iron core reaches a critical mass it collapses, triggering a supernova (this is called an iron-core-collapse supernova). It only takes a few tens or hundreds of years for a very massive star to go from just starting to produce iron, to the iron-core-collapse that ends its life; whence the idea that producing iron is the beginning of the end for such stars.
However, this cannot occur for stars like the Sun. Only very massive stars can generate enough heat to burn heavy elements. Stars like the Sun can fuse hydrogen to helium, and toward the end of their lives burn helium to carbon, but that's as far as they can go. They die over a relatively long period of time (typically several hundred thousand years), during which they are either red giants or "horizontal-branch" stars, which are similar to red giants, but have slightly different details in the way they burn hydrogen to helium and helium to carbon in their deep interiors. (There is a brief discussion of the Late-Main-Sequence life of Sunlike stars, and why they swell up to become red giants, at http://cseligman.com/text/stars/sunms.htm
Usually, such stars lose mass at a fairly rapid rate, because they are very large and as a consequence have a low gravity. They also tend to have normal variations in their size and brightness, which over time tend to become more extreme and somewhat more likely to produce instabilities that throw off large amounts of gas (forming planetary nebulae, or proto-planetary nebulae). Between that and the gradual exhaustion of the fuel in their insides, they eventually do not have enough material pressing down on the regions where nuclear reactions occur to maintain the temperatures required for those reactions. At that point the remaining mass of the star rapidly contracts to become a white dwarf. (There is a brief discussion of the end-of-life stages for red giants that become white dwarfs at http://cseligman.com/text/stars/planetary.htm
In other words, the Sun will never produce iron, so its death will have nothing to do with that. However, for very massive stars, the start of iron production starts the formation of a massive iron core whose collapse will doom the star within a very short period of time, astronomically speaking.
It's not clear whether you wanted any further details, but to summarize how the iron-core-collapse works:
(1)Once the iron core reaches a certain mass, the electron degeneracy that maintains its structure can no longer support its weight and the weight of the star outside the core. The core begins to contract very rapidly (essentially in "free-fall"), and that part of the star just outside the core, which is still full of nuclear fuels of various sorts, contracts with it.
(2)As the core collapses, its density increases. Once it is a few trillion times denser than water, neutrinos are produced in huge amounts as a result of the merging of electrons and protons (in the iron nuclei) to form neutrons, which turns the "iron" core into an incredibly dense "sea" of pure neutrons. This incredibly dense neutron sea is opaque to neutrinos, meaning that all the energy produced by those reactions, which would usually escape from the core as fast as it was produced, is completely blocked, greatly increasing the temperature of the core. This causes a "neutrino shock" that very temporarily halts the collapse.
(3)When the core collapse halts/pauses, all the surrounding material that was falling inward with it slams into the core and also halts. The conversion of its inward energy of motion to heat increases its temperature by a thousand or more times, and increases the rate of nuclear reactions occurring in that material by ten to a power of several thousand times. In the resulting firestorm, several things happen:
(a) All the elements heavier than iron are created (save perhaps for a substantial fraction of the gold, which is thought to be mostly created by collisional merging of binary neutron stars), in what amounts to not much more than an instant.
(b) Incredible amounts of energy are produced, reversing the infall of the surrounding material, and throwing it outward at speeds of tens of thousands of miles an hour.
(c) Over a period of a few hours, as the outward-bound material spreads throughout what used to be the more or less normal outer parts of the star, those outer parts are blown apart, scattering most of the mass of the star into interstellar space.
(d) As the outside of the star is scattered to the winds, so to speak, all the light created in the interior of the star over the past few hundred thousand years, which has been struggling to get through the gases pressing down on the inside of the star throughout that long period of time, suddenly finds nothing in the way and floods out of the star, thereby releasing as much light and energy in a few hours or days as the star would have "normally" released in several hundred thousand years.
(e) Since such stars are tens, hundreds, or even thousands of thousands of times brighter than the Sun before their death, releasing an energy equivalent to several hundred thousand years' worth of their radiation temporarily makes them several hundred billion times brighter than the Sun. This is why supernovae are so bright. We are not seeing the energy produced by the reversal of the central collapse -- that is all used up just reversing the infall. We are seeing the light that was already passing through the star, that might have served to light it up for hundreds of thousands of future years, suddenly released by the disappearance of the outer layers that were blocking its outward flow.
There are also other kinds of supernovae, and I've greatly simplified the discussion, compared to what a theoretical astrophysicist might prefer; but hopefully it gives you a good idea of the basic principles.
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QUESTION: Hypothetically, could someone purposefully kill a star if they shot a large amount of iron at it? Of course, they would need the technology and a huge rocket to accomplish the task. If it is even possible, how much iron would be needed to kill the star quickly?
If it could be done, you'd need a solar mass of iron or more, depending upon what stage of its life the star was in. That's pretty impractical, since you'd have to remove all the iron from about a hundred thousand solar masses of material to have that much iron in one chunk (iron representing only about one part in a hundred thousand or so of the material in typical stars).
However, I doubt that shooting that much iron at a star would "kill" it in the same way that an iron-core-collapse supernova does, because that requires the iron to be in the center, and the temperatures in the region surrounding the iron core in the billions of degrees; and even if you could get such a mass of iron into the center of a star, it probably wouldn't have the proper conditions in the region around it to trigger a typical iron-core-collapse.
On the other hand, shooting a stellar mass of anything at a star at a speed high enough to "insert" it into the core of the star would almost certainly create a shock wave (as it entered the star) that would do a pretty good job of blowing the star to bits even before the mass reached its core (and, most likely, went right out the other side). So if you could shoot that large a mass of anything
at a star you would probably destroy it in one way or another.