When Do Neutron Stars Form?

and

Where Do Neutron Stars Come From?




Alpha Probe Docks Before Renewing Project Viligance


The birth of a neutron star necessarily stems from the death of one or more stars. (A white dwarf in a binary system can pick up some of its neighbor's mass and collapse into a neutron star). The most spectacular event heralding the birth of a neutron star is a supernova. The following will detail the events leading up to the supernova, nature's single most luminescent and energetic event (with exception to the Big Bang).

A recipe for destruction. The making of a neutron star:

First start with a bright, beautiful O or B main sequence star which is more than 8 times as massive but less than 40 times as massive as our sun. Let the star undergo hydrogen fuel burning in its core for about 10 million years until you have a helium core with a hydrogen shell around it. Allow the hydrogen shell to burn and heat up the helium core until the helium suddenly starts its own fusion process. This will happen suddenly, causing the core to expand explosively and raising temperatures in the core up to 350x10^8K. Note also that as the star reaches this stage, its size will have greatly increased due to convection in the hydrogen shell and the heating of the helium core. Some mass may be lost to solar winds, but don't worry, this is to be expected. After you let the helium core fuse for a bit, you'll end up with a carbon-oxygen core with surrounding hydrogen and helium shells. These shells will burn and heat up the C-O core. Here is where it's important that your star is at least 8 times as massive as the Sun. If it weren't, fusion would stop and you'd be left with a white dwarf (and no-one wants that). Because of the larger mass, the core will have a higher pressure and lower volume. Therefore its temperature will be greater as well. So now your C-O core will start the fusion process and create a silicon core which in turn creates an iron core. Now you're at the critical stage - the instant before collapse. You'll notice that your once hydrogen rich star is now composed of skins of heavier elements with iron at its center (see diagram below). The temperature ranges from (few)x10^9K at the core to (few)x10^7K at the outer hydrogen shell. At this point, you should be putting on some eye protection, because it's going to get bright REAL fast. What you have is a very hot (3x10^9 to 9x10^9K or .3-.8 MeV) and very dense (10^9 - 10^10 gcm^-3) core. At these densities, the Fermi energy is in the MeV range and exceeds the electron capture thresholds of iron. Also, the central pressure forcing the atoms together is greater than the pressure holding the core up from collapse. So, what you should now have is a bunch of nuclei floating around with high energy electrons whizzing around (ionized atoms) and gravity pulling all these things together. The result is a core (roughly the size of Mars) collapsing to about the size of New York City (10km). Huge amounts of energy is released as electrons combine with protons in the nuclei to create neutrons and anti-neutrinos. You might be worrying about how you are going to stop such a cataclysmic collapse. Don't fret because the collapse will stop on its own when the nuclear density of the star is about 10^14gcm^-3 and has a radius under 100km. After the collapse and the subsequent explosion from all the energy being released from the atomic reactions, you'll be left with a nice, relatively small ball of neutrons. This is your neutron star! (Some like to think of it as a giant nucleus made of 10^57 neutrons, though). Sorry, but you will probably have to wait a bit for the dust to clear after the explosion so you can see your creation. Be careful, though, because neutron stars are stable only between 1.4 and 3.0 solar masses. Anything less and you're left with a boring white dwarf. Anything more and you have an annoying black hole.

But wait, there's more. For those who prefer creating a neutron star "safely", read on.

In this case, you start with two stars orbiting each other. Both are relatively low mass stars which would never become a neutron star on their own (initial mass less than 8 solar masses) . Follow the same procedure as before for fuel burning in the star but stop with a C-O core and let the star cool to become a white dwarf. The more massive of the two stars will become the white dwarf first since more massive stars burn their fuel faster and live shorter. Let the other one continue its evolution while the other is orbiting as a white dwarf. As you watch the stars orbiting one another, you'll notice that the white dwarf is "stealing" mass from its neighbor star - especially as the other star reaches its Red Giant stage. The white dwarf will continue to take mass until it becomes too massive (>1.4 solar masses). At this point, it will collapse into a neutron star. The other star will later become a white dwarf and the two will orbit each other in the form of an x-ray binary. I hope you aren't allergic to radiation!

So now you're done and you can be proud. You have the knowledge necessary to make your very own neutron star. Good Luck and Happy star collapsing!


Supergiant with high density core seconds before collapse



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