Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: McQueen on 22/09/2017 09:08:27

Title: Why don't neutron stars turn into a cloud of hydrogen?
Post by: McQueen on 22/09/2017 09:08:27
 Black holes have their genesis in neutron stars. Consider the following:  Nuclear reactors emit a lot of neutrons, so theoretically it is possible to fill a small container with neutrons. If placed on a desk and left alone for a few hours all the neutrons disappear! This is because neutrons have a half-life of about 10 minutes. What this means is that a neutron star should not last for more than 10 hours at a maximum (given that the average neutron star has the volume of about a city block). Thus, in theory, no free-standing neutron star should last for more than 10 hours or so. All that should be left (with a little luck) is a small cloud of hydrogen.   
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.mediafire.com%2Fconvkey%2F5cf2%2Fj4n8o7go4o7rowr4g.jpg&hash=21b063c6f07ea5c14ace9238b507fa41) (http://www.mediafire.com/view/j4n8o7go4o7rowr/neutron_star.jpg)
Given the massive distances involved and the (theoretically) extremely short life span of the neutron star, it should  be nothing less than a miracle to spot one. Instead neutron stars appear to last for billions  of years, not composed of neutrons it is true but of protons and electrons closely packed together. The question here is that protons and electrons attract at a distance but in closer proximity they repel each other strongly. So how are electrons and protons packed together in a neutron star?  Granted if accompanied by a giant companion star that feeds it, the neutron star might last for longer in the process creating a black hole. A black hole is a super dense area into which everything enters but from which nothing ever emerges, but as has been conjectured surely this goes against all logic? With so much matter, whole stars for instance ( and perhaps galaxies)  entering into these black holes, what happens to the original gravity producing mechanism, surely it must be affected in some way? To state that this inflow of matter is perpetually resulting in a stronger and stronger gravitational force does not make sense, energy is being used to strip down matter, that matter is, in turn, creating other different types of energy. What then happens to the gravity? Out of what is this perpetually increasing gravitational force created?   In fact with the huge energies and forces involved, a black hole should be the ideal environment for the creation of new matter.  It is even possible that it turns into something like a roman candle ejecting stars and material to great distances, forming new galaxies.
Title: Re: Why don't neutron stars turn into a cloud of hydrogen?
Post by: jeffreyH on 22/09/2017 17:31:42
Neutrons have zero charge but do have mass so that gravity is the dominating force.
Title: Re: Why don't neutron stars turn into a cloud of hydrogen?
Post by: Janus on 22/09/2017 19:43:36
Black holes have their genesis in neutron stars. Consider the following:  Nuclear reactors emit a lot of neutrons, so theoretically it is possible to fill a small container with neutrons. If placed on a desk and left alone for a few hours all the neutrons disappear! This is because neutrons have a half-life of about 10 minutes. What this means is that a neutron star should not last for more than 10 hours at a maximum (given that the average neutron star has the volume of about a city block). Thus, in theory, no free-standing neutron star should last for more than 10 hours or so. All that should be left (with a little luck) is a small cloud of hydrogen.   


Neutron stars do not decay into hydrogen clouds for the following reason:
When a neutron decays, it does so into a proton and electron with a release of some energy.  The proton- electron pair take up more volume than the neutron did.  Thus for a neutron star to decay, it would have to expand to a larger volume.  But that would require energy to lift its mass against it own gravity.  If the mass of neutrons is great enough, the energy needed to expand the mass against it own gravity is larger than the energy involved with the decay of the neutrons. The neutrons can't decay, because the energy needed to allow them to do so is more than the energy they have available. 

If you have a small enough sample of neutrons, then their mutual gravity would not be enough to prevent decay.  What this means is that there is a minimum mass for a stable neutron star, but not that all neutron star have a limited life.  (There is also an upper limit. As the mass increases, it finally reaches a point where the neutron star forms an event horizon and becomes a black hole.)
Title: Re: Why don't neutron stars turn into a cloud of hydrogen?
Post by: evan_au on 22/09/2017 20:16:27
(Oops - overlap with reply from Janus)

Nuclear reactions (plus physical processes and chemical reactions) tend to proceed in the direction that releases energy (I am sure there are more precise definitions involving enthalpy & entropy...).

There are some chemical reactions that proceed more in one direction at high pressure, and more in the other direction at lower pressure. These are usually reactions that show a large change in volume. The most familiar example is probably dissolving carbon dioxide in soda drinks:
- at high pressures (in the factory), the gas is "squeezed" into the liquid. The volume reduces.
- at low pressures (when you open the lid), the gas bubbles out of the liquid. The volume increases.

When a dying star  has burnt all its fuel, it cools down and shrinks, increasing the surface gravity, and increasing the pressure at the core. This increased pressure tends to favor reactions that reduce the volume.

The core is a plasma of iron nuclei, which repel each other by their powerful electrostatic repulsion; this is mixed with a cloud of electrons (not bound to any particular nucleus).

If the same mass of matter were all neutrons, it would take far less volume, and this reduction of volume is strongly favoured by the immense pressures found in the core of these old stars.

This outcome is very different from the situation with an isolated neutron, which has a result like opening a bottle of sada water in a vacuum (it explodes).

See: https://en.wikipedia.org/wiki/Neutron_star#Formation