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Why is a proton like a blueberry muffin?

You Wrote:
Dear Sir/Madam

Are Protons Made from two up quarks and one down or do these Quarks exist inside the Proton as shown in diagrams? If they exist inside, then what is the 'shell' or 'rest' of the Proton made from? (I assume Neutrons are similar other than having one up and two down Quarks). I've heard of a neutron star but I haven't heard of a proton star is there such a thing? If not why?

Andrew Russell


Dear Andrew,

First about proton structure: You are right, the u and d quarks move in a "sea" of other particles that constitute "the rest" of the proton. These other particles are:

gluons:
These are massless and carry the strong "color" force that binds the quarks together. You can think of the gluons as being emitted by, and forming a cloud around, the u and d quarks.

quark/anti-quark pairs:
Because the color force between the u and d quarks is very strong, it can create pairs of quarks and anti-quarks in the cloud of gluons. Anti-quarks have the same mass as their quark companions, but they have the opposite electric, color, and weak charges. Thus, even though these additional quarks are charged, they are always paired up with their corresponding anti-quarks, and the net charge is still due only to the two u's and one d that are "extra".

You should also know that these quark/anti-quark pairs can annihilate each other and turn back into gluons.

So, you can think of a proton as something like a blueberry muffin: The two u's and the d are like the blueberries, the blueness that stains the dough around them is like the pairs of additional quarks and anti-quarks that pop out, and the dough itself is the cloud of gluons that holds the whole thing together. Unlike a muffin, however, this sea of particles isn't static. It is constantly bubbling and rolling as new quark/anti-quark pairs are created and annihilated, emitting and absorbing gluons and pushing the extra u's and d all around inside. Of course, we can only see this when you look really close, which is what our accelerators let us do.

For more information about the color force and gluons and anti-particles, you might have a look at http://particleadventure.org for a nice discussion.

Now, to the second question: Why neutron and not proton stars? This is a really interesting question. First off, there are stars made up mostly of protons. These are just the "usual" luminous stars like our Sun. Our Sun "burns" protons (aka, Hydrogen nuclei) in the process of nuclear fusion, creating heavier nuclei (such as Helium) out of them. It is the heat released by this nuclear burning that prevents the Sun from collapsing under the pull of gravity.

Neutron stars, on the other hand, are very dense collections of neutrons whose weight is supported by so-called "degeneracy pressure" rather than the thermal pressure that supports the Sun. My guess is that you really want to know why there aren't any cold "proton stars" that are supported by proton degeneracy pressure. The short answer is that the proton's electric charge makes this impossible. If one took a whole bunch of cold protons and placed them together in space, their mutual electric repulsion would vastly overwhelm gravity and they would fly apart and not form a compact star.

However, we can go a bit deeper than this and ask what happens when you take equal numbers of protons and electrons. Then the protons still contribute almost all of the gravitational mass, but the collection is electrically neutral as a whole. But this is exactly like a normal star formed from hydrogen clouds that collapse under gravity. Nuclear burning converts the protons (and half the electrons) into Helium nuclei (with two protons and two neutrons). When all the protons are "burned up" into Helium, the star can no longer support its own weight through thermal pressure and we should ask if the there is degeneracy pressure in this case.

The answer is yes, but it turns out not to come from the Helium nuclei or their constituent protons and neutrons. The degeneracy pressure actually comes from the remaining electrons (two per Helium nucleus to keep everything neutral). Such stars are called White Dwarfs. Our Sun will most likely end its days as a white dwarf.

By the way, degeneracy pressure comes from two different aspects of quantum mechanics. The first is the uncertainty principle which says that the smaller the box you put a particle in the faster it will bounce around in that box. The other aspect is the Pauli exclusion principle which says that certain particles called fermions (neutrons, protons, and electrons are all fermions) try to avoid one another, even in the absence of charge based forces. For the white dwarf or neutron star, this means that the large number of fermions present leads to a small available volume for each fermion, and thus a rapid motion for each.

For a short description of degeneracy pressure, try: http://scienceworld.wolfram.com/physics/FermionDegeneracyPressure.html and the links listed there.

I hope this has answered your questions.

Best Regards,
Patrick B. Greene, Ph.D.
Theoretical Astrophysics Group

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last modified 11/25/2002   physicsquestions@fnal.gov