Sunday, February 10, 2013

Degenerate Matter: Exotic Stars

This post deals with hypothetical "exotic" stars and their composition. For an introduction to degenerate matter followed by descriptions of its "first two" stages, see here.

Neutron stars, as with white dwarfs, can only exist at a certain range of masses, before gravity overcomes the neutron degeneracy pressure. Above a certain density, neutron degenerate matter can no longer exist.

Here one enters the realm of speculation, as it is not known exactly where this upper limit is, and what stages a degenerate object undergoes immediately after. By most estimates, neutron degenerate matter cannot exist in an object weighing more than 3 solar masses. Since, by the Pauli Exclusion Principle, no two neutrons can occupy the same location at any amount of pressure, it is reasonable to assume that they, under the pressure of gravity, eventually break down instead into their constituent particles: quarks and gluons.

The quark structure of a neutron. The neutron contains two down quarks and one up quark, and connecting particles called gluons (the wavy lines) that bind quarks together.

As the mass of a degenerate object exceeds three solar masses, the matter at the core will collapse from neutron degenerate matter to quark-degenerate matter, better known as quark-gluon plasma. Such an object would then be called a quark star.

Gluons can effectively be ignored in considering the properties of a quark star; since they have a mass of 0, they only contribute in binding quarks into larger particles. In quark stars, however, quarks are free rather than bound in nuclei, and gluons would therefore have little effect.

Therefore, typical (if any such matter can be called "typical") quark-gluon plasma would contain up and down quarks, the lightest and most common quarks. However, at the extremely high densities of a quark star, the energies may be high enough for a third type of quark, the strange quark, to spontaneously form. Strange quarks, the next-heaviest quarks, are not commonly found but, under normal circumstances, decay into up quarks. Strange quarks, however, do compose more exotic particles that have been synthesized in particle accelerators. Quark stars which include strange quarks in their composition are sometimes called strange stars.

At the present time, there is no conclusive evidence in favor of the existence of quark stars, though they have been proposed as an explanation for certain celestial phenomena. For example, there are a few known stellar remnants whose masses have appeared to those of heavy neutron stars (around 2 or 3 solar masses), but whose radii are only 5 miles! Usual neutron stars, with diameters usually twice that figure, are not as dense as these strange objects. As of now, the measurements are not completely certain, and revisions are possible.

Perhaps the most compelling evidence for quark stars are certain types of gamma-ray sources that defy explanation due to their tremendous brightness. One such event was recorded in 2007, and was classified as a supernova, receiving the designation SN 2006gy.

An X-ray image of SN 2006gy (the bright spot in the upper right). The source is 238 million light years away in the galaxy NGC 1260. At its peak, SN 2006gy outshined its entire home galaxy (bright spot in lower left) and was one of the most luminous objects in the universe.

Though initially thought to be a supernova, the explosion released ten times more energy than other similar supernova events. Therefore, some have hypothesized that the event was a so-called quark nova. A quark nova would mark the collapse of a neutron star into a quark star. The transition of the quarks from bound state (in the neutrons) to a free state (quark-gluon plasma or strange matter) would release tremendous amounts of energy: up to a thousand times a "typical" supernova explosion. Another indicator is the concentration of high-energy radiation; SN 2006gy released an unusual amount of radiation in the X-ray and gamma ray areas of the electromagnetic spectrum.

Despite claims of the existence of quark stars, all evidence thus far, including SN 2006gy, is ambiguous and does not confirm nor deny their existence. When a neutron star's gravity overcomes neutron degeneracy pressure and collapses, the resulting stellar remnant does pass through a quark stage, but it is not known whether such a state is stable, or, if it is, what range of masses a quark star can assume. This uncertainty primarily stems from our ignorance of the properties of such super-dense matter.

However, even if quark stars exist, they are only kept stable by quark degeneracy pressure. Quarks, as with neutrons, resist being forced to occupy the same location. But, if the mass exceeds about 3-3.5 solar masses, gravity (theoretically) overcomes quark degeneracy pressure and the stellar remnant collapses. The next step in the life of the collapsed star is, if anything, even more mysterious. One plausible theory is the formation of yet another type of star, termed an electroweak star.

Following the pattern of a neutron star, the quarks in a quark star, when subjected to enough gravitational pressure, break down. However, quarks have no known component particles, so they may instead undergo a decay, related beta decay. The decay only happens in a small area of the core of an electroweak star, and the resulting situation is a striking analog of normal stellar fusion (see below). The use of fuel in this case is called electroweak burning, rather than fusion.

The above diagram is a visualization of a hypothetical electroweak star (not to scale). Such a star, as with a quark star, would probably have an outer layer of neutron degenerate matter, and would appear from the outside as an overdense neutron star. It would probably be on the order of 2-5 miles in diameter, and would have a miniscule core, only the size of an apple! However, packed into this core would be approximately two earth masses. Here, quarks are converted into leptons (including electrons). In the process of this conversion, very small neutrinos (or antineutrinos) are released. The outward pressure provided by these particles stops the electroweak star from collapsing under the force of gravity.

Another curiosity of electroweak stars is the property that gives them their name. At certain extreme temperatures and densities the electromagnetic force (the force which describes the charge of particles, and whose large scale effects are electricity and magnetism) and the weak nuclear force (the force which controls beta decay) "unify". The unification of these forces at extraordinarily high temperatures and densities (on the order of a billion billion degrees, for instance) means that these forces, under such circumstances, are no longer distinguishable and rather behave as manifestations of a single force, called the electroweak force. The center of an electroweak star is one of the only places in the Universe that the conditions necessary for this unification occur. Such conditions notably occurred a trillionth of a second after the Big Bang. Insight into electroweak stars would correspondingly elucidate aspects of the Big Bang.

One might think that such an electroweak star, since it emits a wealth of radiation in the form of neutrinos, would be easily detectable. However, neutrinos are notoriously elusive particles; they hardly interact at all with normal matter, but rather pass right through it. Efforts to detect neutrino sources in the cosmos, therefore, are difficult, and, to this day, only very low-resolution "neutrino-images" have been achieved.

The key parameter in determining how many electroweak stars are out there is stability. Early models seemed to suggest that electroweak burning would only last for a very short time, maybe only a matter of seconds. However, some more recent research has indicated that electroweak stars may have enough fuel to last millions of years or more. In the latter case, there is likely to be a small, but detectable, population of such stars throughout the cosmos.

It is possible, in addition, that electroweak burning would shed enough mass (through neutrinos) from the stellar remnant to render it stable, i.e. supportable by quark degeneracy pressure. However, in other cases, after production of leptons ceases, electroweak stars would collapse further, powerless against gravity, eventually transforming into the most mysterious objects in the universe (see the next post).

Sources:, Quark Star at Wikipedia,,

1 comment:

bluegreenplant said...

Nice blog. Good presentation.