A strange star or quark star is a hypothetical type of star composed of strange matter, or quark matter. This is an ultra-dense phase of matter that is theorized to form inside particularly massive neutron stars.

It is theorized that when the neutronium which makes up a neutron star is put under sufficient pressure due to the star's gravity, the individual neutrons break down into their constituent quarks. Some of these quarks may become strange quarks and then form strange matter. The star then becomes known as a "strange star" or "quark star".

Strange star

Strange matter is composed of up quarks, down quarks and strange quarks bound to each other directly, in a similar manner to how neutronium is composed of neutrons; a strange star is essentially a single gigantic hadron. A strange star lies between neutron stars and black holes in terms of both mass and density, and if sufficient additional matter is added to a strange star it will collapse into a black hole as well.

Strange matter is one candidate for the theoretical dark matter that is a feature of several cosmological theories.

Recent theoretical research has found mechanisms by which quark stars with "strange quark nuggets" may decrease the objects' electric fields and densities from previous theoretical expectations, causing such stars to appear very much like--nearly indistinguishable from--neutron stars (Jaikumar et al. 2006). However, the team made some fundamental assumptions that led to uncertainties in their theory large enough that the case for it is not yet solid. More research, both observational and theoretical, remains to be done on strange stars in the future.

Quark star

A quark star may be formed from a neutron star through a process called quark deconfinement. It is in this process that produces the quark nova. The resultant star should have free quarks in its interior. The deconfinement process should release immense amounts of energy, perhaps being the most energic explosions in existence. It may be that gamma ray bursts are indeed quark-novae.

Neutron stars with masses of 1.5 - 1.8 solar masses with rapid spin are theoretically the best candidates for conversion. This amounts to 1% of the projected neutron star population. An extrapolation based on this indicates that up to 2 quark-novae may occur in the Milky Way each day. This would mean that there is a large population of quark stars in our galaxy.

Theoretically quark stars may be radio quiet, so radio-quiet neutron stars may be quark stars.

Observed overdense neutron stars

Strange stars and quark stars are largely theoretical at this point, but observations released by the Chandra X-Ray Observatory on April 10 2002 detected two candidates, designated RX J1856.5-3754 and 3C58, which had previously been thought to be neutron stars. Based on the known laws of physics, the former appeared much smaller and the latter much colder than they should, suggesting that they are composed of material denser than neutronium. However, these observations have been under attack by researchers who say the results were not conclusive; it remains to be seen how the question of strange star or quark star existence will play out.

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