A strange star or quark star is a hypothetical type of star composed of strange 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 and their constituent quarks form strange matter. The star then becomes known as a "strange star" or "quark star". Strange matter is composed of 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 nucleon. 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 hypothetical dark matter that is a feature of several cosmological theories.
Strange stars are largely theoretical at this point, but observations released by the Chandra X-Ray Observatory on April 10, 2002 detected two candidates, designated RXJ1856 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 existence will play out.
Therefore, strangestars -- so called because they are believed to consist almost entirely of strangequarks -- are scientists' only chance to observe a sizable and stable "chunk" of quark matter.
This stability led scientists to believe that stars made of neutrons, which measure about 20 kilometers in diameter, were the final stage in the evolution of a "massive" star (whose mass is at least 1.4 times greater than that of the sun).
However, the theoretical strangestars would represent an even further stage in stellar evolution: according to Usov, when the core of a neutronstar is sufficiently dense, neutron matter can be converted into quark matter.
Strange matter is composed of strangequarks bound to each other directly, in a similar manner to how neutronium is composed of neutrons; a strangestar is essentially a single gigantic nucleon.
Some theories suggest that strange matter, unlike neutronium, may be stable outside of the intense pressure that produced it; if this is so, then small substellar pieces of strangestars (sometimes called "strangelets") may exist in space in a wide range of sizes all the way down to atomic scales.
Strange matter is largely theoretical at this point, but observations by the Chandra X-ray Observatory in 2002 detected two candidate strangestars, designated RX J185635-3754 and 3C58, which had previously been thought to be neutronstars.
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