Astronomers and particle physicists are investigating the possible discovery of stars that are made up of a different form of matter - a type of matter that had not even been expected to occur in nature until its existence was first theorized in 1984. The discovery is now linked to the theory of stellar evolution. Now theory and observation may reveal the existence of a strange new type of object in the cosmos.What Are Strange Stars?
Depending on their final stellar mass, stars collapse into white dwarfs, neutron stars (or pulsars), or if massive enough, into black holes. Neutron stars are formed from the remnants of stars whose stellar corpse is greater than 1.44 times the mass of the Sun. If less than this, the final state of the star is a white dwarf, and if greater than 3.0 solar masses, the star's ultimate fate is to become a black hole. In the case of neutron stars and pulsars the pressure in the center of these objects can be several times the density of ordinary atomic nuclei. At these densities it is theoretically possible for the neutrons to "melt", permitting the deconfinement of the quarks that make up the neutrons and this forms what is called a quark-gluon plasma. From the up and down quarks that once constituted the neutrons of the star, theory holds that about half of the down quarks from the neutrons will change into more massive strange quarks and hence the name, "strange star". Strange stars would be denser and therefore smaller than neutron stars.
All this leads to the possibility that there are stars made almost entirely of quarks or neutron stars possessing a core of this quark-gluon plasma and having a crust of more conventional nuclear material, called a quark-hybrid star. This state of matter, comprised of up, down, and strange quarks may be the most stable form of matter, exceeding that of the most stable isotope of iron. Theory predicts that deconfined quarks (that is quarks not paired with other quarks) exist only momentarily as they interact to form other particles. We are only able to catch short glimpses of deconfined quarks in particle accelerators so an entire star composed of quarks would provide scientists an exciting opportunity to study this exotic material.
High rotation rates and small diameters have lead some researchers to consider that many pulsars are not neutron stars as generally thought but may actually be strange stars. Radio pulsars have been analyzed and researchers are able to define their size and mass within a narrow range by measuring the rate of their pulses and the size of their emissions. Their results limit the mass of the radio pulsars studied to less than 2.5 solar masses and their size to under 10.5 km radius. This places the size of pulsars within the range of strange stars.
In 1984 Professor Edward Witten of Princeton first proposed the existence of quark-matter stars and since then Prof. Witten's theoretical quark stars have acquired additional advocates. Among those supporters is V. V. Usov of the Weizman Institute of Science. He not only offers analytical support but also has proposed some properties of strange stars. Referencing Witten's 1984 work, he surmises that some of the dense astronomical objects, such as pulsars, x-ray bursters, soft gamma-ray repeaters, and others, may be strange stars rather than neutron stars. He further offers that at the time the strange star is created, the high initial temperatures would drive off any atmosphere, leaving a bare surface. As the star cools, the eventual accretion of normal matter to the strange star would form an optically thin atmosphere. At the surface of the strange star, captured ions and electrons are bound to the star by gravity but may be prevented from reaching the surface by a residual electromagnetic barrier of the quarks and electrons in the dense plasma. He states that "strange quark matter may be the absolute ground state of the strong interaction" (which holds quarks, nucleons, and atomic nuclei together) and he expects that state to exist throughout the strange star.
The properties that Usov has attributed to a potential strange star should be observationally discernable. His three properties are: First, their x-rays should be 10 to 100 times more energetic that a neutron star. Secondly, those x-rays will have a pulse rate of 1 millisecond. Thirdly, strange stars will release high-energy gamma rays from electron-positron annihilation.Observational Evidence
In April of 2002, NASA announced the discovery of what they believe to be two strange stars. One of the two stars, RXJ1856.5-3754 is in the constellation Corona Australis, 400 light-years from Earth. Analysis of its light determined its temperature and hence an estimate of how many x-rays it emits. The Chandra X-ray Observatory used the star's x-ray brightness to estimate its size and found it to be only half the size of a neutron star. Theory doesn't allow such dense matter anywhere except in a strange star. RXJ1856 appears to be between 7.6 and 16.4 km in diameter, too small for a neutron star but allowed for the quark model. It exhibits no spectral or temporal features when monitored by Chandra's spectroscopic observation instruments or from earlier X-ray analysis. Neither was Chandra able to detect a pulse.
Data from the Chandra X-ray Observatory was compared to Hubble Space Telescope data and the combined information resulted in a determination that RXJ1856 radiates like a solid body with a temperature of 700,000 degrees Celsius and has a diameter of 11.3 km - too small to be a neutron star, according to the Chandra science team. Being too small and too cool to be a neutron star lends credence to the idea that this is a strange star, indeed.
However, previous observations of RXJ1856's optical and ultraviolet emissions do not support the quark star idea. The researchers acknowledge that there may be other explanations for the data. Possibly the Chandra observations could be explained if RXJ1856 is a normal-sized neutron star with an x-ray hot-spot or possesses an atmosphere that is distorting its emissions. However, such a star should pulsate and RXJ1856 does not pulse as would be expected in the hot-spot model.
When NASA announced the RXJ1856 findings it also reported another possible strange star, called 3C58, located 10,000 light-years away in Cassiopeia. Chinese astronomers observed this star in 1181 when it exploded in a supernova. With the time known since it was a supernova, 3C58 should now have a temperature of about 2 million degree Celsius. However, data from Chandra place its temperature at less than 1 million degrees Celsius. It is losing temperature and luminosity faster than neutron star models predict. Quark crystals composed of pions or kaons may be responsible for this behavior. Chandra did not detect x-ray radiation emanating from 3C58 as anticipated, indicating that the star's temperature is too cool to produce x-rays. In the case of 3C58 it might be made of a quark core surrounded by a neutron crust. "Our observations of 3C58 offer the first compelling test of models for how neutron stars cool, and the standard theory fails," says David Helfand from Columbia University in New York, a member of the Chandra team; "It appears that neutron stars aren't pure neutrons after all - new forms of matter are required."
Here too, other explanations are possible. For instance 3C58 may just be an especially dense neutron star. Improvements in the neutron star models are needed to guide the efforts to prove or disprove whether RXJ1856, 3C58, as well as future candidates, are neutron stars, quark stars, or some other entity.
Chandra and XMM-Newton, (the European Space Agency's X-ray Multi-Mirror satellite, launched in 1999) has recently observed compact x-ray sources that don't exhibit spectral lines in the thermal component of their spectrum. This condition resembles the bare surface of strange stars that Usov theorized. Investigators conclude that due to the special evolutionary aspects attributed to quark stars, specifically the rapid cooling and thermal photon radiation expected from bare strange stars, that locating objects with a "thermal featureless spectrum", as was observed with RXJ1856, may be a way to detect strange stars.Conclusion
Because of their similarities in size and mass, stars that appear to be neutron stars or pulsars are now being considered, at least as a starting point, the best prospects for strange stars. Importantly, the smaller radius of strange star candidates leans away from the neutron star model. Therefore, careful analysis is needed to determine if a particular candidate fits the neutron star model or the quark model. Continuing investigation of RXJ1856 and 3C58 may prove that stars composed of quarks do exist. Until that happens, however, other explanations are being offered. Research is subject to differences in interpretation. Therefore, observation will play a crucial role in mediating the theoretical arguments.
Future prospects that offer additional help in the identification of strange stars may lie in the detection of gravitational waves from neutron stars, the prime candidates for strange stars. Gravitational wave detectors such as LIGO (Laser Interferometer Gravitational-Wave Observatory, sponsored by the National Science Foundation) and the Virgo Gravitational Observatory in Italy may soon be fully operational. Add this to the continued use of such instruments as orbital observatories like Chandra and XMM-Newton, plus improvements in the neutron and quark star models, then perhaps if strange stars exist and become better known to us that they will no longer seem quite so strange.