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Important notices





It is known that inside neutrons and protons there are more fundamental particles called Quarks. There may exist a more compact stellar object which consists of mainly free quarks. This kind of stellar object is called strange star.

Though some ideas of quark stars were proposed as early as in 1969, no great progress had been made until the 1980’s. In 1986, Alcock et al. studied the structures of the strange stars. They found that a typical strange star basically consists of two components, a solid crust and a core filled with quarks which are likely superconducting liquid. But the basic global properties of strange stars are very similar to those of neutron stars including mass, radius and magnetic field etc. One of the main differences is that the crust of a strange star is much thinner than that of a neutron star.

Are there strange stars? Are some known neutron stars actually strange stars ? All these questions should be answered finally by observations. The possible observational phenomena include rotation, cooling and burst phenomena etc. 

Rotation

About two years after the discovery of the supernova explosion SN1987A, Kristian et al. (1989) claimed to have discovered within this SN1987A a 0.5 ms optical pulsar which was disproved about one year later. This wrong discovery, however, promoted the progressive development in the field of strange star. Due to the existence of high viscosity in strange matter, strange stars can reach much shorter period than neutron stars. If the pulsar with submillisecond period would exist, it should be a strange star rather than a neutron star.


Cooling

The thermal radiation from the surface of a star is a very promising discriminate between strange stars and ordinary neutron stars. Because the neutrino energy loss rate in strange matter is much higher than that in neutron matter, the surface temperature of a young strange star is lower than that of an ordinary neutron star of the same age. Scientists further compared the cooling curves of neutron stars and strange stars with the observed surface temperature of PSR 0656+14, and concluded that PSR 0656+14 might be a strange star by assuming that other rapid cooling mechanisms in neutron stars are absent.


Gamma-ray burst

Gamma-ray bursts are considered as one of most mysterious phenomena of modern astronomy. Gamma-ray bursts are believed to arise at cosmological distances in the merger of binaries consisting of either two neutron stars or a neutron star and a black hole. However, it is now proposed that the conversion of neutron stars to strange stars is another possible origin of gamma-ray bursts. Such a process can take place in the low mass X-ray binary systems where the mass transfer from the companion to the neutron star can be larger than 0.5 solar masses. If the neutron star in the low mass X-ray binary accretes the matter, the central densities can reach to a state that strange-matter seeds are formed in the interiors of the star. After a strange-matter seed is formed, the strange matter will begin to swallow the neutron matter in the surroundings. The star will then cool by the emission of neutrinos and antineutrinos, and the total energy deposited due to this process is sufficient to explain the energy requirement of the gamma-ray bursts even they are located in cosmological distance.

In December 1995, a special X-ray transient source, GRO J1744-28 was discovered. It possesses hard X-ray bursts the properties of which differ markedly from those of other known high-energy bursts. Study of the burst energy, duration, interval and spectrum suggested that this source could be a strange star accreting matter from its low-mass companion.

Although there are quite a number of evidences to support the strange stars, more detail studies in experiments and observations are necessary to make a very sound confirmation about their existence stars. After predicting neutron star theoretically, people had waited for 35 years to really discover it. If strange stars actually exist, people perhaps could wait not so long to confirm it.


Related Links
About Gamma Ray Burst
http://science.msfc.nasa.gov/newhome/headlines/ast15dec97_1.htm
About GRO J1744-28
http://science.msfc.nasa.gov/newhome/headlines/ast01dec98_1.htm

Photo courtesy:NASA