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Astronomers have found a very strange neutron star in a supernova remnant, according to an article published in the journal Nature Astronomy in October 2022. Its mass is so small that it has exceeded the known lower limit of a neutron star. Either the existing neutron star model needs to be improved, or it may not be a neutron star at all, but a strange star, such as a quark star.

High-mass stars often experience supernova explosions when they die. In the process of outward expansion, the material ejected from the explosion will interact with the interstellar medium to form special nebula-like celestial bodies, which is called supernova remnant (SNR).

The most famous supernova remnant is the Crab Nebula, which is 6500 light-years away. The pulsar in the middle of it is the neutron star left over from the explosion more than 900 years ago.

The Crab Nebula and the central pulsar neutron star, as the remnants of massive stars, usually have a mass of no less than 1.44 times the mass of the sun, the Chandra Seka limit. If it is below this mass, then the compact star will generally be a white dwarf star that has not experienced a supernova explosion, just like the future outcome of our sun.

One of the difficulties in the study of neutron stars by white dwarfs supported by electronic degenerate pressure is the measurement of distance. For example, the supernova remnant mentioned today (HESS J1731-347) was previously calculated to be more than 10, 000 light-years away. But recently it was discovered that another bright star was hidden here, so the researchers used ESA's Gaia telescope data to recalculate the distance of the relic. It turns out that it is much closer than we thought, only about 8,000 light-years.

The result of supernova remnant HESS J1731-347 has a direct impact on the previous estimation of the characteristics of neutron stars. So the researchers recalculated the characteristics of the neutron star using data from the Newton X-ray telescope. As a result, the neutron star has a mass of only 0.77 times the mass of the sun and a radius of about 10.4 kilometers.

This result indicates that it is different from all known neutron stars, which means that either there is something wrong with our existing neutron star model, or it is an unknown strange star, such as a quark star.

But at present, there are no reliable theories and models about quark stars, and even in many cases, some neutron stars are considered to conform to some characteristics of quark stars.

We know that astronomers discovered the predicted pulsar, the rotating neutron star, in the 1960s.

The first pulsar, CP1919, was not known much about the quark structure smaller than neutrons at that time, except that the neutron degenerate pressure could continue to resist the gravitational collapse of stars after electron degeneracy pressure.

Neutron stars supported by neutron degeneracy pressure if the mass of the star debris is larger, breaking the Oppenheimer limit, then the neutron degeneracy pressure will not be able to withstand, so the star can only continue to collapse until it becomes a black hole. However, there is still no definite answer to whether there is an intermediate process between a neutron star and a black hole.

Black holes and neutron stars (imaginary pictures) even for neutron stars, we do not fully understand its specific physical form at present. Because the internal structure of the neutron star depends on the equation of state of matter at this time. However, there is still uncertainty about the interaction model between nucleons. Especially in the ultra-high density environment of the core of the neutron star, where the composition state of matter may change, which will make matter appear new structure and state, which is reflected in the equation of state, there will be more degrees of freedom.

The structures of neutron stars given by different equations of state are quite different, but the mainstream models believe that there should be special matter forms in the core of neutron stars, such as hyperon matter.

The matter as we know it is made up of protons and neutrons, which is made up of the upper and lower quarks of the first fermion. In addition to the upper and lower quarks, the hyperon contains at least one second-generation fermion, the odd quark.

In some quark star models, the odd quarks here may come from the lower quarks in neutrons, which are accompanied by a huge energy release when they become odd quarks. To some extent, this can make up for the explanation of the energy source of supernova explosion by the traditional neutron star model.

In this model, there is not only hyperon matter containing odd quarks in the neutron star, but also some free quark matter that breaks through quark confinement.

It can be seen that there is no clear boundary between neutron stars and quark stars, they are just models with different emphasis. Neutron stars are not simply composed of neutrons, and quark stars are not quark balls composed entirely of free quarks, they may have complex internal structures and matter forms.

However, there are still some differences between a neutron star and a quark star. For example, for a neutron star, its mass is inversely proportional to the cube of its radius, that is, the larger the mass, the smaller the neutron star.

M ∝ 1max R ³

Because the greater the mass, the greater the gravity, the easier it is for matter to be pulled to the center. There is a lower limit for the radius of the neutron star, about 10 kilometers. If the mass were higher, the neutron star would be compressed into a black hole. So the mass range of a typical neutron star is usually between the Chandra Seka and Oppenheimer limits, which is about 1.4 to 3 times the mass of the sun.

But for a quark star, its mass is proportional to the cube of the radius, that is, the larger the mass, the bigger the size.

M ∝ R ³

Although compared with the traditional neutron star model, the quark star has not only strong gravitational bondage, but also strong interaction bondage between nucleons, which is theoretically denser. However, for quark stars, it is very different from traditional neutron stars in terms of mass composition.

As mentioned before, in general, the Higgs mechanism provides only a fraction of the mass of matter. Most of the mass sources of matter are actually given by various interactions between quarks and gluons. But for the strange matter such as quark star, it no longer has the normal material structure of quark confinement, so the actual mass of quark star will need to be revised in the model.

In addition, unlike neutron stars, 10 km is the lower limit of the radius for neutron stars, but 10 km is the upper limit for quark stars, and the lower limit of quark stars may not exist at all. This means that in a sense, the range of quark stars is so broad that even if there are tiny strange matter wandering among the stars, you can call them "quark stars".

How to form a quark star? In general, in addition to evolving from neutron stars, quark stars may also evolve directly from other massive stars. For example, some red supergiants, from which they evolved into II supernovae or Wolf-Rayet stars, may develop into quark stars.

At present, many neutron stars with abnormal properties are suspected to be quark stars. In addition to what we are talking about today, like the nearest known neutron star (RX J1856.5-3754), it is also thought to be a quark star because it is too small.

RX J1856.5-3754 also has the millisecond pulsar 3C58 in the constellation Cassiopeia, which cannot be explained by the neutron star model because of its unusually high cooling rate, so it is also suspected to be a quark star.

3C58 also has SN 1987A in the large Magellanic Galaxy, a supernova explosion discovered outside the Tarantula Nebula in 1987. But the strange thing is that so far no collapsing neutron star has been found here. Astronomers have put forward various conjectures to explain the missing neutron star, one of which is that it may have become a quark star.

There are also the brightest supernovae in history, SN 2005ap and SN 2006gy, which are very unusual in terms of brightness, hundreds of times brighter than the average II supernova. This unusually bright supernova means that there is a lot of extra energy there, which also raises the suspicion that some kind of quark star may have been born there.

SN 2005ap

SN 2006gy, so does quark star exist? As just said, at this stage, our understanding of quark stars (including neutron stars) is very limited, and there is not a unified reliable model to explain all phenomena. Even if there are such celestial bodies as quark stars in the universe, it is believed that it will not be suddenly discovered by us one day. It should be a new concept that, like neutron stars and black holes, is gradually accepted with the development of science and the deepening of understanding.

This article comes from the official account of Wechat: Linvo says ID:linvo001, author: Linvo

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