A quark is defined as any component of a set of primary subatomic particles that interact via the strong force and are thought to be among the fundamental components of matter. Protons and neutrons are formed by quarks interacting with one another via this strong force, much like atomic nuclei are formed by the latter particles combining in various proportions. Quarks are classified into six kinds, known as flavours, based on their mass and charge properties. There are three pairings of quark flavours: up and down, charm and weird, and top and bottom. Quarks appear to be actual elementary particles, with no discernible structure nor the ability to be resolved into smaller particles. However, quarks appear to invariably combine with other quarks or antiquarks, which are their antiparticles, to generate all hadrons—the so-called strongly interacting particles that include both baryons and mesons.

Quark (Particle)

James Joyce isn’t generally the first name that comes to mind when we think about particle physics. In 1963, when physicist Murray Gell-Man offered a term for his hypothesis of a fundamental particle of matter smaller than a proton or a neutron, Joyce was not on his mind. There was no spelling for the phrase he pronounced “quork” since it had never been written down.

James Joyce

According to Gell-Man’s own account, he had a propensity of calling strange items “squeak” and “squork,” and “quork” was one of them. He stumbled upon a phrase from Joyce’s ‘Finnegan’s Wake‘ a few months later:

“Three quarks for Muster Mark!

Sure he has not got much of a bark

And sure any he has it’s all beside the mark.”

Joyce clearly intended quark to rhyme with Mark, bark etc. However, this didn’t sound anything like the “kwork” in Gell-Mann’s thoughts. The physicist used some imagination and recreated the statement as a request for drinks at the bar:

“Muster Mark gets three quarts!”

Pronouncing the term like kwork “might not be wholly irrational” with this change, notes Gell-Mann in his 1994 book ‘The Quark and the Jaguar’. “The recipe for producing a neutron or proton out of quarks is, roughly speaking, ‘Take three quarks,” thus the allusion to three seemed appropriate.

Murray Gell-Man

The name quark comes from an old English phrase that means “to croak.” In the tale of Tristan and Iseult, a bird choir mocks King Mark of Cornwall, and the above-quoted verses are about that. However, there is a persistent mythology, particularly in German-speaking regions of the world, that Joyce got it from the word Quark, a German word of Slavic origin that is often translated as “cottage cheese” but is also a slang phrase for “trivial nonsense.” According to folklore, he heard it in a peasant market in Freiburg during a visit to Germany.

Quark (Dairy Product)

The universe is full of unexplainable things and new theories and discoveries emerge every year that have astrophysicists puzzled. Stellar structure has always been a dynamic topic in the field, but in 2006, star XTE J1739-285, was proposed as a candidate for a quark star, a star composed of quark matter. Since then, many other stars have been proposed as candidates, but astrophysicists are still uncertain about the possibility of such bodies to exist. So, do quark stars really exist?


As the name suggests, quark stars are composed of quark matter, or in other words, free quarks. Quarks are one of the fundamental particles in physics. Quarks combine to form hadrons, such as protons or neutrons, and are key to understand atomic and nuclear processes. There are six types of quarks, which are known as flavours: up, down, charm, strange, top and bottom. Protons and neutrons are one of the most stable particles in the universe, and it would take a big amount of energy to separate them into individual quarks. If individual quarks were obtained, it is thought that they would instantly recombine to form another hadron. However, this paradigm would not hold in the case of quark stars, in which quarks are believed to be free.

Figure 1: Types of Quarks. Credit: [1], CERN, no date.  


To understand what quark stars are, we first need to know how they are formed. Stars are formed from a collapsing cloud of gas and dust. Matter starts accumulating at the centre and such high temperatures and pressures are reached that stars begin to fuse hydrogen. When this process is achieved, the star enters the main sequence. The mass of the star will determine the next process once hydrogen in the core has run out, but we are only concerned in those stars that might end up as quark stars. Stars with more mass than half the mass of our Sun, will continue to collapse this time fusing elements up to iron. Once reactions end, gravity makes the star collapse. If the mass of the star is bigger than 8 solar masses, the collapse will end in a supernova. The remnant of the supernova might be either a black hole or a neutron star, or if the hypothesis is correct, a quark star.


Most of the candidates for quark stars are currently classified as neutron stars. Neutron stars are extremely dense objects. A simple teaspoon weighs 10 million tonnes! ([2] Universe Today, 2016). Gravitational forces are so strong that electrons and protons combine to create neutrons. Neutron stars hold due to neutron degeneracy pressure. Like electrons, two neutrons cannot occupy an identical state even at high pressures. The main reasoning behind quark stars is that they are between neutron stars and black holes, in which they are not massive enough to produce a black hole but have enough mass for neutrons to not be stable. These neutrons would then be broken down into their individual quarks ([2] Universe Today, 2016). Quark stars are composed with three quark flavours: up, down, and strange. Strange quarks come into play since they are formed when up and down quarks are compressed together.

Figure 2: Quark Star Structure. Credit: [2] Universe Today, 2016

Any net positive quark charge must be balanced with a negative charge, hence electrons. The centres of quark stars are expected to be electrically neutral, hence no presence of electrons would be found. However, if quark stars are more massive, low-density regions will occur around the surface and condensates will be formed ([3] Weber et al., 2012).  A condensate is a state of matter formed when quark gas in low-density regions is cooled to very cold temperatures. These conditions are likely to occur in the surface of the quark stars. Condensates formed in these conditions have been theorised by astrophysicists and all condensates formed contain electrons. The presence of these electrons offers the possibility that quark stars are surrounded by electrons and/or ions, hence having a nuclear crust. The maximum possible density of this crust is estimated to be 4.3 × 1011 g/cm3 ([3] Weber et al., 2012).

Figure 3: Quark Star Compared with Neutron Star. Credit: [3] Weber et al, 2012.

The electrons at the surface of the quark star are held electrostatically with the free quarks. This shell of electrons in the surface is only a few fermis thick. Most of the quark stars candidates that have been found have a radius smaller than that of neutron stars. Due to the smaller radius of these objects, these objects obtain very rapid rotation speeds, theorised to be smaller than a millisecond ([3] Weber et al., 2012).


Since we now know the origin and composition of quark stars, we can now analyse two of the candidates that have been proposed. The first official candidate was neutron star XTE J1739-285.  This neutron star is the fastest spinning neutron star known with a frequency of 1122 Hz, which suggests rotation speeds smaller than a millisecond. This star is measured to have a radius between 9 and 12 kilometres and a mass of 1.2 solar masses ([4] Xiaoping et al., 2007). Models have shown that the only possibility for the star to reach such speeds is if its core is composed of quark matter together with an ionic envelope ([4] Xiaoping et al., 2007). ASASSN-15lh is the most luminous recently discovered supernova, which could have been triggered by a very fast rotating pulsar. It is theorised that if the pulsar had been a neutron star, the high rotational energy would have quickly dissipated. However, if it had been a star composed of quark matter, this rotational energy would not have dissipated due to the interactions between quarks ([5] Dai et al., 2018).

Figure 4: Artist’s impression of Supernova ASSASN-15lh. Credit: [6] Sky & Telescope, 2016.

It is clear to see that quark stars are an ongoing debate in theoretical astrophysics. Several phenomena found in several neutrons star cannot be explained with the current models. However, if the stars were composed of free quarks, models show that these properties could then be explained. Even though these models might work, the existence of free quarks within a body is still a puzzle and the properties of matter at very high densities and very cold temperatures are not yet fully understood. Astrophysics will certainly keep an eye if more candidates for quark stars are found, and hence, decide if they can really exist.



[1] CERN (no date). The Standard Model. Available at: <> [accessed 12/05/2022]

[2] Universe Today (2016). What Are Quark Stars?. Available at: <> [accessed 10/05/2022]

[3] Weber et al. (2012). Structure of Quark Stars, arXiv: 1210.1910 [astro-ph.SR]

[4] Xiaoping et al.(2016). Is XTE J1739-285 a quark star masquerading as a neutron star, arXiv: 1610.08770 [astro-ph.HE]

[5] Dai et al.(2018). The Most Luminous Supernova ASASSN-15LH: Signature of a Newborn Rapidly-Rotating
Strange Quark Star, arXiv: 1508.07745 [astro-ph.HE]

[6] Sky & Telescope (2016). The Resurgence of the Brightest Supernova. Available at: <> [accessed 12/05/2022]