Particles called quarks hold the key to the nal fate of some stars
In neutron stars, the strength with which the core collapses fuses protons and electrons into neutrons. Neutron stars are extremely dense, creating immense pressure that could be forcing the neutrons into a new state of matter. An old open problem asks wh
e know that all matter is composed of atoms, and atoms are made of protons and neutrons inside the nucleus and electrons outside. But unlike electrons, protons and neutrons are composite particles because they are further made up of quarks.
Quarks can’t exist in isolation. They can only be found in groups of two or three, if not more. Such clumps of quarks are called hadrons. Protons and neutrons are common examples. Physicists have mostly studied quarks based on the behaviour of hadrons, and are also interested in how quarks clump together.
WWhen quarks clump
Two recent ndings revealed new insights on this count. One, published on
February 20, reported that three-quark clumps are more likely to form than two-quark clumps when a particular type of quark is more densely surrounded by some other particles. According to the international team of researchers that conducted this study, the nding rejects “conventional particle-physics models in which the consolidation of quarks is independent of the particle environment”.
Another study, published on March 15, reported observing clumps composed entirely of the heavier quarks. Protons and neutrons are clumps of lighter quarks and are thus more long-lived.
Heavy-quark clumps are very short-lived and harder to study, requiring more sophisticated tools and computing power. Yet understanding them is important to complete our understanding of all quarks, and by extension how these elusive particles aect what we know about nuclear fusion and the fate of stars.
In fact, in the particular and unusual case of quark stars, understanding quarks could have a more direct impact.
The tension of every star
A star is a globe of matter that has found a way to strike a balance between two forces. The force of gravity — arising from the star’s mass — encourages the star to collapse under its own weight and implode. The nuclear force, expressed in the explosive energy released by fusion reactions at its core, pushes the star to blow up and outwards. In a star, the two forces are equally matched and it shines in the sky.
But once a star runs out of material to fuse, nuclear fusion weakens and gravity starts to gain the upper hand. Eventually, the star will ‘die’ and implode. Its fate in its afterlife depends on how large and massive it was when it lived, as a result forming a white dwarf, a neutron star or a black hole.
Scientists have estimated that if the Sun were 20-times more massive, it may collapse into a black hole when it dies. If it were only eight-times heavier, it could become a neutron star. But could there be stars that are too heavy to form a neutron star yet not too heavy to form a black hole, and thus form a quark star?
Enter ‘quark matter’
In neutron stars, the strength with which the core collapses will fuse all protons and electrons inside into neutrons, thus its name. Physicists understand neutron colour charge. Then there are also antiquarks, their antimatter versions. A quark-antiquark clump is called a meson (they don’t annihilate each other because they are of dierent types, e.g. up + anti-down). Three-quark clumps are called baryons and they form the normal matter surrounding us.
Quarks are further held together by another set of particles called gluons. Because nuclear forces are very strong, quarks are always tightly bound to each other and are not free, even in the vacuum of empty space.
The nuclear force that holds quarks together is explained by a theory called quantum chromodynamics. It predicts that at suciently high (by all means extreme) energies, nuclear matter can become ‘deconned’ to create a new phase of matter in which quarks don’t have to exist in clumps.
Physicists have been able to obtain evidence of deconnement by smashing lead ions against each other at very high energies in machines like the Large Hadron Collider. In these experiments, a state of matter called a quark-gluon plasma exists for a brief moment; the ‘plasma’ means the quarks are independent. According to the Big Bang theory, the universe was lled with this plasma before the particles clumped and formed the rst blobs of matter.
This clumping process may release energy or modify its surroundings in a way that astrophysicists can look for, and eventually discover a quark star. Until then, the possibility will live on as one of the many open problems of physics.
(Qudsia Gani is an assistant professor in the Department of Physics, Government Degree College Pattan, Baramulla.)