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Quark and neutron stars: how do we tell them apart?

Quark and neutron stars: how do we tell them apart?

Matter found in the densest stars in the Universe – initially known as radio pulsars – poses a frustrating problem for astrophysicists. Theoretical projections allow for two possibilities: they are either neutron stars or quark stars (also known as strange stars). To tell these two types apart through observation was previously thought impossible. However, physicists from the Jagiellonian University and Cracow University of Technology have recently discovered new features of quark stars that may help discern them from neutron stars.

When Jocelyn Bell, a PhD student at the University of Cambridge, first observed a radio pulsar in 1967, she opened a new chapter in the history of astrophysics. Pulsars were quickly identified as rapidly rotating neutron stars, celestial objects scientists have speculated about in the 1930s, ever since they discovered the neutron and learned the structure of the atomic nucleus. To put it very simply, neutron stars are giant atomic nuclei composed of neutrons with a smattering of protons. It is the most dense type of matter in the Universe. A neutron star containing the same amount of matter as the Sun would be a sphere with a diameter of about 25 kilometres (the diameter of the Sun is equal to 1.5 million kilometres). Hence, such a star would be over 200 trillion times denser and smaller in volume than the Sun. The mass of one millilitre (1 cm3) of the matter from that star would weigh 280 million tonnes on Earth.

Confirming the existence of neutron stars was the final element of the theory of stellar evolution developed by astrophysicists throughout the 20th century. It describes the slow and inevitable process in which a star increases the density of its core due to thermonuclear synthesis. The ultimate outcome of this evolution for a broad array of stars is the formation of extremely dense neutron stars.

Understanding stellar evolution was a great success for nuclear physics. The 1970s and 1980s saw a surge in the development of particle physics, resulting in theories on the structure of neutrons, protons and other hadrons made up of fundamental constituents of matter – quarks. Six types of quarks have been discovered, out of which three – marked as u, d and s – are relevant  for astrophysics. However, laying the groundwork for quark models and the theory of strong reactions based on them, called quantum chromodynamics, took a lot of time. The properties of quarks differed vastly from what the researcher previously imagined. Notably, the electric charge of quarks is not an integer multiplier (2/3 for u quark and -1/3 for d quark), and quarks themselves are subject to colour confinement, meaning that they are always found in threes, never in isolation. A proton is made up of uud quarks, while a neutron is made up of udd quarks.

The quark structure of neutrons doesn’t affect the astrophysical properties of neutron stars. Nevertheless, it might be more noticeable in the innermost core of such stars, where a ‘jumble’ of u and d quarks might be a little closer to reality than whole neutrons. Sadly, there seemed to be no way to peer into the centre of a neutron star to confirm that hypothesis. This opinion on the role of quarks has fundamentally changed with the introduction of the strange quark matter hypothesis. In 1984, American physicist Edward Witten pointed out that the quark theory allows for the existence of matter that is more strongly bound than that found in atomic nuclei, composed of three quarks: u, d  and s. Because the s quark is known as the strange quark, this hypothetical matter is called Strange Quark Matter hypothesis. The most important property of SQM when it comes to astrophysics is that it is at zero pressure in its base state. It behaves similarly to a drop of water at zero gravity – it keeps its shape. A similar amount of gas will fill its whole volume. The density of SQM is 4x1014 (400 trillion) times higher than that of water (1 g/cm3).

Stars like the Sun are built from gases and held together by the gravitational pull of the particles of those gases. Neutron stars are similar. In both cases, if it wasn’t for the gravitational pull, the matter would simply dissolve in space. However, it is not the same for strange quark stars. These objects would be stable even if there was no gravity, since they are held together by the strong interactions between quarks. The physical nature of neutron stars and quark stars is different (see image on the right), but their astrophysical properties are similar. Their size is also quite alike: quark stars are a little bit smaller (about 20 kilometres vs. about 24 kilometres in diameter). Such minute differences are impossible to detect with the current astronomical equipment.

For a very long time, the inability to discern the nature of observed stars was frustrating for astrophysicists. Luckily, the latest study conducted by Prof. M. Kutchera and T. Kędziorek from the JU Institute of Theoretical Physics in collaboration with Dr hab. Ł. Bratek, Dr hab. J. Jałocha and Dr hab. S. Kubis, published in The Astrophysical Journal, proves that quark stars have hitherto undiscovered properties that can be used to differentiate them from neutron stars. This is because they behave differently when they form a binary star system.

About 80% of the stars in the Milky Way Galaxy can be found in binary systems, revolving around one another. We know of a number of cases in which two pulsars are not separated by a great distance. In these ‘tight’ systems, stars interact very strongly with each other, causing internal oscillations. It turns out that radial oscillations of quark stars are much different than those of neutron stars. In case of the latter ones, their energy slowly dissipates without affecting the evolution of the binary system. The converse is true for quarks stars: the energy of the oscillation immediately radiates away, significantly influencing the evolution of the system.

The speed at which oscillations of a quark star lose energy is directly tied to the star’s structure. The density of mass on the surface of a quark star, where the pressure is no longer a factor, is equal to the density of SQM: 4x1014 g/cm3. The surface of a neutron star is entirely different. It is made up of atoms of iron that form a crystal, just like pure iron on Earth, the density of which is 8 g/cm3. Radial oscillations of a star can basically be described as rhythmic ‘shrinking’ and ‘expanding’. The amplitude of these oscillations is usually extremely small (usually fractions of a permille). However, in extreme cases, the amplitude may be as high as several dozen percent. During the expansion, the radius of the star increases, which means an increase in volume and a decrease in density. An increase of radius by one permille causes a decrease of surface density by three permille. For a neutron star, that means a slight expansion of iron that transitions into a slight shrinkage, and so on. These oscillations recede over time. In the case of a strange quark matter star, the situation is nothing alike. A 0.3% drop in the density of quark matter causes it to enter an excited state, which turns it into a completely different form of matter. The entirety of the oscillation energy is transformed into latent heat, and the oscillations disappear over the course of the first cycle. Physically, it means that the energy of a quark star dissipates immediately. The paper published in The Astrophysical Journal describes in detail the mechanism of conversion of mechanical energy into internal energy of a star.

The abovementioned property of quark stars has not been previously known to researchers, and so it wasn’t taken into account when preparing models of evolution of compacted binary quark stellar systems. It was commonly thought that quark stars and neutron stars looks exactly the same: both stars get closer and closer to each other until they form a black hole. But if we consider the newly discovered mechanism of energy dissipation in quark stars, then the evolution of both types of stellar system will prove to be entirely different.

The paper Oscillating Strange Quark Matter Objects Excited in Stellar System, written by Marek Kutchera, Łukasz Bratek, Joanna Jałocha, Sebastian Kubis and Tomasz Kędziorek, was published in the latest issue of The Astrophysical Journal.

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