We 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.

When quarks clump

Two recent findings 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 finding 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 affect 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 stars well — on paper. The problem is they can’t run any direct experiments on them in any laboratory on the earth. They also don’t know either the masses or the radii of most neutron stars in the universe. So astrophysicists are very interested in studying them.

The matter inside neutron stars is extremely dense. For example, two Suns’ worth of mass is packed into a sphere only 25 km wide. This creates an immense pressure that could be forcing the neutrons into a new state of matter.

An old open problem in physics asks whether this state could be quark matter — when there are no longer any neutrons, only quarks.

In December 2023, researchers from the University of Helsinki reported in the journal Nature Communications that the insides of most massive neutron stars have an 80-90% chance of being made of quark matter.

The research team combined astrophysical observations with theoretical ab initio (from scratch) calculations to develop a model that they ran using a supercomputer, and arrived at this result. However, these astrophysical observations were small in number, meaning the result is not so reliable. Astrophysicists need more observational data to understand quark matter and how exactly it forms.

The need for quarks

A popular way of calculating the bulk properties of any material is to use an equation of state — an equation that, when solved with data about some of a material’s physical properties, reveals the values of other properties. For neutron stars, this is the Tolman-Oppenheimer-Volkoff equation: it is very complex but it assigns a probability to the presence of quarks within neutron stars.

Physics has a rich tradition of giving quirky names to things physicists find. For example, quarks come in six ‘flavours’ — three are called charm and strange; quarks themselves have a property called colour charge; and so on. The name ‘quark’ itself is courtesy physicist Murray Gell-Mann, who named these particles after a line in James Joyce’s 1939 masterpiece, Finnegan’s Wake.

Protons are positively charged and therefore have a magnetic moment (a turning force exerted by a magnetic field) associated with them. But neutrons have a magnetic moment, too, yet they are neutrally charged. So physicists in the 1960s figured neutrons must be made of smaller particles that gave rise to the magnetic moment but whose electric charges cancel themselves out. Gell-Mann called them quarks and their existence was confirmed in the 1970s.

Setting quarks free

There are six types of quarks: up, down, top, bottom, strange, and charm. Each quark can have one of three types of 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 different 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 sufficiently high (by all means extreme) energies, nuclear matter can become ‘deconfined’ 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 deconfinement 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 filled with this plasma before the particles clumped and formed the first 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.

An artist’s impression of a type of neutron star called a magnetar. Magnetars are the cosmic objects with the strongest magnetic fields ever measured in the universe.
| Photo Credit: Reuters

Magnetars are among the universe’s most extreme objects – a class of the compact stellar remnants called neutron stars that possess immensely strong magnetic fields. Once in a while, they produce enormous eruptions of gamma rays in the strongest nondestructive release of energy known in the cosmos.

Scientists have now detected the most distant-known instance of one of these eruptions, called a giant flare, from a magnetar residing in a galaxy called Messier 82, or M82. This surge of gamma rays, the most energetic form of light, unleashed in just a tenth of a second the amount of energy our sun would emit in a span of roughly 10,000 years, they said.

Only two confirmed giant flares have been observed in our Milky Way galaxy, in 2004 and 1998, and only one previous one in another galaxy, in 1979 in the Milky Way’s neighboring Large Magellanic Cloud, the researchers said.

“Giant flares are very rare events,” said astrophysicist Sandro Mereghetti of Italy’s National Institute for Astrophysics (INAF) in Milan, lead author of the research published on Wednesday in the journal Nature. “The Milky Way contains at least 30 magnetars, possibly many more, which have not been seen to emit giant flares.”

M82, nicknamed the “cigar galaxy” because when viewed edge-on it has an elongated and cigar-like shape, is 12 million light-years from Earth in the constellation Ursa Major. A light year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km). The magnetar giant flare from the Large Magellanic Cloud was about 160,000 light-years from Earth.

The M82 giant flare was the most distant known but not the most energetic. The one spotted in 2004 had the energy equivalent to about a million years of output from the sun.

While there are more energetic cosmic events such as supernova explosions at the end of a massive star’s life and gamma-ray bursts caused by two neutron stars merging, those involve destruction, unlike giant flares. Magnetars also emit occasional surges of gamma rays and X-rays at lower energy levels than giant flares.

Neutron stars are born in the explosion and collapse of stars eight to 25 times the mass of the sun at the end of their life cycle. They compress one or two times the sun’s mass into a sphere only the size of a city.

“They are the most compact and dense astrophysical objects. They are as dense as atomic nuclei,” INAF astrophysicist and study co-author Michela Rigoselli said of neutron stars.

The main trait that sets magnetars apart from other neutron stars is a magnetic field 1,000 to 10,000 times stronger than an ordinary neutron star’s magnetism and a trillion times that of the sun.

“We can say that magnetars are neutron stars powered by their own magnetic energy. This does not happen in ordinary neutron stars,” Mereghetti said.

“A giant flare originates from a reconfiguration and a reconnection of the magnetic field of the magnetar,” Rigoselli added.

The magnetar in this research is believed to spin rapidly, perhaps completing a rotation every few seconds. Its giant flare was detected by the European Space Agency’s Integral space observatory on Nov. 15, 2023, in M82, a galaxy boasting a star formation rate much higher than the Milky Way’s – called a “starburst galaxy.”

“The fact that Messier 82 is so active in star formation is relevant for our finding,” Rigoselli said. “In such an active galaxy, there are many young, massive stars like those which evolve into supernova explosions and give birth to neutron stars. It would have been suspicious to detect a magnetar giant flare coming from a quiescent galaxy.”

A spinning neutron star periodically swings its radio (green) and gamma-ray (magenta) beams past Earth in this artist’s concept of a black widow pulsar. The pulsar heats the facing side of its stellar partner to temperatures twice as hot as the sun’s surface and slowly evaporates it.
| Photo Credit: Reuters

How fast can a neutron star drive powerful jets into space? The answer, it turns out, is about one-third the speed of light, as our team has just revealed in a new study published in Nature.

Energetic cosmic beams known as jets are seen throughout our universe. They are launched when material – mainly dust and gas – falls in towards any dense central object, such as a neutron star (an extremely dense remnant of a once-massive star) or a black hole.

The jets carry away some of the gravitational energy released by the infalling gas, recycling it back into the surroundings on far larger scales.

The most powerful jets in the universe come from the biggest black holes at the centres of galaxies. The energy output of these jets can affect the evolution of an entire galaxy, or even a galaxy cluster. This makes jets a critical, yet intriguing, component of our universe.

Although jets are common, we still don’t fully understand how they are launched. Measuring the jets from a neutron star has now given us valuable information.

Jets from stellar corpses

Jets from black holes tend to be bright, and have been well studied. However, the jets from neutron stars are typically much fainter, and much less is known about them.

This presents a problem, since we can learn a lot by comparing the jets launched by different celestial objects. Neutron stars are extremely dense stellar corpses – cosmic cinders the size of a city, yet containing the mass of a star. We can think of them as enormous atomic nuclei, each about 20 kilometres across.

In contrast to black holes, neutron stars have both a solid surface and a magnetic field, and gas falling onto them releases less gravitational energy. All of these properties will have an effect on how their jets are launched, making studies of neutron star jets particularly valuable.

One key clue to how jets are launched comes from their speeds. If we can determine how jet speeds vary with the mass or spin of the neutron star, that would provide a powerful test of theoretical predictions. But it is extremely challenging to measure jet speeds accurately enough for such a test.

A cosmic speed camera

When we measure speeds on Earth, we time an object between two points. This could be a 100-metre sprinter running down the track, or a point-to-point speed camera tracking a car.

Our team, led by Thomas Russell from the Italian National Institute of Astrophysics in Palermo, conducted a new experiment to do this for neutron star jets.

What has made this measurement so difficult in the past is that jets are steady flows. This means there is no single starting point for our timer. But we were able to identify a short-lived signal at X-ray wavelengths that we could use as our “starting gun”.

Being so dense, neutron stars can “steal” matter from a nearby orbiting companion star. While some of that gas is launched outwards as jets, most of it ends up falling onto the neutron star. As the material piles up, it gets hotter and denser.

When enough material has built up, it triggers a thermonuclear explosion. A runaway nuclear fusion reaction occurs and rapidly spreads to engulf the entire star. The fusion lasts for a few seconds to minutes, causing a short-lived burst of X-rays.

One step closer to solving a mystery

We thought this thermonuclear explosion would disrupt the neutron star’s jets. So, we used CSIRO’s Australia Telescope Compact Array to stare at the jets for three days at radio wavelengths to try and catch the disruption. At the same time, we used the European Space Agency’s Integral telescope to look at the X-rays from the system.

To our surprise, we found the jets got brighter after every pulse of X-rays. Instead of disrupting the jets, the thermonuclear explosions seemed to power them up. And this pattern was repeated ten times in one neutron star system, and then again in a second system.

We can explain this surprising result if the X-ray pulse causes gas swirling around the neutron star to fall inwards more quickly. This, in turn, provides more energy and material to divert into the jets.

Most importantly, however, we can use the X-ray burst to indicate the launch time of the jets. We timed how long they took to move outwards to where they became visible at two different radio wavelengths. These start and finish points provided us with our cosmic speed camera.

Interestingly, the jet speed we measured was close to the “escape speed” from a neutron star. On Earth, this escape speed is 11.2 kilometres per second – what rockets need to achieve to break free of Earth’s gravity. For a neutron star, that value is around half the speed of light.

Our work has introduced a new technique for measuring neutron star jet speeds. Our next steps will be to see how the jet speed changes for neutron stars with different masses and rotation rates. That will allow us to directly test theoretical models, taking us one step closer to figuring out how such powerful cosmic jets are launched.