Strange Quark Matter: Neutron Star Cores

Neutron stars are among the densest objects in the universe, packing the mass of our sun into a city-sized sphere. However, astrophysicists and nuclear physicists have gathered compelling evidence suggesting our current understanding of these stars might be incomplete. Recent observations point to the possibility that the cores of these stars—or perhaps the entire star—are made of “strange quark matter,” a substance far more stable and dense than standard atomic nuclei.

Understanding the Quark Soup

To understand strange matter, we first have to look at normal matter. Everything you see around you is made of atoms, which contain protons and neutrons. These particles are not fundamental; they are composite bundles made of smaller particles called quarks.

• Protons are made of two “up” quarks and one “down” quark.
• Neutrons are made of one “up” quark and two “down” quarks.

In standard physics, quarks are always confined inside these protons and neutrons. You cannot find a quark floating freely in nature. However, the “Strange Matter Hypothesis,” popularized by physicist Edward Witten in 1984, suggests that under extreme pressure, this rule changes.

Inside the core of a neutron star, the pressure is so intense that the boundaries between neutrons may break down. The particles dissolve into a uniform soup of free quarks. At these energy levels, some of the down quarks spontaneously convert into heavier “strange” quarks. The result is a mixture of up, down, and strange quarks known as Strange Quark Matter (SQM).

The Evidence: HESS J1731-347

The theory of strange matter has moved from pure mathematics to observational astronomy thanks to specific anomalies found in the sky. The most prominent piece of evidence arrived recently via a study published in Nature Astronomy.

Researchers analyzing the object known as HESS J1731-347 found that it defies standard neutron star physics. Located in the constellation Scorpius, this object appears to have a mass of approximately 0.77 times that of the Sun. This is significantly lighter than the theoretical minimum mass for a neutron star, which is usually calculated to be around 1.1 to 1.2 solar masses.

If HESS J1731-347 were a normal neutron star composed of nuclear matter, it would have decompressed and exploded at such a low mass. The fact that it remains stable suggests it is much denser and more bound together than nuclear physics allows. This points directly to strange quark matter, which can remain stable at lower masses due to the strong interaction between the three types of quarks.

The Mass Gap Mystery: GW190814

Another piece of evidence comes from gravitational wave detectors like LIGO and Virgo. In August 2019, these observatories detected a merger event named GW190814. This event involved a black hole colliding with a mystery object that had a mass of roughly 2.6 solar masses.

This object sits in the “mass gap.” It is too heavy to be a standard neutron star (which should collapse into a black hole above 2.2 or 2.3 solar masses) but arguably too light to be a typical black hole.

Astrophysicists hypothesize that this object could have been a “strange star.” Because strange quark matter is more compressible and has a different equation of state (the relationship between pressure and density) than neutron matter, a strange star could theoretically support more mass without collapsing. This allows for stable stars that are heavier than the maximum limit predicted for normal neutron stars.

Hybrid Stars vs. Strange Stars

Scientists are currently debating two main configurations for these celestial bodies:

1. Hybrid Stars: These look like standard neutron stars on the outside with a crust of ions and electrons. However, as you go deeper toward the center, the pressure rises until phase transition occurs, creating a core of strange quark matter.
2. Strange Stars: If the hypothesis of “absolute stability” is true, strange matter is the most stable form of matter in the universe. If a neutron star converts to strange matter, the conversion could consume the entire star all the way to the surface. These objects would have no crust (or a very thin suspended crust) and would radiate X-rays with a distinct spectrum different from neutron stars.

How We Detect the Difference

Distinguishing a ball of neutrons from a ball of quark soup is difficult from thousands of light-years away, but NASA’s NICER (Neutron star Interior Composition Explorer) mission is providing the necessary data. NICER measures the X-rays emitted from hotspots on the surface of rapidly spinning pulsars.

By timing these pulses with nanosecond precision, scientists can map the gravitational bending of light around the star. This allows them to calculate the star’s radius to within a few percent.

• Radius Constraints: If a star has a mass of 1.4 solar masses but a radius significantly smaller than 11 or 12 kilometers, standard nuclear physics models fail. Quark matter, being more compact, allows for smaller radii.
• Cooling Rates: Quark matter emits neutrinos (ghostly subatomic particles) much more efficiently than normal neutron matter. Therefore, a young strange star should cool down much faster than a standard neutron star. If astronomers find a young supernova remnant with a central star that is surprisingly cold, it is a strong candidate for composed strange matter.

The "Strangelet" Contagion

One of the most fascinating aspects of this theory is the concept of “strangelets.” A strangelet is a small fragment of strange matter. According to the Bodmer-Witten hypothesis, if a strangelet were to come into contact with normal matter (like a standard nucleus), it would convert that matter into strange matter instantly, releasing energy in the process.

While this sounds like science fiction, it plays a role in astrophysics. If two neutron stars collide (a “kilonova”), they spray material into the cosmos. If they release strangelets, these fragments could bombard other stars, slowly converting them over millions of years. This remains theoretical, but it highlights how fundamental the discovery of strange quark matter would be for our understanding of the universe’s stability.

Frequently Asked Questions

Is strange matter dangerous to Earth? No. Even if strange matter exists in the cores of neutron stars, it is confined there by immense gravity. “Strangelets” floating through space are purely hypothetical and, even if they exist, the likelihood of them hitting Earth with enough energy to cause a reaction is statistically negligible.

What is the difference between a quark star and a neutron star? A neutron star is composed mostly of neutrons (neutral subatomic particles). A quark star is composed of the constituent parts of those neutrons (quarks) moving freely. Quark stars are denser and potentially more stable.

Has a strange star been confirmed 100%? Not yet. Objects like HESS J1731-347 are very strong candidates, but definitive proof requires more precise measurements of the mass and radius relationship. Future gravitational wave observations will likely settle the debate.

Who discovered strange matter? The concept was first proposed by physicists A.R. Bodmer in 1971 and later expanded upon by Edward Witten in 1984. The search for physical evidence has been ongoing since then.