The Quark Star: The Star We’ve Never Seen

As a kid with myopia or short-sightedness, I was recommended by the doctors to gaze upon the stars in the night sky in order to improve my vision. I spent hours tracing major stars like Polaris and constellations like Ursa Minor in the sky. Within a few months, I could name nearly all major stars in constellations. But some stars escaped not only my memory but also my vision, for they are nowhere to be found to anyone! These stars are called the quark stars.

Every object in our universe is made of atoms. Deep inside the atoms are particles called protons and neutrons, which are further composed of “quarks,” one of the fundamental blocks of matter. Multiple scientists like Walter Bodmer (1971) have theorized that quark matter consists of up, down, and strange quarks. Up quarks are oriented upward, down quarks point downward, and strange quarks are named for their strangely long lifetime—so long that they eventually decay into up and down quarks. Another type of subatomic particle called gluons holds the quark matter together. A combination of quarks and gluons in a chemically stable composition is referred to as QGP and is believed to be distributed throughout the universe in its early stages.

Among the numerous variety of existing stars, one of the most peculiar is a neutron star. They are huge interstellar objects with an average mass of 1.4 times the mass of the sun. Despite their enormous size, they pack their nuclear matter in an extremely small space, increasing their density by so much that a teaspoon of their nuclear matter would weigh billions of tons. The gases inside such an extremely dense star create a lot of pressure, which can cause the protons and neutrons inside the star to smash into each other with a ton of energy. This collision forms a minuscule fireball in which everything “melts” into a quark-gluon plasma, creating an ultra-dense phase of quark matter, known as a quark star. If the pressure is high enough, this collision will create enough quark matter to form a quark star—a star that is 99 to 100% quark matter.

Scientists have been trying to confirm a quark star’s existence for decades. While they haven’t yet found a full-blown quark star, they have detected quark matter in neutron stars. When two stars orbit each other due to gravitational pull, they send ripples through the fabric of space which is recorded as gravitational waves. One factor that influences the intensity of the gravitational waves is the amount of quark matter in the stars. Scientists have detected faint gravitational wave patterns that indicates the orbit of two neutron stars. 

However, stars are not the only things in our universe that give off gravitational waves. When two black holes merge, they also send out gravitational waves—and since both quark stars and black holes are extremely dense celestial objects, the gravitational wave pattern indicating quark matter is very similar to the one resulting from the merging event. One observation that could help to distinguish quark stars from black holes is a behavior unique to stars called stellar pulsation. When a star tries to maintain balance, its outer layer expands and contracts. These fluctuations in stars cause variations in how much light a star gives off, creating pulsation that is reflected in the gravitational wave image.

So where does that leave us? Unfortunately, no quark stars have been detected so far, though we have some promising contenders. Cosmologists have identified several stars previously categorized as neutron stars with stronger gravitational wave patterns to see if they could actually constitute quark stars. Additionally, many radiation records from objects that were assumed to be black holes are being rechecked to confirm that they were not quark stars. If the scientists discover something promising, the next step would be to secure authorization to send a drone to capture pictures of what might be the first quark star.

Scientists are also currently using a large hadron collider, a particle collider that accelerates subatomic particles and makes them crash into each other to reveal new subatomic particles, to find some unique properties which will help us better differentiate quark stars from neutron stars and black holes.

Although gazing upon the night sky was intended to help my vision, it ultimately allowed me to realize that the great mystery of our universe lies in the things we cannot see.