Can a quark star exist? It's an open question in the astronomy community, but there appears to be an argument for quark stars if we examine the physics of dying stars in more detail, argues astrophysicist and Space.com columnist Paul Sutter. He goes deep into the details in this week's episode of "Ask a Spaceman."
In Episode 11 of the Facebook Watch series, Sutter continues the topic of quark stars, which he first began last week in Episode 10. It's best to watch last week's episode to get the full story, as Sutter touches upon a critical concept called degeneracy pressure in great detail. For a quick, one-sentence recap: Degeneracy pressure stops the collapse of an object (such as a white dwarf, which we'll get into in a moment) from collapsing because the fundamental particles within the object are crammed into a tiny space.
Sutter goes deep into physics in this week's episode, but he says it's necessary — the topic of quark stars is so complicated that it needs three parts to explain. "That's how intense it is," he says in the video. Here, he traces stellar evolution from a dying star through to smaller and smaller star variants: white dwarfs, neutron stars and the theoretical quark stars.
First, let's briefly talk about what happens when a star similar in mass to our sun reaches the end of its life. It sloughs off layers of gas and leaves behind a cooling white dwarf star. These are Earth-size objects with immensely strong gravity, some 350,000 times the gravity of Earth. White dwarfs hold off collapse through degeneracy pressure. The star's electrons — negatively charged particles — will cram into a small space and resist being squashed further, which stops the collapse.
But what if the genesis star was much larger than our sun? As Sutter explains, astrophysicist Subrahmanyan Chandrasekhar theorized that after a star is roughly 1.4 times the mass of our sun, this electron degeneracy pressure can be overridden. So the star will keep shrinking into something called a neutron star, which is about the size of a city. (Chandrasekhar's theories took a while to be accepted, but he eventually co-won the 1983 Nobel Prize. Also, NASA's Chandra X-Ray Telescope is named after him.)
In a neutron star, the degeneracy pressure acts a little differently. Some of the electrons are pushed against another fundamental particle, called a proton (which has a positive charge, and is found in atoms' centers). The electron and the proton pushed together — a negative and a positive charge — end up combining and creating a neutral particle called a neutron. Neutrons can be "crammed much more tightly than electrons" Sutter explains. That's why a neutron star is so small.
So what if you override the neutrons' degeneracy pressure? In most cases, the star would collapse into a singularity — a stellar-mass black hole. That's an accepted path in stellar evolution for stars that are at least three times the mass of our sun. Black holes pull in gas, dust and any other objects nearby and have such a strong gravitational well that even light cannot escape.
But can you make a quark star? Well, neutrons are not the smallest fundamental particle. Each neutron is made up of even smaller particles called quarks. Quarks (and their antimatter counterparts) come in six types, or "flavors": up, down, top, bottom, strange and charm.
A quark star — if it actually exists — would happen if somehow you could collapse the neutron star even further. Not so far that it becomes a black hole, but into an intermediary stage. In this stage, the neutrons would be disassociated and there would be quarks balled up that are supported by their own degeneracy pressure, or their resistance to crunching even smaller. Hence, a quark star.
We can study quarks using huge colliders that smash up small particles, but the mathematics and physics are pretty complicated, Sutter says. For example, quarks tend to arise in swarms — not as individuals — which makes them hard to study.
To learn more about quark stars, stay tuned for next week's episode. The episodes will be released weekly on Wednesdays at 12 p.m. EDT (1600 GMT), so like the Facebook page or check back later to see more. Sutter also responds to reader questions in every episode. Check the page to learn more about past topics the show covered, such as the Big Bang, Pluto and galaxy collisions.
Sutter is a cosmologist at Ohio State University and chief scientist at the Center of Science and Industry in Columbus, Ohio. He has a long-running podcast, also called "Ask A Spaceman." You can catch all past episodes of that podcast here.