Quark Star
A quark star is a theoretical type of stellar remnant that may form after a supernova explosion, particularly from high-mass stars that cannot stabilize as neutron stars due to their extreme mass. When the core of such a star collapses, the pressure is so immense that the neutrons within begin to convert some of their down quarks into strange quarks, leading to a state where quarks are no longer confined to individual baryons like neutrons. This results in a star composed primarily of quark matter, capable of resisting further gravitational collapse through the outward pressure exerted by the quarks.
The concept of quark stars emerged as a third possibility beyond the well-established neutron stars and black holes. Some scientists suggest that quark stars may arise from events called quark-novas, which could occur shortly after a supernova. While no quark stars have been definitively identified, astronomers are on the lookout for massive neutron stars that may actually be quark stars. Observations of gravitational waves, such as those from the merger of neutron stars, are being studied to explore the possibility of quark transitions during these energetic events. Understanding quark stars could provide valuable insights into particle physics, particularly since strange quarks cannot be produced in laboratories.
Quark Star
FIELDS OF STUDY: Stellar Astronomy; Astrophysics
ABSTRACT: Quark stars are stars that are too large to be classified as neutron stars but not large enough to collapse into black holes. Although quark stars have not yet actually been observed, scientists have enough evidence to believe that they exist. Quark stars are important because they may provide information about quark particles, which have yet to be replicated in laboratories.
Death of a Star
Over millions or billions of years, high-mass stars use up the energy-producing material in their core and die in a spectacular explosion known as a supernova. Supernovas result when the star’s gravitational force overwhelms its energy-depleted core, causing the core to collapse until the matter inside becomes so dense that the pressure forces it apart again. Much of the star’s matter then explodes out into the universe at speeds approaching ten thousand kilometers per hour.
Not all of the star’s matter is blown away in the supernova, however. A supernova occurs when the star’s carbon core has fused into iron, at which point fusion would consume more energy than it would produce. Instead, the protons and electrons in the core are compressed so much that they merge and become neutrons. The resulting object is called a neutron star. Neutron stars are generally about 10 to 20 kilometers (6.25 to 12.5 miles) in diameter.
If a high-mass star is more than three times the mass of Earth’s sun, it will leave behind matter from its core. This core remnant cannot produce any energy. Thus, it will experience extreme pressure from the star’s gravitational forces and eventually collapses into a black hole.
Scientists long believed that a supernova produced by a high-mass star could result in only one of two options, a neutron star or a black hole. But modern astrophysicists theorize that a third option may exist: quark stars. A quark star would be composed of small particles known as quarks, or elementary particles that make up certain types of subatomic particles. Two quarks (or, more accurately, one quark and one antiquark) form a type of particle called a meson. Three quarks form a type of particle called a baryon, a category that includes neutrons. The quarks that form mesons and baryons are held together by the strong interaction, one of four fundamental forces in the universe. Forcing these quarks apart would produce resistance similar to that experienced when stretching a piece of elastic. In a quark star, the quarks would exert enough outward pressure to prevent the star’s total collapse.
The Third Option
When the material left over after a supernova is too massive to form a neutron star, it becomes difficult for the star to resist collapsing from its own gravity. At this point, the quarks come into play. Each neutron contains two down quarks and one up quark. (There are six types of quarks; the other four are strange quarks, charm quarks, bottom quarks, and top quarks.) As the gravitational pressure builds, some down quarks become strange quarks. The star is then considered a quark star or, if it contains strange quarks, a strange star.
Some scientists have theorized that quark stars may be formed by quark-novas. They propose that just days after a massive star explodes in a supernova, it experiences a second explosion—the quark-nova—and becomes a quark star. It is possible that the gases and debris from the supernova may obscure this second explosion. Scientists have not observed quark-novas and have no direct proof that they occur. However, there is some evidence that the phenomenon may exist. In particular, scientists have noticed that some stars thought to result from supernovas contain less iron and more titanium than expected. If these stars underwent quark-novas, this might explain the discrepancy.
The Importance of Quark Stars
Astronomers have not identified any quark stars, but they continue to look for them. Some scientists theorize that if they find large neutron stars with a mass 2.5 times greater than that of Earth’s sun, it might actually be a quark star. This discovery would be particularly valuable to astrophysicists. Because strange quarks cannot be replicated in a laboratory, a quark star would be an important source of information for particle physicists as well.
In 2017, astronomers observed the merging of two neutron stars for the first time. The event was labeled as GW170817 by the astronomers. The pressure and energy generated by the event were, in theory, large enough to split the neutrons in quarks and form a quark star. Two international research groups independently studied the gravitational wave spectra of GW170817 to validate this hypothesis.
Researchers fed the gravitational-wave analysis data into computer model simulations. Most of the simulation models suggest the transition of star-matter into quark-matter does occur during such events. However, these results still require verification with experimental observations. Gravitational-wave detection is a relatively new concept in astronomy. Improvements in detection methods should help astronomers understand more about such events and whether the quark-transition is sustained to birth a quark star or collapses into a singularity.
PRINCIPAL TERMS
- neutron star: a dense, quickly spinning star made of the matter left behind after a star has experienced a supernova, or intense, bright explosion.
- quark: an elementary particle that is part of every hadron (proton or neutron). It is found in pairs or triplets.
- quark-nova: an explosion believed to occur after a high-mass star experiences a supernova, resulting from a second collapse of its core.
Bibliography
Bernoskie, Brandi, Heather Deiss, and Denise Miller. "What Is a Supernova?" NASA Education. NASA, 4 Sept. 2013. Web. 6 Apr. 2015.
Hall, Shannon. "Quark Nova Spotted in Cas A?" Sky & Telescope. F+W Media, 29 May 2014. Web. 6 Apr. 2015.
"High Mass Star." Space Book. Las Cumbres Observatory Global Telescope Network, 2012. Web. 6 Apr. 2015.
Nave, Carl R. "Quarks." HyperPhysics. Georgia State U, n.d. Web. 7 Apr. 2015.
"Neutron Stars: Incomprehensible Density." Science and Space. Natl. Geographic Soc., n.d. Web. 6 Apr. 2015.
O’Neill, Ian. "Why Are Quark Stars So Strange?" Discovery News. Discovery Communications, 19 Jan. 2010. Web. 6 Apr. 2015.
Petkov, Alex. "Gravitational Waves Could Reveal the Birth of a Quark Star." Physics World, 19 Feb. 2019, physicsworld.com/a/gravitational-waves-could-reveal-the-birth-of-a-quark-star. Accessed 21 July 2021.
"Quark." Cosmos: The SAO Encyclopedia of Astronomy. Swinburne U of Technology, n.d. Web. 6 Apr. 2015.
Sutter, Paul. "Are Quark Stars Possible?" Space.com, 19 Feb. 2018, www.space.com/39717-are-quark-stars-possible.html. Accessed 21 July 2021.