HARVARD GAZETTE ARCHIVES
Quark stars signal unstable universe
By William J. Cromie
A star in the constellation Corona Australis emits X-ray signals that some astronomers think come from an object made not out of atoms, or even the protons and neutrons that make up atoms, but a strange combination of quarks. Called "strange quark matter," all existing knowledge points to it as the most stable form of matter that can possibly exist. If so, then all other materials must be relatively unstable.
"If strange quark matter really exists, it implies that normal matter is not ultimately stable," says Jeremy Drake of the Harvard-Smithsonian Center for Astrophysics. "One would think that, given enough time, heavier normal matter, like iron, could eventually revert naturally to the more stable strange quark form. Then if a piece of this matter collides with normal stuff at high enough speed, it could convert it into strange quark matter. Such collisions release a lot of energy, so that in the distant future they might be used as a new form of energy."
But before speculation gets too intense, astrophysicists must prove that strange quark stars are really out there. Evidence in their favor is good, but by no means conclusive.
In orbit around Earth, a satellite called the Chandra X-ray Observatory surveys the universe for sources of X-rays, which come from hot, active places. Such places include neutron stars, the still energetic corpses of burnt out stars once more massive than the Sun. When such stars use up their hydrogen fuel they explode into bright supernova, then their cores collapse into an extremely heavy ball of neutrons enveloped in a thin atmosphere containing iron and other debris from the explosion. In the core of the dying star, extreme pressure breaks atoms down into protons, neutrons, and electrons. The protons and electrons combine into neutrons, and the remaining material is so heavy that one tablespoon of it weighs about four trillion pounds.
Such weighty objects are barely visible in optical telescopes, but they emit distinct patterns of X-rays picked up by instruments aboard the Chandra satellite. However, when astronomers and astrophysicists looked at the data recorded by these instruments, they didn't see what they thought they'd see.
Small, cool, and bizarre
The star in Corona Australis was thought to be the nearest neutron star to Earth. But its X-rays give off no hint of a thin atmosphere with iron or other elemental debris. And it appears to be only half the size that, according to theory, a neutron star can possibly be - seven miles across instead of 12-20 miles.
Another bizarre star, much further away, was first seen by Chinese and Japanese sky-watchers as a supernova in August 1181, in the constellation Cassiopeia. When stars blow up like this, astrophysicists have a formula to predict how fast they cool off. Temperatures of neutron stars can be measured by the intensity of X-rays they emit. When astronomers check its temperature, the object in Cassiopeia turns out to be much too cold to fit the definition of a neutron star.
What are these weird objects then?
The short answer is that astronomers don't really know. But when they try to figure out how to put all the mass left over from an exploded star bigger than the Sun into a package that small and cool, they conclude that its neutrons may have been broken down into quarks.
Quarks were first named by Dr. Murray Gell-Mann, who won the Nobel Prize in physics in 1969. James Joyce, who liked to play with words, used "quark" in his book "Finnegans Wake." Gell-Mann adopted the word, and further research turned up six kinds, or "flavors," of quarks - up and down, top and bottom (also known as truth and beauty), charm and strange. (Physicists have a sense of humor, too.)
According to this theory, neutrons consist of three quarks, two downs and one up. If pressure in a collapsing star becomes great enough, neutrons may burst, freeing the quarks and causing the liberated particles to pack themselves more tightly. In this process, strange quarks are created, and the result is an immensely heavy ball of up, down, and strange quarks, known as strange quark matter.
"A strange quark star would be like a single huge particle, the size of a small city, rather than a neutron star made of trillions and trillions of particles," Drake says.
It ain't necessarily so
The pressures needed to make such matter cannot be duplicated on Earth, so there's no way to check this scenario directly. Some scientists insist that other ways exist to explain the X-ray data.
Patrick Slane, an astrophysicist who works down the hall from Drake, points out that x-rays from the Corona Australis star may come from a hot spot on an ordinary neutron star. Instead of originating from a small strange quark star, the energetic rays may emanate from a hot area on the surface of a neutron star.
Drake admits to that possibility, but says the probability is not too likely. Neutron stars rotate rapidly, so that a hot spot on a larger star would generate pulsations in the x-ray signal. No such pulsations have been observed.
As far as the abnormal cooling rate of the other supposed strange quark candidate, "there are many processes that could lead to faster than expected cooling," notes Slane. "I would put strange quark matter fourth or fifth on the list."
How can the mystery be solved? "With more observations of (the star in Corona Australis) we may be able to rule out more confidently that it's a hot spot we're seeing," says Drake. "But I don't think it would be too fruitful to keep looking at the same objects. The best way forward is to find more of these stars and try to work out their essential properties."
A total of seven candidates for strange quark bodies now exist, and astronomers may find many more when they look more closely at other objects now labeled neutron stars.
"If strange quark stars do exist, the conclusion that they are very common in our galaxy would be inescapable," Drake comments. Because it would be so stable, strange quark matter would survive collisions with neutron stars as well as the birth and death of ordinary stars. But as far as taking over the galaxy and turning everything into heavy bags of quarks, Drake doesn't believe that will happen. "It would take much longer than the age of the universe (about 13 billion years)," he notes.
Any strange quark matter that exists is likely to be floating around our galaxy in small pieces. "It would have come from collisions between strange quark stars and other dense objects like neutron stars," Drake explains. "Strange quark nuggets may even be part of Earth." If so, they would be present in very, very small amounts and nothing to worry about, at least for the next 15 billion years or so.
And if it turns out that there are no strange quark nuggets on Earth and no strange quark stars in our galaxy, Drake and his colleagues won't be too disappointed. "We have demonstrated that we can build facilities like Chandra and use them to determine the detailed properties of objects hundreds of trillions of miles away," he notes. "That gives us the ability to gain insights into the tiniest scales of nature that make up the cosmos."
Three quarks for Muster Mark.