"We haven't found strange stars yet," explains Prashanth Jaikumar at the Argonne National Laboratory. "But that doesn't mean they don't exist. Maybe we have found them. Maybe some of these neutron stars are really strange stars. According to our theory, it would be very difficult to tell a strange star from a neutron star."
Research suggests that neutron stars may gradually transform into 'strange' stars - i.e. in stars made up primarily from the 'strange' quark. The conventional wisdom is that the electric field of a such a hypothetical strange star (made up from strange matter) at its surface would be so huge and its luminosity so big that it would be impossible to confuse it with anything else.
One of the most interesting aspects of neutron stars is that they are not gaseous like usual stars, but they are so closely packed that they are liquid. Strange stars should also be liquid with a surface that is solid.
However, Jaikumar and his colleagues challenge that. Strange stars are usually assumed to exhibit huge electric fields on their surface precisely because they are assumed to have a smooth surface. But according to the scientists neither neutron stars nor strange stars have such a smooth solid-like surface.
"It's like taking water," Jaikumar says, "with a flat surface. Add detergent and it reduces surface tension, allowing bubbles to form. In a strange star, the bubbles are made of strange quark matter, and float in a sea of electrons. Consequently, the star's surface may be crusty, not smooth. The effect of surface tension had been overlooked before."
One consequence is that a strange star wouldn't have large electrical field at surface or be super-luminous. It also allows for a strange star to be less dense than originally thought, although such stars are definitely unusually dense compared to regular stars.
Much of the matter in our Universe may be made of a type of dark matter called weakly interacting massive particles, also known as WIMPs. Although some scientists predict that these hypothetical particles possess many of the necessary properties to account for dark matter, until recently scientists have not been able to make any definite predictions of their mass. In a new study, physicists have derived a limit on the WIMP mass by calculating how these dark matter particles can transform neutron stars into stars made of strange quark matter, or "strange" stars.
WIMPs are thought to be largely located in the halos of galaxies. Although galaxy halos (image above) are not visible, they contain most of a galaxy's mass in the form of the heavy WIMPs.
Dr. M. Angeles Perez-Garcia from the University of Salamanca in Salamanca, Spain, along with Dr. Joseph Silk of the University of Oxford and Dr. Jirina R. Stone of the University of Oxford and the University of Tennessee showed that, when a neutron star gravitationally captures nearby WIMPs, the WIMPs may trigger the conversion of the neutron star into a strange star.
One important issue is whether at high density 'strange' quark matter is more stable than regular matter (which is comprised of 'up' and 'down' quarks). Jaikumar and colleagues think that as a neutron star spins down and its core density increases, it may convert into the more stable state of strange quark matter, forming a strange star.
Theorists cannot say with absolute certainty whether or not a neutron star gradually converts into a strange star. The conversion occurs, according to new research, as a result of the WIMPs seeding the neutron stars with long-lived lumps of strange quark matter, or strangelets. WIMPs captured in the neutron star's core self-annihilate, releasing energy in the process.
According to Jaikumar, making the distinction is rather tricky: "There might be a slight difference. You'd look at surface temperature and see how stars are cooling in time. If it is quark matter, the emission rates are different, so the strange star may cool a little faster."
It's the astronomers' job to discover whether strange stars exist or not. Either discovery will have important implications for the theory of Quantum Chromodynamics (QCD) -- which is the fundamental theory of quarks. "Finding a strange star would improve our understanding of QCD, the fundamental theory of the nuclear force. And it would also be the first solid evidence of stable quark matter", Jaikumar said.
In 2009, Kwong-Sang Cheng of the University of Hong Kong, China, and colleagues have presented evidence that a quark star formed in a bright supernova called SN 1987A (above), which is among the nearest supernovae to have been observed.
Observing a quark star could shed light on what happened just after the Big Bang, because at this time, the Universe was filled with a dense sea of quark matter superheated to a trillion °C. While some groups have claimed to have found candidate quark stars, no discovery has yet been confirmed.
The birth of a neutron star is known to be accompanied by a single burst of neutrinos. But when the team examined data from two neutrino detectors - Kamiokande II in Japan and Irvine-Michigan-Brookhaven in the US - they found that SN 1987A gave off two separate bursts.
"There is a significant time delay between [the bursts recorded by] these two detectors," says Cheng. They believe the first burst was released when a neutron star formed, while the second was triggered seconds later by its collapse into a quark star. The results appeared in The Astrophysical Journal (www.arxiv.org/abs/0902.0653v1).
"This model is intriguing and reasonable," says Yong-Feng Huang of Nanjing University, China. "It can explain many key features of SN 1987A." However, Edward Witten of the Institute for Advanced Study in Princeton, New Jersey, is not convinced. "I hope they're right," he says. "My first reaction, though, is that this is a bit of a long shot."
High-resolution X-ray observatories, due to fly in space in the next decade, may have the final say. Neutron stars and quark stars should look very different at X-ray wavelengths, says Cheng.
The image of SN 1987A at top of the page combines data from NASA's orbiting Chandra X-ray Observatory and the 8-meter Gemini South infrared telescope in Chile, which is funded primarily by the National Science Foundation.
The X-ray light detected by Chandra is colored blue. The infrared light detected by Gemini South is shown as green and red, marking regions of slightly higher and lower-energy infrared, respectively. The core remains of the star that exploded in 1987 is not visible here. The ring is produced by hot gas (largely the X-ray light) and cold dust (largely the infrared light) from the exploded star interacting with the interstellar region. Credit: Gemini/NASA
"Supernova 1987A is changing right before our eyes," said Dr. Eli Dwek, a cosmic dust expert at NASA Goddard Space Flight Center in Greenbelt, Md. For several years Dwek has been following this supernova, named 1987A for the year it was discovered in the Large Magellanic Cloud, a neighboring dwarf galaxy. "What we are seeing now is a milestone in the evolution of a supernova."
The Daily Galaxy via NASA and newscientist.com