Weizmann Institute Astrophysicist Hot on the Tracks of a New "Strange" Star

REHOVOT, Israel - January 12, 1998 - A new "strange" star is suspected to be lurking in our galaxy - and a Weizmann Institute astrophysicist is hot on its tracks.

In research published in the January 12 issue of Physical Review Letters, Prof. Vladimir Usov of the Institute's Condensed Matter Physics Department outlines the last of three characteristics that may enable astronomers to finally identify examples of strange stars, whose existence was predicted nearly 15 years ago. The first two characteristics were described by Usov in the June 1, 1997 issue of The Astrophysical Journal Letters.

"Only a handful of the heavenly bodies observed in our galaxy are probably strange stars, but they would constitute a totally new class of celestial objects with extraordinary properties," Usov says.

Usov's three unique features would distinguish the strange star from the more common neutron star, which is outwardly very similar.

There are approximately 1,000 observed objects classed as neutron stars in our galaxy, but, according to Usov, as many as one percent of these may in fact be strange stars. So maybe some of the 20 or so enigmatic bodies which astronomers believe may be black holes.

A champion of density and stability

The positive identification of these shadowy celestial denizens would settle an important question for particle physics, concerning the existence of matter made up of quarks - the most stable matter in the universe.The existence of such matter was posited in 1984 by Prof. Edward Witten of the Institute for Advanced Study in Princeton, and its crucial component is the strange quark - one of six types of quarks, the tiniest building blocks of matter.
However, quarks do not normally exist as separate entities, and it's only possible to get a brief glimpse of their existence in high-energy particle accelerators.Therefore, strange stars - so called because they are believed to consist almost entirely of strange quarks - are scientists' only chance to observe a sizable and stable "chunk" of quark matter."The discovery of a strange star would prove that quark matter can indeed exist," Usov says.

Before Witten proposed his quark matter theory, it was assumed that under the extremely high pressure existing in some astronomical objects, matter made up of neutrons, which have no electric charge, was the most stable of any matter type. This stability led scientists to believe that stars made of neutrons, which measure about 20 kilometers in diameter, were the final stage in the evolution of a "massive" star (whose mass is at least 1.4 times greater than that of the sun). Neutron stars form when massive stars collapse, and because they are exceedingly stable, they are highly unlikely to collapse further.

However, the theoretical strange stars would represent an even further stage in stellar evolution: according to Usov, when the core of a neutron star is sufficiently dense, neutron matter can be converted into quark matter.Both neutron and strange stars are not only extremely stable but also improbably dense: one cubic centimeter of strange quark matter would weigh about 1 billion tons.

The three criteria

Although neutron and strange stars are similar in size and density, Usov used theoretical calculations to search for unique behaviors that would set a quark star apart from its neutron "cousin."

These three unique behaviors are as follows:

First, the energy of X-rays emitted by a strange star is about 10 to 100 times greater than that of X-rays emitted by a neutron star.

Secondly, the X-rays emitted by strange stars are fired in pulses, each lasting around 1 millisecond.

Finally, the strange star, while comprising mostly quarks, also contains a small quantity of electrons. As negatively-charged electrons try to escape from the star, a very strong electric field is created over its surface. This electric field causes spontaneous creation of pairs consisting of electrons and their positively charged counterparts, called positrons. The electrons and positrons can annihilate each other when they meet, leading to the release of high-energy gamma radiation. This so-called annihilation gamma-ray emission can be detected by astronomers.

"If a compact object fits these three criteria, chances are high that you've found a strange star," Usov concludes.

A prime candidate

According to Usov, one likely candidate for a strange star has already been detected by astronomers. It's a mysterious X-ray source located near the galactic center, referred to by scientists as 1E1740.7-2942.

The powerful source is currently believed to be a black hole, but Usov suggests it may be a strange star because it meets all three of his criteria.

Usov is now working on a further refinement of the strange star's profile in order to enable astronomers to identify other potential candidates in the cosmos.

The Weizmann Institute of Science is a major center of scientific research and graduate study located in Rehovot, Israel.