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Binary Star Kicks Up Stellar Gale
By Robert Roy Britt
Senior Science Writer
posted: 07:00 am ET
14 November 2000
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*** When a normal star like our Sun teams up with a superdense neutron star, gas and dust gets stripped from one, sucked toward the other, and then slammed into one of the densest object in the universe. Then things get wild. Think 4.5 million miles (7.2 million kilometers) per hour. | About Neutron Stars | | Big stars end their lives in a giant explosion called a supernova. If the star was massive enough and the conditions right, the core implodes, forminga very dense state of matter. So dense that electrons are squeezed into the protons, forming neutrons. A neutron star can pack the mass of 1.4 Suns into an area 7 to 12 miles (11 to 20 kilometers) across. | That's the speed of the stellar wind -- a cosmic stream of charged particles -- emanating from a pair of stars known as Circinus X-1. Although Circinus X-1 was discovered in 1971, much about it remains a mystery because it resides in the densest part of our Milky Way Galaxy, called the galactic plane; where most telescopes can't get a good look at it through all the obscuring gas, dust and stars. But using the orbiting Chandra Observatory, researchers measured the wind in the X-ray spectrum, which is not visible in normal telescopes or to the eye.Making a gale The two stars, known as a binary system, orbit each other every 16.5 days. They are about 20,000 light-years from Earth. One is a normal star, much like our Sun, while the other is an ancient corpse of a star, known as a neutron star. The incredibly dense neutron star sucks the life out of the normal star, stealing gas and dust and pulling it into a flat cloud called an accretion disk. [See click-to-enlarge illustration.] When this matter smacks into the surface of the neutron star, the radiation and rotational force of the star blast some of the material back into space, creating a stellar wind. Our own Sun produces a constant "solar wind," but it comes from the star itself and is nothing compared to the gale coming from Circinus X-1. "The wind is significantly faster than the solar wind and also denser," said Niel Brandt, assistant professor of astronomy and astrophysics at Penn State. The Sun's solar wind is typically around 1 million miles (1.6 million kilometers) per hour, but varies dramatically moment by moment.A surprising find Brandt and a colleague measured what scientists call a P Cygni spectral line. The stellar wind was analyzed by a prism-like device aboard Chandra, which separated the X-rays into their full spectrum -- much like visible light can be separated into all its colors. This information, produced in snapshots each time the binary star system completed an orbit, was then plotted on a graph, which revealed the pulsating P Cygni signature. This signature had been observed before, coming from other star systems, in visible and ultraviolet light. But this was the first time it had been measured in the X-ray spectrum. "It caught us totally by surprise to observe a complex emission structure like a P Cygni profile in high-energy X-rays," said MIT researcher Norbert Schulz. "This detection clearly marks a new area in X-ray astrophysics, where we will be able to study dynamical structures in the universe like we currently do at ultraviolet or optical wavelengths." P Cygni profiles make it easier to study the speed, density and composition of a stellar wind. "The wind coming out of Circinus X-1 is composed of gas that contains highly ionized atoms of silicon, neon, iron, magnesium and sulfur," Brandt said. The researchers said the P Cygni profile looks very similar to the profiles of certain types of quasars, which are distant, extremely powerful galaxies thought to have supermassive black holes at their centers. The neutron star in Circinus X-1, however, is only slightly more massive than our Sun, but is packed into a ball less than 12 miles (19 kilometers) in diameter."Many quasars are known to drive powerful outflows, and these outflows are seen via the P Cygni spectral lines they produce," Brandt told SPACE.com. "We now see a similar outflow from Circinus X-1." The researchers called the finding a possible " microquasar," a term that has been used to describe other luminous, pulsating objects that are far smaller than true quasars. It is not clear what causes the stellar wind to pulsate, the researchers said.The results will appear in Astrophysical Journal Letters and were presented November 8 at a meeting of the High-Energy Astrophysics Division of the American Astronomical Society in Honolulu. Click here for more headlines and information on neutron stars.
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