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X-Rays Reveal Gigantic Star May Be Twins

One of the Milky Way Galaxy's largest stars may in fact be a double star system, according to recent research by a team of astronomers using NASA's Rossi X-ray Timing Explorer (RXTE) satellite.

The team based its conclusions on unusual variations in the intensity of X-rays emitted by hot gas near the star Eta Carinae about 7,500 lightyears from Earth. They suggest the variations are caused by the presence of a massive companion star in orbit around Eta Carinae.

The new work offers insight into the origin and evolution of a class of stars called luminous blue variables, which are the most massive stars known. Such stars shine so intensely they sometimes become unstable and blow their outer layers off.

During the mid-1800's, Eta Carinae blasted an amount of material equivalent to the mass of our entire Solar System into space. The gas and dust in the material formed a shell that surrounds the star and now blocks it from direct view. The astronomers took what amounted to an X-ray of the shell and found what's inside may really be two stars.

A binary system?

While using RXTE to monitor the X-ray emission from Eta Carinae every week for a period of two years, the team found X-rays emitted by hot gas near the star initially increased over a period of months and then rapidly diminished in intensity in a matter of days. Such variability is highly unusual and has never been observed before for Eta Carinae. One simple explanation may be that the variability of the X-ray energy coming from Eta Carinae is due to the presence of a massive stellar companion orbiting the star. They could be bound to each other by the force of gravity.

The presence of such a companion had been suggested by other astronomers, however the presence of a companion star is controversial since the spectrum -- a means of measuring the properties of objects by splitting their light into its component colors -- of Eta Carinae is notoriously variable, and since the spectral features originate in a very complex medium. As a result, the suggestion that Eta Carinae is a binary star has not been generally accepted by the astronomical community. The X-ray variations may change that.

Stellar shock wave

The research team said the orbit of the companion star is an ellipse which moves it closer to and further away from Eta Carinae over a five-and-a-half year period. When the stars are close, the two stellar winds slam together creating a shock wave that heats the gas to about 60 million degrees and emits large amounts of X-rays. When they are farther apart, the shock wave diminishes along with the X-rays.

The spacecraft's data may have resolved one mystery, but at the same time the data has opened another. Strange peaks in the intensity of X-rays from Eta Carinae seem to occur every 85 days. While the first peaks detected were relatively weak, their strength rose as the X-ray emission from Eta Carinae has brightened.

The puzzle

The puzzle is what causes the 85 day X-ray period. It may be the rotation of the star, or the star may pulsate in that time, or it might even be the orbit period of a third object in the system, a possibility that makes some astronomers uncomfortable.

With at least 50 times more mass than our Sun, luminous blue variable stars like Eta Carinae are the most massive known. If Eta Carinae is a double star system, each star is estimated to be 70 times more massive than the Sun.

Rossi X-ray Timing Explorer

The Rossi X-ray Timing Explorer is a NASA Goddard Space Flight Center satellite launched to earth orbit in 1995. RXTE is designed to study time variability of X-rays from sources with moderate spectral resolution. Time scales from microseconds to months are covered. RXTE was designed and built by the Engineering Directorate at GSFC for a lifetime of two years, with a goal of five years. RXTE into is in a low-earth circular orbit at an altitude of about 360 miles where it circles Earth about every 90 minutes.

Images from from high-energy astrophysics satellites:
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