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Canada's Space Scope Cracks Open A Massive Star

Hubble image of WR124 : WR124 is a close cousin of WR123. This Hubble Space Telescope image in the light of excited hydrogen gas reveals the gas being ejected at high speed from the central star (overexposed and blotted out by a white dot) as it catches up and collides with the slower wind from a previous phase. Although both stars are almost identical, WR123 must be slightly older, having already dissipated its colliding gas. Credit: Yves Grosdidier (Universite de Montreal and Observatoire de Strasbourg), Anthony Moffat (Universite de Montreal), Gilles Joncas (Universite Laval), Agnes Acker (Observatoire de Strasbourg), and NASA.
Montreal QC (SPX) May 18, 2005
The MOST space telescope has given astronomers new clues about an exotic star, at least ten times more massive than our Sun, spewing gas into space at a rate of more than 100 trillion tonnes per second.

And according to results presented today at the Canadian Astronomical Society meeting in Montreal, Canada, the star - with the misleadingly bland name of WR123 - is even weirder than astronomers ever suspected.

The new findings, by Laure Lefevre and Anthony Moffat (Universite de Montreal), Sergey Marchenko (Western Kentucky University) and the international MOST science team, are based on over five weeks of non-stop monitoring of the light variations of WR123.

WR123 is a member of the relatively rare class of Wolf-Rayet stars (named after two French astronomers who discovered their telltale strong plasma winds using a simple spectroscope in the Paris suburbs in 1867, ironically the same year Canada became a nation).

Wolf-Rayet stars like WR 123 have long been known to exhibit complex - seemingly chaotic - brightness variations associated with the turbulent high-speed winds they eject into space.

But the nearly continuous coverage possible with the MOST (Microvariability & Oscillations of Stars) satellite has revealed a clock in the chaos - a stable variation repeating every 10 hours (see figure illustrating WR123 variations).

"Finding a clock in a star like WR123 is like finding the Rosetta stone for astronomers studying massive stars," explained Ms. Lefevre, a PhD student at the Universite de Montreal.

"However, although WR123 may vary like clockwork, it must be a very strange mechanism indeed."

The only theories to explain the 10-hour clock in WR123 would be:

(1) the rotation of the star itself, (2) the orbit of another small star around WR123, or (3) vibrations in the structure of WR123 that are transmitted to its dense enveloping wind.

All of these ideas are equally strange.

If WR123 is spinning at that rate, the surface would be moving so fast (about 2000 kilometres per *second*, or over 7 million kph) that the star should throw itself apart, unless that is the actual source of the wind!

If the star is in a tight binary system, it's so tight that its companion would be orbiting inside the star itself. If pulsations are the right answer, theoreticians will have to completely revise their current understanding of this class of massive stars.

The same period was hinted at in spectroscopic data obtained from an Earthbound observatory a year earlier, but the MOST results leave little doubt as to the bizarre timing of this stellar clock.

One hundred times fainter than what the unaided eye can see, WR123 is located about 19,000 light-years from Earth, in the direction of the constellation Aquila ("the Eagle").

WR123 and other similar Wolf-Rayets (see the Hubble image of WR124) are believed to have had very violent births, ejected by a supernova explosion in a binary system, or by a gravitational slingshot from a dense star cluster.

"Either way, WR123 was probably kicked out from the nest rather abruptly," jokes Dr. Moffat, who helped develop these formation theories in the late 1970's.

Stars that start off their lives with ten or more times the Sun's mass are capable of "burning" hydrogen into helium, helium into carbon, and so on up to the final nuclear ash, iron, before the iron-rich core collapses on itself in less than a second and produces the greatest of all stellar explosions, a supernova.

Since H-burning lasts by far the longest, some 90% of stars that shine are actually consuming hydrogen in their cores at a prodigious rate.

Then, somewhat under 10% of stars are in the next stage, that of He-burning, while only a miniscule fraction occurs in the subsequent, ever-faster evolving stages. WR123 represents the fleeting final stages of helium-burning, before the rapid death-spiral to supernova.

The gases ejected from stars like WR123 will enrich the interstellar medium, and contribute to future generations of stars. Understanding such stars is vital if we are to properly understand the evolution of the Milky Way and other galaxies.

"We may be seeing an example of one of the key stages in the stellar lifecycle that led to the Sun, Earth, and us, being here," noted Ms. Lefevre.

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Canadian Space Telescope Detects Puzzling Brightness Variations In Dying Star
Montreal QC (SPX) May 17, 2005
Astronomers presented on Sunday new results from the Canadian MOST ('Microvariability and Oscillations of Stars') satellite at the Canadian Astronomical Society Meeting held at the Universite de Montreal.


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