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The stellar wind reveals its secrets
1/25/13

Massive stars are the masters of the galaxy.  At least ten times bigger than the sun, four times hotter and a million times brighter, they are the architects of the stellar universe. Although little is known about them, Yaël Nazé has just completed a piece of the puzzle. Her discovery could be described as a “Rosetta Stone ” which should contribute to a better understanding of stellar winds emitted by these stars and which constantly eject matter into space. Her discovery has just been published in the journal Astronomy and Astrophysics.

It is known as Cyg. Cyg OB2#9, to give it its full name. Could it be a barbarian name? In fact this stellar system has a somewhat manageable name. Some of its “neighbors” out there in the universe have inherited much more difficult names : HD 269810, LBV 1806-20, LH54-425 A + B, IRAS 05423-7120, NGC 3603 A1…  2MASS J00060337+63404(EN)-Cyg-OB267  1RXS J163352.2-480643, and so on and so forth.

As its scholarly name suggests (for those who can read between the lines), CYG OB#9 is part of the Swan constellation, Cygnus in Latin.  Anecdotally, it owes its name to its cross-like shape which the astronomer Ptolemy saw as the outline of a swan. The letters “OB” refer to the main stars of the group (according to their spectral type) while “#9” quite simply signifies that it is the ninth star of this group. 

For a long time, Cyg OB2#9 was just an ordinary massive star, neither more remarkable nor interesting than the others and thus did not attract the attention of researchers. Like all stars in this category, it is ten times bigger than the sun. It is also a lot hotter: 40 000 degrees Celsius on its surface, whereas the temperature on the surface of our “fire-star” does not exceed 5 500 °C. Consequently it is a million times brighter than our sun. However, it would be futile to try and detect it in the night sky; this group of stars is situated at some 4 000 light years from the Earth and can only be observed with the aid of powerful telescopes.

Finally, like most massive stars, Cyg OB2#9 actually is... a couple of stars. The two stars that comprise it (one being slightly bigger than the other) turn around each other and, unless there is a separation which is an unlikely event in the galaxy, they should spend their lives together. Yet, even though the researchers suspected this to be the case, they were only able to confirm it in 2008 following a long period of observations directed by the astrophysicist Yaël Nazé, a researcher at the FNRS with GAPHE (High Energy Astrophysics group ) of the University of Liege.

Collision or no collision?

At the time, this discovery was a first step towards a better knowledge of the stellar system. However some questions still remained, particularly in terms of the collision between its stellar winds. Massive stars constantly lose matter during the entire course of their existence and this matter is ejected into space. It is as though they are being progressively “peeled”. These winds are extremely powerful: they can reach speeds of several thousands of kilometers per second. By way of comparison, the solar wind ejects more than ten million times less matter and is ten times slower on average!

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