What Happens When the Winds of Giant Stars Collide?

by Nancy Atkinson on October 12, 2012

Want to stay on top of all the space news? Follow @universetoday on Twitter

XMM-Newton observation of the core of the very massive cluster Cyg OB2 located in the constellation of Cygnus, 4700 light-years from Earth. Credit: ESA/G. Rauw

From an ESA press release:

Two massive stars racing in orbit around each other have had their colliding stellar winds X-rayed for the first time, thanks to the combined efforts of ESA’s XMM-Newton and NASA’s Swift space telescopes. Stellar winds, pushed away from a massive star’s surface by its intense light, can have a profound influence on their environment. In some locations, they may trigger the collapse of surrounding clouds of gas and dust to form new stars. In others, they may blast the clouds away before they have the chance to get started.

Now, XMM-Newton and Swift have found a ‘Rosetta stone’ for such winds in a binary system known as Cyg OB2 #9, located in the Cygnus star-forming region, where the winds from two massive stars orbiting around each other collide at high speeds.

Cyg OB2 #9 remained a puzzle for many years. Its peculiar radio emission could only be explained if the object was not a single star but two, a hypothesis that was confirmed in 2008. At the time of the discovery, however, there was no direct evidence for the winds from the two stars colliding, even though the X-ray signature of such a phenomenon was expected.

This signature could only be found by tracking the stars as they neared the closest point on their 2.4-year orbit around each other, an opportunity that presented itself between June and July 2011.

As the space telescopes looked on, the fierce stellar winds slammed together at speeds of several million kilometres per hour, generating hot plasma at a million degrees which then shone brightly in X-rays.

The telescopes recorded a four-fold increase in energy compared with the normal X-ray emission seen when the stars were further apart on their elliptical orbit.

“This is the first time that we have found clear evidence for colliding winds in this system,” says Yael Nazé of the Université de Liège, Belgium, and lead author of the paper describing the results reported in Astronomy & Astrophysics.

“We only have a few other examples of winds in binary systems crashing together, but this one example can really be considered an archetype for this phenomenon.”

Unlike the handful of other colliding wind systems, the style of the collision in Cyg OB2 #9 remains the same throughout the stars’ orbit, despite the increase in intensity as the two winds meet.

“In other examples the collision is turbulent; the winds of one star might crash onto the other when they are at their closest, causing a sudden drop in X-ray emission,” says Dr Nazé.

“But in the Cyg OB2 #9 system there is no such observation, so we can consider it the first ‘simple’ example that has been discovered – that really is the key to developing better models to help understand the characteristics of these powerful stellar winds. ”

“This particular binary system represents an important stepping stone in our understanding of stellar wind collisions and their associated emissions, and could only be achieved by tracking the two stars orbiting around each other with X-ray telescopes,” adds ESA’s XMM-Newton project scientist Norbert Schartel.

Read the team’s paper: The 2.35 year itch of Cyg OB2 #9 – I. Optical and X-ray monitoring

NASA press release

About 

Nancy Atkinson is Universe Today's Senior Editor. She also works with Astronomy Cast, and is a NASA/JPL Solar System Ambassador.

katesisco October 13, 2012 at 11:53 AM

i had to go to the report and then to look up adiabetic to see that it means a change in temperature without a loss or gain in heat energy. shouldn’t that have been explained in the public blurb?

Richard_Kirk October 13, 2012 at 1:05 PM

I would have not included an explanation if I was writing the article, just as I would not define ‘light-year’ (too simple for an astronomy website), ‘planet’ (not quite as simple, and provokes arguments), or even ‘heliopause’ (not a widely used word but easy to look up). Personally, I have a lot more trouble with ’6 million horsepower’ static rocket tests. It is a difficult balance. If you have ever seen Sir Patrick Moore speak, he is (or was) very good at throwing in explanations for terms like this without disturbing the flow; but that is not easy and he has hd a lifetime getting good at it.

There are other issues here. If you have a gas atom travelling thorough space at a million miles an hour, what temperature is it? A single atom does not really have a temperature. If it is part of a cloud of other atoms that have interacted enough that they are in thermal equilibrium, then you have a temperature, but that temperature may be much lower than the atomic speed would suggest. If your atom was part of such a cloud but had a bit higher energy than the others due to random processes, and has raced into space ahead of the others, and is no longer interacting with them, then it may have a velocity but not a temperature again.

This does not mean we cannot calculate the temperature of gas atoms in space. In general we can, but we have to be quite careful to define we are talking about. Here, ‘adiabatic’ has a precise thermodynamic definition, but it can also be loosely understood as ‘something that happens so quickly that no significant heat escapes’; and the opposite of ‘isothermal’ or ‘something that happens so slowly that everything has had time to reach a uniform temperature’. Real processes lie somewhere between these two extremes.

My real answer to your question is: IMHO ‘adiabatic’ is a good word to know, and you looked it up, so now you do, yay you. Better than a quick gloss of the word in the public text, which would either be too brief to be correct, or too long to be memorable.

Torbjörn Larsson October 13, 2012 at 7:26 PM

To add what can breed (too much) familiarity, having the two idealized processes of adiabatic and isothermal processes in classical thermodynamics means that as soon as you have to apply it to real heat engines you will never, ever forget them. They are the ’0′ (zero) and ‘oo’ (infinity) of the field.

Comments on this entry are closed.

Previous post:

Next post: