For those of us astronomers who’re crazy enough to study the stuff between the stars, there are a few dramatis personae dotted about the sky which are particularly noteworthy. Zeta Ophiuci is one of those stars. So consistent are the interstellar light absorbers between us and Zeta Oph, that it can actually be used as an “ISM standard” — a way of calibrating absorption strengths in the interstellar medium (ISM). Zeta Oph is also quite an interesting star in itself, mind you…

The image you’re seeing is a bow shock. Named after the bow of a ship, and the wake a ship leaves as it travels through the water, this bow shock means that Zeta Oph is plowing through this big cloud of gas and dust. Plowing through at a rate of about 24 kilometres every second*. The image is taken NASA’s WISE instrument in infrared light. All of that dusty looking haze you’re seeing is actually completely invisible in optical light.

Why exactly a star would be travelling at speeds and cutting through an interstellar cloud has to do with its history. Zeta Oph is a hefty star, weighing in at around 20 solar masses. And massive stars like this tend to form together in groups. They also have a nasty habit of exploding as supernovae. It’s most likely that Zeta Oph had a massive companion once upon a time, which did exactly that. Supernovae, being amongst the most powerful events in the Universe, tend to deliver quite a kick to any objects too close to them when they go off. The end result is that even a massive star like Zeta Oph can end up being hurled away into space.

History aside though, bow shocks are interesting things. These big ripples in the interstellar medium glow under infrared light, because the gas and dust contained within them gets all stirred up. Between dust grains smashing into each other at speed, and ultraviolet light from the hot star, they start to become warm. Perhaps this bow shock has something to do with how reliably interstellar absorptions can be seen in this star’s spectrum, who knows?

In any case, it never fails to interest me how even stars which we think we know a lot about can still yield surprises. There’s a lot out there that you would simply never notice until you looked for it…

*That sounds like quite a lot, but it’s not really all that fast as stars go…

Image credit: NASA/JPL-Caltech/UCLA

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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