A fast-moving star is colliding with interstellar gas, creating a spectacular bow shock

Zeta Ophiuchi has had an interesting life. It started out as a typical large star about twenty times more massive than the sun. It spent its days happily orbiting a large companion star until it exploded as a supernova about a million years ago. The explosion ejected Zeta Ophiuchi, so now it’s speeding away through interstellar space. Of course, the supernova also ejected the outer layers of the companion star, so instead of empty space, our plucky star is also speeding through the remaining gas. As they say on Facebook, it’s complicated. And that’s great news for astronomers, as a recent study shows.

Zeta Ophiuchi is most famous for beautiful images like the one above. By passing through interstellar gas, the star has created hot shock waves that glow in everything from infrared to X-rays. The physics of these shock waves is tremendously complex. It is governed by a set of mathematical equations known as magnetohydrodynamics, which describes the behavior of fluid gases and the magnetic fields that surround them. Modeling these equations is pretty bad, but when you have turbulent motion like shock waves, things get even worse. That is why Zeta Ophiuchi is so important. Since we have a great view of its shock wave, we can compare our observations with computer simulations.

In this latest study, the team created computer models that simulated the shock wave near Zeta Ophiuchi. They then compared these models with infrared, visible, and X-ray observations. Their goal is to determine which simulations are the most accurate so that the models can be further refined. Of their three models, two of them predicted that the brightest region of the X-ray emission should be at the edge of the shock wave closest to the star, and this is what we observed. But all three models also predicted that the X-ray emissions should be weaker than what we observe, so none of the models is completely accurate. But these models are difficult to get right, and this work is a good start.

The difference in X-ray brightness is probably due to turbulent motion within the shock wave. The team plans to include some of this turbulent motion in future models. Through multiple iterations, they should be able to create a simulation that closely models this interstellar shock wave.

Magnetohydrodynamics is a central part of many astrophysical processes, ranging from solar flares to the formation of planets, to the powerful black hole engines of quasars. Most of these interactions they’re hidden by distance or dust, so it’s great that Zeta Ophiuchi can give astronomers powerful insight into this complex physics.