Not all stars are happy to stick around, orbiting the galactic center with all the other stars. Some stars go rogue, thrown through the Milky Way with significant force. These are the runaway stars, and we can trace their trajectories to understand the violent events that can take place in the Universe.
One of these stars, and one of the most famous, is Zeta Ophiuchi. Located about 440 light-years from Earth in the equatorial constellation Ophiuchus, it is also one of the strangest stars in the sky.
Not only is it unusually fast, with a speed of 30 to 40 kilometers (roughly 20 to 25 miles) per second, but it’s an odd kind of star to watch streaking through space.
Zeta Ophiuchi is a main sequence star; that is, one that is still fusing hydrogen into helium at its core. And it is a hot, massive O-type star: about 20 times the mass of the Sun, glowing blue with its intense heat.
These stars also have relatively short lives; Zeta Ophiuchi is about halfway through its projected main sequence lifetime of 8 million years.
This means that they are not very common in the Milky Way; but these stars are also usually born, and spend their lives, in groups known as associations.
Zooming through space, however, Zeta Ophiuchi is all alone, raising questions about where it came from and how it got to its current state.
Scientists now believe that Zeta Ophiuchi was hurled through space by the supernova explosion of a companion binary star. A pulsar, also zooming through space, has a path that would have crossed that of Zeta Ophiuchi a few million years ago.
This suggests that the pulsar was the star that went supernova, sending both stars flying.
(NASA/CXC/Univ. of Cambridge/J. Sisk-Reynés et al.; NSF/NRAO/VLA; PanSTARRS)
Above: A composite optical, infrared and X-ray image of Zeta Ophiuchi.
Because Zeta Ophiuchi is so well known, we know quite a bit about it. For example, the images show a colossal arc shock in the thick cloud through which the star is traveling. This is created by material blowing off the star and colliding with the gas.
And X-ray emission around the star was detected in Chandra observations in 2016: thermal emission, created by shock-induced heating.
A new study led by computational astrophysicist Samuel Green of the Institute for Advanced Study in Dublin, Ireland, has delved into the data at various wavelengths to see if bow shock dynamics can explain the observed cloud, as well as thermal emission. This includes observations across optical, infrared, radio and X-ray wavelengths.
They ran simulations and found that their results did not match observations. The brightest X-ray in the Chandra data is emitted from a bubble surrounding the star. In the simulations, the brightest X-rays were in the bow shock itself.
This suggests that something is missing in the simulations or in our understanding of the strange star and its environment.
Future simulations will introduce more physical processes into the mix or be performed at higher resolutions to better model the turbulence involved.
As for other remarkably fast stars, the fastest runaway main-sequence star discovered so far is S5-HVS1, thrown across the galaxy at about 1,700 kilometers (about 1,056 miles) per second by an interaction with Sagittarius A*, the galaxy in our galaxy. supermassive black hole.
The fastest dead stars are a pair of white dwarfs traveling at 2,200 kilometers (about 1,370 miles) per second, powered by a double-detonation supernova.
The fastest star identified so far in our galaxy is S4714, which reaches speeds of 24,000 kilometers (14,900 miles) per second as it orbits Sgr A*
The team’s paper has been accepted into Astronomy and Astrophysics. A large version of the composite optical, X-ray, and infrared observations of Zeta Ophiuchi can be found on the Chandra website.