Telltale Shock Waves from Runaway Stars

Astronomers are now finding dozens of fast-moving runaway stars by searching for the bow shocks they create in interstellar gas and dust.

Zeta Ophiuchi's infrared shock wave

Zeta Ophiuchi, the blue star in the center of the image, and its accompanying bow shock, as seen by the WISE space telescope.
NASA / JPL-Caltech / UCLA

The apparent stillness of the night sky is deceiving. In reality, some stars zip through the cosmos quiet fast — at least 30 kilometers per second relative to their surroundings. These speedsters tend to be moving in isolated regions of the galaxy, though their paths often lead directly away from stellar clusters or even the supermassive black hole at the center of the galaxy.

Through pure luck, astronomers have discovered more than 20 so-called runaway stars. But there is strength in numbers. If astronomers could systematically search for — and therefore discover more — runaway stars, they might be able to better understand what boosted those stars to such high velocities in the first place.

Enter Zeta Ophiuchi, a hot, massive O star traveling around the galaxy at a distance from us of about 366 light-years. Its relative motion is fast enough, roughly 24 km/s (54,000 miles per hour), to be "supersonic." As this massive star plows through space, its strong outflowing wind causes interstellar gas and dust to stack up in front of it, like water that piles ahead of a ship or air that piles ahead of a supersonic jet. This arc-shaped gas then compresses, heats up, and shines with infrared light — a direct signature of the mass and speed of the star coursing through it.

With this example in hand, a team of astronomers decided to probe bow shocks in search for runaway stars. They turned to archival infrared data from the Spitzer and WISE space observatories and found more than 200 images of fuzzy red arcs such as the examples below. They then used the Wyoming Infrared Observatory — a 2.3-meter telescope perched atop Mount Jelm — to search for the culprits behind the shocks. The team was surprised to find that “more than 95% of these bow shocks have a hot, massive, runaway candidate at their center,” said William Chick, a graduate student at the University of Wyoming, when he presented the results at the American Astronomical Society meeting in Kissimmee, Florida. “It may be that our Milky Way is swarming with these hot runaway stars.”

Infrared shock waves around runaway stars

Three cosmic bow shocks and their accompanying runaway stars found in Spitzer and WISE data. (The image on the far-right actually contains two runaway stars and two bow shocks.)
NASA / JPL-Caltech / University of Wyoming

Moreover, he added, “It appears that bow-shock nebulae are an extremely efficient method at locating these hot runaway stars.” His team plans to extend its search to include the entire galactic plane. With more runaways spotted, the researchers will be able to trace the stars’ motions backward in order to find the source of their accelerating kicks.

Two mechanisms have been offered to fling a star into the void with that much speed. If a binary star falls close to the supermassive black hole at the Milky Way’s center, then it can be disrupted such that one star remains trapped around the black hole and the other gets slung away rapidly. Alternatively, a supernova in a binary system might free the companion, flinging it outward into the galaxy. Chick suspects that both mechanisms likely play a role.

5 thoughts on “Telltale Shock Waves from Runaway Stars

  1. Anthony BarreiroAnthony Barreiro

    What is the speed of sound in an interstellar medium? Since sound travels faster in solids than liquids, and faster in liquids than gases, I expect the speed of sound would be quite slow in a highly rarefied medium.

    1. Tom-Fleming

      Two thoughts….. The O types hurtled from the vicinity of the SMB have to be close to it since their Main Sequence lifetime is so short. Other O types being sponsored by launches from binary systems should be widely scattered. Do the data so far obtained support these suppositions?
      High speed launches should not be limited to the O class. They’re just easier to find. Are other spectral classes being identified with associated bow shocks?

    2. Corey Rueckheim

      I too want to hear more about what “supersonic” speed means in space. Isn’t it true that sound simply won’t travel in these rarefied areas because there just isn’t enough matter to transmit it? Maybe the word “supersonic” is being used as an analogy here rather than as an exact description of what is really happening? In the flight of a jet plane, as the jet travels faster and faster, doesn’t the air piled up in front of the jet get denser and denser, and get hotter and hotter due to compression? Even after passing the speed of sound, the air continues to get denser and denser and to heat up – even causing a fiery re-entry for orbital vehicles re-entering the earth’s atmosphere. Isn’t it this “snowplow” effect that’s causing the fuzzy red arcs mentioned in the article, rather than shock-waves caused by sound?

    3. Peter WilsonPeter Wilson

      Speed of sound in a gas is the average velocity of its atoms, no matter how highly rarefied. In a sense, “nothing happens,” until the speed of sound is exceeded, then a shock-wave develops. It is an extremely tenuous “shock;” you would not hear it if you were in a space-craft and it hit. Nonetheless the “shock-wave” causes ionization, and it glows to the extent that it can be seen in long-exposure photos.

    4. Portermac

      The speed of sound in any medium, solid, liquid, or gas, is the speed of a compression wave of the atoms within the material, rather than the speed of the atoms themselves, although the two are correlated in a gas (speed of atoms = temperature). Turns out the speed is largely independent of pressure, at least as far as the ideal gas model is concerned (see link below). Speed depends on the size and temperatures of the atoms, and the frequency (wavelength) depends on the mean free path, which is very large in space. That implies the pitch of the sound is much lower than audible, so even though we call it “sound,” it’s really not something we could actually hear.

      Speed of Sound in Air

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