Invading Stars Faster Than Speeding Bullet


A team of astronomers looking for pre-planetary nebulae using the Hubble Space Telescope instead came across some renegade stars screaming through space. These runaway stars are moving along at 50 km/s (112,000 miles an hour), and have traveled an estimated 160 light years from where they originated. “We think we have found a new class of bright, high-velocity stellar interlopers,” says astronomer Raghvendra Sahai from JPL. “Finding these stars is a complete surprise because we were not looking for them. When I first saw the images, I said ‘Wow. This is like a bullet speeding through the interstellar medium.’ Hubble’s sharp ‘eye’ reveals the structure and shape of these bow shocks.”

Just as a speedboat on a lake creates a wake, as these speedy stars plow through the interstellar “ocean” they create brilliant bow shocks as streams of matter flowing from the stars slam into the surrounding dense gas.

So far, 14 of these runaway stars have been found. What kind of stars are they? Astronomers can only estimate the ages, masses, and velocities of these renegade stars. The stars appear to be young — just millions of years old. Their ages are inferred from their strong stellar winds. Most star produce such winds either when they are very young or when they are dying, and Sahia said it is clear they are not dying. Massive dying stars produce flowing clouds of ionized gas around them and that type of gas is not around these interlopers and old stars are almost never found near dense interstellar clouds.

They appear to be medium-sized stars that are a few to eight times more massive than the sun.
The research team believes the interloper stars were dynamically ejected from their natal environments which were probably massive star clusters. There are two way this could have happened. One possible scenario is that a star in a binary system exploded as a supernova and the partner got kicked out. Another possibility is a collision between two binary star systems or a binary system and a third star. One or more of these stars could have picked up energy from the interaction and escaped the cluster.

Depending on their distance from Earth, the bullet-nosed bow shocks could be 100 billion to a trillion miles wide (the equivalent of 17 to 170 solar system diameters, measured out to Neptune’s orbit). The bow shocks indicate that the stars are traveling fast, more than 112,000 miles an hour (more than 180,000 kilometers an hour) with respect to the dense gas they are plowing through, which is roughly five times faster than typical young stars.

Runaway stars have been seen before. The Infrared Astronomical Satellite (IRAS), which performed an all-sky infrared survey in 1983, spied a few similar-looking objects. The first observation of these objects was in the late 1980s. But those stars produced much larger bow shocks than the stars in the Hubble study, suggesting that they are more massive stars with more powerful stellar winds.

“The stars in our study are likely the lower-mass and/or lower-speed counterparts to the massive stars with bow shocks detected by IRAS,” Sahai explains. “We think the massive runaway stars observed before were just the tip of the iceberg. The stars seen with Hubble may represent the bulk of the population, both because many more lower-mass stars inhabit the universe than higher-mass stars, and because a much larger number are subject to modest speed kicks.”

Astronomers have not spotted many of these stellar interlopers before because they are hard to find. “You don’t know where to look for them because you cannot predict where they will be,” Sahai says. “So all of them have been found serendipitously, including the 14 stars we found with Hubble.”

“One of the questions that these very showy encounters raise is what effect they have on the clouds,” says team member Mark Morris of the University of California, Los Angeles. “Is it an insignificant flash in the pan, or do the strong winds from these stars stir up the clouds and thereby slow down their evolution toward forming another generation of stars?”

Sahai and his team used Hubble’s Advanced Camera for Surveys to examine 35 objects that appeared as bright infrared sources in the IRAS archive. The team is planning follow-up studies to search for more interlopers, as well as study selected objects from this Hubble survey in greater detail in order to understand their effects on their environment.

Source: NASA, AAS Press Conference

15 Replies to “Invading Stars Faster Than Speeding Bullet”

  1. Don’t wish to be rude, but goodness they look like exactly like some kind of swimming cosmic spermatozoa!

    As for ” One possible scenario is that a star in a binary system exploded as a supernova and the partner got kicked out. Another possibility is a collision between two binary star systems or a binary system and a third star. One or more of these stars could have picked up energy from the interaction and escaped the cluster.”

    Of course, another possibility theory is that the stars were once in a very young trapezia or quadruple multiple star system, (like theta 1 orionis in the heart of the Orion Nebula), and one was then ejected from the tussle for gravitational dominance. This creates a “run away” star and leaves a stable triple star – like a closer binary with a fainter/ smaller distance companion. This has the bonus of happening near a star formation region, and the improved possibility of interstellar material to interact with.

    Again, thanks for the interesting report from the AAS conference.

  2. G’Day Feenix

    There re many references to what you want to know.
    I’d suggest you look at and article at the NASA/IPAC Extragalactic Database. Here you find a nicely written article on some of the questions you ask. This is John Hurcha of the Harvard-Smithsonian Centre of Astrophysics (how absolutely appropriate). This can be found at;
    Extragalactic Redshifts
    Slightly confusing is the various distances calculations, as what is relative to what is not very easy to state simply. You can see this yourself at; NASA/IPAC EXTRAGALACTIC DATABASE : Search for Objects by Object Name
    Input an NGC galaxy. I.e. NGC 253 in Sculptor and see the list yourself.

    Hope this helps.

  3. the quality of the articles has been excellent since they all went to the conference. their minds must be in a highly excited state.
    mind you, i think nancy is holding the fort back at home 🙂

  4. Mr. Too Obvious
    Velocity, you mean velocity, not speed. I.e. The speed of something in a given direction.
    If you need a lesson, I do suggest you read The Physics Classroom It might help.

  5. A question springs to mind – I’ve often wondered, and I cannot find an answer via Google:

    Relative to _what_ do astronomer measure the speed at which they tell us stars and galaxies move through space? Is it the centre of our own galaxy? If somebody knows – I’m curious…

  6. Feenixx
    The moderator? deleted one of my post as I think it had two links.
    Suggest you go to the NASA/IPAS Extragalactic Database, which has lots of information.
    Go to links;
    Literature>Knowledgebase (Level 5)>Table of Contents
    Here you will find your questions answered under;
    >Cosmology then the sub-menu Basics
    I sure most of you question can be found.

  7. Feenix
    Actually Wikipedia has an article that might help you. Read the wiki on the Milky Way, and read the section “Velocity.”
    Although the summary is fairly simplistic, it might satisfy your curiosity.

  8. Speed is calculated via the typical RT=D. If you know the distance traveled within a specified amount of time, you can easily figure out the speed of an object. This is normally relative to the object itself.

    The two (or more points) are typically triangulated from other known objects/locations in order to increase accuracy. Sounds complicated, but it really isn’t…. unless the object is picking up speed or bleeding it off; a key it may be orbiting something… That is another lesson entirely!

    Keep in mind most speeds are not absolute, but approximations, (calculating distance between points when they are far away isn’t easy or perfect; but does get better with longer observations) and rounded off.

  9. thanks, Salacious BC, I think I got it – for galaxies, motion is relative either to the Hubble Flow or to CMB….
    Actually, I found the article about the Hubble Flow really good. The authors of the articles I read don’t really tell which of those two frames they observe from, but the results seem close enough together for what matters here and now… and yes, it’s velocity, not speed.

    Simplistic? Ah, well… see, physics is only a hobby for me.

  10. Here’s another question for some smartypants to answer:

    When we speak of dense gas clouds what does it mean?

    I know that density is defined as weight or mass per unit volume but I don’t understand why they never say what the relative density is.

    They’ll always say how massive an object is in terms of so many solar masses or Jupiter masses.

    They’ll tell you how fast an object is moving, how large it is, how bright, how distant, how energetic, etc. and always with some standard of measurement or scale.

    Most articles that discuss interactions between large structures of gas and dust with stars, galaxies, galaxy clusters and so forth refer to the density of the gas clouds or dust but they neglect to say what it means relative to the vacuum of space.

    Heck, Earth’s atmosphere is “super dense” compared to a vacuum!

    The only time they define it is when they say that a teaspoon of black hole matter would weigh as much as the earth, and then it’s mostly for the wow factor.

    Oh well, it wasn’t meant to be a rant. I’d appreciate if someone could give me a frame of reference for the “super dense” gas near the galactic core and maybe a link to a primer on the subject.


  11. Runaway stars are probably quite common. Many star clusters are tightly packed and the limited space would mean that a star would be ejected. Best example is AE Aurigae, ejected from the Trapezium in the Orion Nebula and crashed into a gas cloud, which is the Flaming Star Nebula!

  12. My first thought when reading this article are the well known ‘runaway’ stars AE Aurigae (mentioned by Sakib above), Mu Columbae & 53 Arietis known to astronomers since the 1950’s. The interesting aspect of this trio of stars is the radial motion of all three leads back to the Orion Nebula and are presumed to have been ejected from the forming cluster at its center, similar to what is proposed for these new ‘runaway’ stars. Pages 283-289 in Burnham’s Celestial Handbook give a excellent review of the three Orion runaway stars with an included diagram tracing their past movements. ‘Runaway stars’ are probably more prevalent in our galaxy, but conditions must be right to pick out these celestial interlopers with any certainty. Who knows, these stars may play key roles in the evaporation of clusters and stellar associations.

  13. I wonder would it be possible for life to develop on a planet that orbits a runaway star?

    Imagine then if the stars velocity was greater than the galactic escape velocity.

    How unfortunite for any intelligent beings to be faced with that, no nearby stars, you could be thousands of lights years away from the nearest star.

    The sky at night would dark except for the huge galaxy in the sky, kinda like the end of the second star wars film.

    Anyway thats what I think of, when someone mentions runaway stars.

  14. What are the chances one of these rouge, speeding stars could come near our solar system?

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