Galaxy forms raptured stars: Left Behind

By Phil Plait | September 20, 2007 1:31 pm

Astronomers have found a galaxy forming stars in a long tail behind it. The stars are being left behind as the galaxy plows through a galaxy cluster.

Coooool. The image is a composite of an X-ray observation (purple, from Chandra), an image that detects hydrogen gas (red) and an image of plain ol’ optical light (white; red and white from the Southern Astrophysical Research telescope in Chile). The galaxy is a member of a cluster of galaxies called Abell 3627. The galaxies in the cluster all orbit the cluster center of mass, usually at pretty high speed. The cluster has lots of hot gas in between the galaxies, and as they move through it they ram into it. Any gas in the galaxy itself can get stripped off by this ram pressure, where it will merge with the gas between the galaxies.

Any given galaxy plowing through all that crap will leave behind a tail of gas as does. The galaxy in question — poetically named ESO 137-001 — is doing just that. The purple in the image shows the gas being heated to millions of degrees by shock waves as the galaxy does the Superman thing through the cluster (in fact, I see what looks a little like a bow front off the galaxy in white light, which is interesting, but may not be real). The tail is about 200,000 light years long– twice the size of our own Milky Way Galaxy.

But what happens to the gas as it’s stripped off? The pressure from the collision with the intergalactic gas will compress it, and when you compress gas, you get stars forming. The galaxy is too far away (220 million light years) to detect individual stars directly. However, as massive stars form from the cooling gas they ionize it, and that can easily be detected: the red color shows that. There are 29 detected areas where the cooler gas is ionized, and these indicate sites of star formation.

We know that these regions are not just hot gas because they are clumped; the superhot X-ray emitting gas is more evenly distributed, and you wouldn’t expect the kind of emission seen in red by that kind of gas.

This means that the galaxy is actively forming stars, but they’re orphans! They aren’t gravitationally bound to the galaxy. Over time, the stars formed will move apart, and are doomed to wander the space between galaxies in the cluster. It’s pretty lonely out there, with gaps of hundreds of thousands of light years between galaxies (the nearest star to the Sun is just over 4 light years away).

If planets form around those stars — and there’s no reason I can think of that that wouldn’t happen — then the view would be amazing: hundreds, maybe thousands of galaxies in the sky, glowing like ghosts, apparitions in an otherwise smooth black vista, unbroken by any other stars. If there are a few other stars nearby, perhaps siblings, then that would only punctuate the emptiness of the region.

That would be pretty cool to see, but I think I like our sky better. We get to see galaxies easily enough (like Andromeda, or the Magellanic Clouds, all of which are relatively easy to spot with the unaided eye) and we get stars too. Things are more crowded here, but there’s still plenty of room to breathe.


Comments (18)

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  1. In Which We Go Back To Middle School « This Recording | April 22, 2008
  1. MichaelS

    Your title is a little off: the galaxy would have to turn around and go through the tail, picking up some stars and leaving others behind. The ones getting picked up would be “raptured”.

    As for the stars left behind, I’m guessing the galaxy is part of a cluster or supercluster or something, and the stars will start orbiting the center of that mass (whatever the parent galaxy was orbiting), but since they got slowed by the friction, they’d be on a rather different orbit.

    At the distance and speed they’re at, would they have a chance to rejoin another galaxy, or will they just burn up in the middle of nowhere before anything exciting happens?

  2. MichaelS

    Helps if I read the article you linked to: the parent galaxy is “plunging into [the Abell] cluster”. So I guess the stars have better odds of bumping through another galaxy, since they’re already headed in that general direction.

  3. ChrisW

    That’s pretty cool, but I definitely wouldn’t want to be orbiting some vagrant star lost in the reaches of intergalactic space …

  4. Quiet_Desperation

    Wait… I live in a largish city. What are these “stars” of which you speak?

  5. Michael H

    If the disenlightened can steal and misuse our technobabble it’s only fair we can steal theirs.

  6. Phil, I thought you didn’t believe in the rapture.

    I’m going to cry.


  7. Greg B


    When you’re done with the book you’re currently writing you really should consider writing another book with a collection of your cool astronomy photo blogs.

    I’m thinking of the kinds of books that your man-crush Wil Wheaton does where he re-writes his best blog entries and publishes them in book form. (BTW, “Man-crush” is your term :) and I kind of agree. Wil’s cool)

    Seriously, you have tons of material here on your blog. Imagine, pretty pictures AND cool science commentary. Always a good combination for a book.

  8. occam's comic

    Gas at millions of degrees????
    Bull, you can’t have a gas at a million degrees!!!
    You can have plasma at that temperature but not gas. Plasma does not behave like a gas, you must deal with moving electric charges and the interactions between the moving electric charges and the magnetic fields that they create.

    We are stuck with truly bad astronomy because astronomers don’t understand (or can’t incorporate) basic scientific understanding of the structure of matter.

  9. Buzz Parsec


    I’m not absolutely sure of that. Under “normal” conditions, a plasma will stay a plasma (the electons won’t recombine with the ions) because the mean time to the next (energetic) collision is much less than the mean time to recombine with an electron. And any collision in a plasma that hot has more than enough energy to re-ionize any atoms that have become deionized. But the ISM is different, because the density is slow low. The mean time to collision can be hundreds of years. (But maybe at this low density, the mean time to recombine with an electron is also proportionately long, and it cancels out.) Anyway, at extremely low densities, very wierd things can happen.
    (E.G. “forbidden” spectral lines.)

    Astrophysicists do know all about plasmas… So this could be just a case of dumbing down – use “gas” instead of “plasma” in the press release because everyone knows what a “gas” is. This sounds like an opportunity for BA to educate us :-)

    P.S. the red is hydrogen in the picture… presumably, H-alpha or H-beta, which are visible, though much less intense than the Lyman lines would be in a partially ionized Hydrogen cloud. However the Lyman lines are all in the ultraviolet, so would be invisible to ground-based telescopes. Spitzer images, any one?

  10. Magnus

    Don’t think there will be any rocky planets around those stars. Rocky material is made in supernovas, and out in intergalactic space there is not many supernovas happening…

  11. Nigel Depledge

    Another nice piece, BA.

    I, too, would appreciate hearing your take on the gas / plasma controversy.

    Plus: OOOhhh! Pretty!!!

  12. There’s a great novel by Ian Banks, Against a Dark Background, where the action takes place in an isolated system between galaxies, in which the whole culture, even after having achieved space flight, is chronically depressed, ’cause they know they’ll never be able to visit any other star system, no matter how clever they get.

  13. RoaldFalcon

    Didn’t someone recently release a picture of the first known star with a tail. Now someone else comes along and says “That’s nothin’. Look at this.”

  14. blizno

    # Quiet_Desperationon 20 Sep 2007 at 1:55 pm

    Wait… I live in a largish city. What are these “stars” of which you speak?”

    “Stars” look a lot like passenger jets except they don’t blink and they don’t move (unless you take pictures months or years apart).
    Try to imagine a streetlight, but it’s very small, very sharp and waaay up in the sky. Then imagine a trillion of them…

    I joke because I have lived in a huge city (Chicago, Illinois) where you were lucky to see six stars during the clearest of nights. I have also spent lots of time camping in one of the last great wildernesses of the USA, the Boundary Waters Canoe Area, BWCA, (a huge, wild, pristine national park spanning a gigantic area of northern Minnesota, pressed against the border with Canada.
    Too much information:

    I’ve sat bolt-upright with my heart pounding in my ears while moose the size of houses thundered within feet of my tiny tent during the wee, dark hours. I’ve slumped on a boulder slack-jawed while the Aurora Borealis squirmed across the sky over my head. I’ve seen the Milky Way splashed so bright across the sky that my heart ached to witness such grandeur.

    That such a gigantic piece of one of the best US states to live in (Minnesota) is still kept carefully pristine encourages me for the future of humanity. Maybe there can remain a few pockets of Earth unspoilt. I so, so wish it.

  15. blizno

    I still don’t understand why a diffuse plasma where each atom is essentially floating alone can keep such incredible temperatures. Surely a particle at millions of degrees would shed photons into interstellar space very quickly. Why doesn’t a solitary nucleus radiate enough energy to cool itself from millions of K to thousands or hundreds of K within a year or so (or a fraction of a second)? Is it because local space is full of photons shed by other millions-of-degree particles and a local particle that sheds a high-energy photon is likely to be hit by somebody else’s high-energy photon, re-energizing it?

  16. occam's comic

    Sometimes its useful to think energy, instead of temperature. And again its not an atom that is floating there but protons (positively charged) and electrons (negatively charged) that are moving. And moving electric charges make a current, electrical currents create magnetic fields which shapes the movement of electrons and protons. This feedback loop can create long filamentary (stringy) structures.
    So you see there are more ways for plasma (as opposed to gas) to ‘store’ energy: the movement of the ions, the magnetic fields that are created, the reduction in entropy by forming structure.

  17. I love the stars and this page is so cool !!!!!!!!!!!!!!!!!!!!!!!!!


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