Half the stars in the universe may exist outside of galaxies

Every so often you get a reminder of how little we know about the universe: Rogue stars outside galaxies may be everywhere | Science/AAAS | News.

You’ve heard of rogue planets, floating through the universe untethered to any solar system. Now meet rogue stars, which drift through space with no galaxy to call home. A new study has come to the startling conclusion that as many as half of all stars in the universe may be rogue, having been ejected from their birthplaces by galaxy collisions or mergers.

The article goes on to discuss that this is just a possibility at this point.  Not everyone is convinced.

“We’ll have to confirm, but they are hard to accommodate with the star model,” he says. Also, he points out, if there is a huge population of stars outside galaxies, we should see a noticeable number of supernovas occurring out in the middle of nowhere as those rogues stars die. “There are ways to test in the near term. It’ll be an enthusiastically pursued question,” Moseley says.

But if this does turn out to be true, what a revision to our view of the universe!  All this time we’ve been thinking that the space between galaxies was this vast emptiness with only the occasional gas or dark matter particle, but now it may be filled (relatively speaking) with stars.  (Yeah, I know “filled” is probably not the right word since even within galaxies there is an unimaginably vast emptiness between stars.  But intergalactic emptiness would still seem a lot less empty than we had previously understood.)

So, in recent years, we’ve learned that there are billions of rogue planets between the stars, and now we may be headed toward learning that half of all stars are between galaxies.  If nothing else, this should keep us humble about what we think we know about the universe.

12 thoughts on “Half the stars in the universe may exist outside of galaxies

  1. Reblogged this on S.A. Barton: Seriously Eclectic and commented:
    An interesting thought. Still, galaxies themselves are small compared to the non-galaxy spaces around them; my semi-educated perception would be that if you place a pea representing a galaxy in the middle of the playing field in a sports stadium, the next galaxy is a pea somewhere beyond the stadium walls, perhaps a few streets over, sitting on someone’s coffee table. Lots of room for wandering stars.

    I have used a rogue star as a passing setting in an as-yet unpublished story, “Dawn Across The Craze” — presented as a tourist destination. People come to see the utter dark of the night sky.

    Maybe there would be more lights in that sky than I depicted, if this idea turns out to be accurate!


    1. Thanks for the reblog!

      You’re definitely right that galaxies are small compared to the space around them, but I think the pea analogy actually sounds more like a description of the relationship between the size of a solar system and the typical distance to the next solar system, at least within our part of the galaxy.

      For galaxies, the Milky Way galaxy is about 100,000 light years in diameter. The closest spiral galaxy, Andromeda, is about 2.5 million light years away and around 200,000 light years in diameter. So, even ignoring all the small galaxies around them, if our galaxy was a pea, the next similar pea would only be about 25 pea lengths away.

      Of course, given the ongoing expansion of the universe, in time, the pea analogy will be accurate for galaxies, but we’re talking about tens (possibly hundreds) of billions of years.


      1. It still means intergalactic space is very sparse of stars! The volume increases as the cube of radius, so a galaxy 25 pea-lengths away means a volume of over 15,000 cubic peas around the Milky Way pea. That’s a huge volume in which to lose even a lot of pea pieces.

        Liked by 1 person

        1. Excellent point. You inspired me to do a little calculating (probably poorly). Stars in our region of the galaxy are typically around 5 light years apart. Stars in intergalactic space would average something like 500 light years apart. (Assuming a sphere 2.5 million light years in radius and 100 billion stars both within and outside of our galaxy within that sphere.)

          The night sky on a world around one of those stars would be an all enveloping darkness with maybe a star or two in the sky and the occasional barely perceptible smudge. Given our discussion on the other thread, intelligent aliens would be hopelessly isolated with virtually no hope of visiting other stars.


          1. Would a star ejected from a galaxy retain its planetary disk? I dunno. Does the whole system come along, Oort cloud and all?

            But, yeah, considering that every star we can see with our eyes is a star in the Milky Way, it would be dark skies out in the intergalactic. Maybe they have 27 moons or something to lighten things up!


        2. I tend to think the planets would come along. Gravitationally they’re pretty tightly bound to the star. The outer Oort cloud (more than a light month or so away) might be a different story depending on the intensity of the mechanism that slings the star out of its galaxy.


  2. A rouge solar system sounds kinda cool. Your likely to have a weirder experience being alive on one of the planets. But as long as you have your loving creator hugging you with warmth everything should be alright.


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