
Half of stars lurk outside galaxies - scott_s
http://www.nature.com/news/half-of-stars-lurk-outside-galaxies-1.16288
======
diziet
The concept of a solitary star without any 'nearby' neighbors features in Iain
Banks' Against A Dark Background:
[https://www.goodreads.com/book/show/422452.Against_a_Dark_Ba...](https://www.goodreads.com/book/show/422452.Against_a_Dark_Background)

~~~
pavel_lishin
I read a short story, not too long ago, about a solitary planet on approach to
a solar system. Their society was stagnating due to some cultural predilection
to having a once-every-ten-year science fair, and having no technological
progress outside of that. It was framed as a murder mystery, too. Wish I
remembered what it was...

~~~
teraflop
Sounds like "The Science Fair" (1971) by Vernor Vinge.

~~~
pavel_lishin
Yes, you're right!

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scott_s
I think it's worth noting that this is the conclusion one group of scientists
came to based on data they collected. That's true of all published articles,
but my point is that other scientists have not necessarily come to the same
conclusion - yet. But that's how science works!

At the end of this submission, they mention that they are planning follow-up
experiment which should get more data.

I first heard about this on the Skeptics Guide podcast
([http://www.theskepticsguide.org/](http://www.theskepticsguide.org/)), and I
was surprised because it's counter to how I think about the universe.

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paul_f
Anyone living on planets orbiting those stars would not see many other stars
at all, right?

~~~
jessriedel
Right. Almost all the stars we see in the night sky are within 1,000 light
years. You could guess this from the fact that the Milky Way disk is about
~1,000 light years thick and ~100,000 light years in diameter. If we could
easily see stars further than the thickness, then we'd see a lot more stars
along the Galactic disk. But instead, the only sign of the disk is the diffuse
glow of very distant, undifferentiated stars where the Milky Way gets it's
name.

Presumably stars outside of galaxies are at much lower density, so they
wouldn't see hardly any points of light in the sky. If most such stars are
still within the Galactic groups, then they would probably just see faint
smudges of other galaxies like the very dim Magellanic Clouds we can see.
(Note that these are still pretty big, though, in terms of angular size.
Several moons wide.) On the other hand, planets around lone stars in voids
away from Galaxy clusters would presumably have nothing visible to the naked
eye from their surface.

(Someone please correct me if I'm wrong. I'm not an astronomer.)

~~~
throwaway_yy2Di
I think that's right (also not an astronomer). I looked this up in the
Hipparcos catalog [0][1] -- that's one that measures parallax distances to
bright stars. It's supposed to be complete to at least +7.3 magnitude [0].
Cutting off at +6.5 magnitude [2] (the faintest visible stars according to
[3]), Hipparcos reports

* 7,943 visible stars

* 1,078 (14%) beyond 1,000 light years;

* 34 beyond 10,000 l.y. (these must be supergiants!)

If I calculated right, a +6.5 magnitude star would demand an _absolute_
magnitude of -0.9 at 1,000 light years, and -5.9 at 10,000 light years.
Looking this up on the Hertzsprung–Russell chart [4], this would include only
very bright giants, and supergiants, respectively.

[0]
[https://en.wikipedia.org/wiki/Hipparcos](https://en.wikipedia.org/wiki/Hipparcos)

[1] ftp://cdsarc.u-strasbg.fr/pub/cats/I/311/

(hip2.dat.gz for data; intro.pdf for field descriptions)

[2] Hp magnitude is a 340-850 nm visible band, which diverges slightly from V
band magnitude,

[http://heasarc.gsfc.nasa.gov/W3Browse/star-
catalog/hic.html](http://heasarc.gsfc.nasa.gov/W3Browse/star-catalog/hic.html)

[3]
[https://en.wikipedia.org/wiki/Apparent_magnitude#Table_of_no...](https://en.wikipedia.org/wiki/Apparent_magnitude#Table_of_notable_celestial_objects)

[4]
[https://en.wikipedia.org/wiki/Hertzsprung%E2%80%93Russell_di...](https://en.wikipedia.org/wiki/Hertzsprung%E2%80%93Russell_diagram)

~~~
jessriedel
OK, but most of those 1,077 are quite dim. WIkipedia's list of the 93
brightest stars (magnitude <2.5) lists only 6 that are further than 1,000 ly.

[http://en.wikipedia.org/wiki/List_of_brightest_stars](http://en.wikipedia.org/wiki/List_of_brightest_stars)

------
DanielBMarkham
Also of note, from another group of researchers word is that there may be more
planets in-between stars than there are circling stars.

If this continues, the universe is going to end up being nothing like we
expected only a few years ago.

~~~
fragsworth
I am pretty sure this is a widely acknowledged assumption among most
scientists, considering our current understanding of the formation of planets,
stars, and galaxies.

~~~
antognini
The free-floating planets result is still controversial and there is a
substantial fraction of the astronomical community that doesn't believe it. If
it is true, it introduces problems in our understanding of planet formation
because planet-planet scattering as we currently understand it is unable to
produce free floating planets at the rate claimed. It is also possible that
planets form in isolation (so really they're very small brown dwarfs), but
given our understanding of star formation it is difficult to form isolated
objects that small.

~~~
fragsworth
> "given our understanding of star formation it is difficult to form isolated
> objects that small."

No, that is demonstrably false. You can try this with any particle gravity
simulator - Place a bunch of particles, and watch how many get ejected. Star
systems behave similarly.

~~~
antognini
The problem is that you cannot treat star formation as a set of collisionless
particles. In general you can estimate the scale of star formation through the
Jeans mass, which is the mass of a gas cloud at which the dynamical time
becomes equal to the sound crossing time. This is essentially the scale at
which the cloud becomes unstable to perturbations --- pressure is no longer
able to support the cloud against gravitational oscillations.

The Jeans mass depends inversely on the square root of the density of the
cloud. The main problem in forming very low mass objects is that you need very
large densities. For a Jupiter mass object you need a density of ~5 x 10^4 /
cm^3. Even in molecular clouds it is difficult to achieve densities that
large. There are ways around it --- turbulence can produce small regions of
very high density, for instance. But like I said, they're not well understood.
In general it's much easier to create a low mass object in the vicinity of
something bigger because then it can form by fragmenting out of the disk that
surrounds the larger protostar.

------
Zarathust
Is all of this accounted for in the "missing mass" of the observable universe?

In other words, could those unseen before stars explain a part of the dark
matter problem?

~~~
Florin_Andrei
No, dark matter is something that happens _within_ galaxies, affecting their
rotation. It's not something outside.

~~~
db48x
Sorry, that's wrong. Matter, dark or otherwise, is scattered throughout the
universe, with variations in density at all scales. Ordinary and dark matter
are gravitationally attracted to each other, so they clump together. Anywhere
you find matter you'll find a proportional amount of dark matter, on average;
naturally there are variations. There's some in between you and your computer
monitor right now.

The first good indication that dark matter must exist was in fact that the
rotation rates of the observable galaxies were too large for the amount of
visible mater they contain. Of course the first explanation was that this
matter was ordinary gas, dust, rocks, planets (which are just larger dust
particles, when compared with the size of even a small galaxy.), etc that
wasn't emitting any light. However, there would have to be so much of it that
it would occlude the distant galaxies, something that obviously isn't
happening. Thus the modern theory is that dark matter is a form of matter that
interacts gravitationally with ordinary matter, but has no interaction with
the electromagnetic force (light).

------
StephenFalken
One day we will eventually realize that we are indeed traveling through space
on "Spaceship Earth".

------
mrfusion
Could this be a solution to the Fermi Paradox?

~~~
pavel_lishin
I don't think so. It may alter the equation, but it wouldn't really be a
"solution". For one, considering how difficult interstellar travel should be
based on what we know, intergalactic travel would be much more difficult, even
given von Neumann's self-replicator ideas. (It's like the difference between
crossing a river, and crossing the Pacific.)

On the other hand, maybe galaxies are just naturally inimical to life, and
extragalactic stars are the only places where long-term biological survival is
possible.

~~~
VieElm
> On the other hand, maybe galaxies are just naturally inimical to life

Gamma ray bursts would be more likely in a galaxy and those are inimical to
life.

~~~
logfromblammo
Also, catastrophic collisions with other bodies would be more frequent when
those bodies are closer together. Even if there is no direct interaction,
nearby massive objects could still perturb the orbits of distant objects--
similar to those in our Oort Cloud and Kuiper Belt--enough to make them impact
planets orbiting closer to the star.

------
comrade1
Our own solar system is outside the central part of the galaxy and a bit off
the ecliptic plane. There isn't that much around us. The central core of the
Galaxy could be populated, wars raging around us, and we're the equivalent of
a mountain village in Laos.

But could you imagine being one of these floater solar systems? We at least
can reach some star systems in generation ships. They would would need much
more.

If 1/2 of stars are outside of feasible colonization distances that changes
the Fermi paradox formula quite a bit.

~~~
pavel_lishin
I don't think you even need generation ships, if you can maintain a constant
acceleration. And with constant acceleration, going to our nearest potentially
habitable neighbor isn't that much further, subjective-time-wise, than it is
to Andromeda.

Assuming acceleration of 1g, from here to Alpha Centauri, it would take 6
years objective, 3.5 subjective. To the center of the galaxy, it would take
27,000 years objective, but only 20 years subjective. Magellanic clouds? 162k
years objective, but only 23 years subjective! 28 years to Andromeda.

Fuel may be in short supply, but relativity really works for you - ignoring
abrasion and impacts, if you can get to one galaxy, you can get to most others
in your neighborhood.

(All calculations derived from
[http://spacetravel.nathangeffen.webfactional.com/spacetravel...](http://spacetravel.nathangeffen.webfactional.com/spacetravel.php),
because I'm too lazy to do it by hand.)

~~~
readerrrr
And the last paragraph is the main problem. The ship would get torn apart from
lone atoms and to actually stop yourself you need an exponential amount of
fuel, because you need to accelerate ( much more than )half of it just to stop
yourself.

Even if you protect yourself from relativistic atoms, they still create a
drag. And that happens before you reach 0.99c.

~~~
pavel_lishin
__edit __: Nevermind - you 're totally right, and I'm totally wrong - I didn't
think it through, and didn't realize that since relativity helps you less and
less as you decelerate, the amount of fuel required does increase hugely if
you actually want to stop at your destination.

Unedited original post follows:

> to actually stop yourself you need an exponential amount of fuel, because
> you need to accelerate half of it just to stop yourself

Hang on, I don't understand this part. First, you're right, I didn't think
about deceleration - but that only doubles the trip length at most, since you
have to accelerate halfway, then decelerate halfway. And probably not even
exactly that if you're carrying your fuel, since deceleration will be a little
bit easier - you've burned some gas, so there's less mass to push around.

And you're right, I didn't think about the drag - but that actually works out
better for extragalactic visitors! There isn't as much stuff there to stop
them when they're taking off, and once they hit a galaxy, it actually helps
them decelerate. (Again, my dreamy eyes are ignoring the practical hazards of
this "help" which might just turn them into a fast moving gas cloud.)

~~~
readerrrr
I'm not talking about the time it takes, but the mass of the fuel. Which is
more than just doubled.

That is what equations tell you:
[http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.h...](http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html)
( scroll to _How much fuel is needed?_ )

~~~
pavel_lishin
Argh, thank you, you're right, I edited my post to reflect that.

I wonder if you could accelerate the whole way in a giant ship, and only slow
down a tiny capsule at your 'destination' \- maybe just a tiny self-
replicating robot factory and some data storage. Decelerate that, land it,
have it build you a new body, some tools, etc., etc.

------
transfire
So they actually "lurk"?

