
Rare system of five stars discovered - Someone
http://www.bbc.com/news/science-environment-33428506
======
Someone
One more and a planet to go
([https://en.wikipedia.org/wiki/Nightfall_(Asimov_short_story_...](https://en.wikipedia.org/wiki/Nightfall_\(Asimov_short_story_and_novel\)),
[https://www.uni.edu/morgans/astro/course/nightfall.pdf](https://www.uni.edu/morgans/astro/course/nightfall.pdf))

~~~
yodon
Or it could be the Pupetteer migration fleet! (yes, Larry Niven did garble a
2n-body Klemperer rosette into a 5-body Kemplerer rosette[1])

[1]
[https://en.wikipedia.org/wiki/Klemperer_rosette#Misuse_and_m...](https://en.wikipedia.org/wiki/Klemperer_rosette#Misuse_and_misspelling)

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lagadu
Ok, so objects orbit not other objects but rather centres of gravity. This
means that the system in question could have planets with small orbits (like
the earth's for example) orbiting that system's centre of mass despite the
stars being many AUs away from it.

It may be unlikely but it's certainly possible, right? Am I missing something
that'd make this impossible? Because that would be incredibly cool!

edit: I'm assuming the masses of the two groups of stars would be roughly
equal, in order for the centre of gravity to be in the "middle".

edit2: philosophical question: can this system still be described as
heliocentric?

~~~
restalis
"This means that the system in question could have planets with small orbits
(like the earth's for example) orbiting that system's centre of mass despite
the stars being many AUs away from it. It may be unlikely but it's certainly
possible, right?"

No, this does not mean that at all! An orbit asks for a centripetal force. In
heliocentric systems this force is the gravity of the star (considering the
gravity of the orbiting body to be negligible). Here, pairs of stars are kept
into orbits and for each star the force that keeps it around is not only the
force of gravity of the other star, but also that of its own (since it's not
negligible any more). A third body can not have a stable motion around the
"centre of mass" between those stars because there does not exist any force to
keep it there. Any body between the orbiting stars will be under forces that
will pull it towards the stars. It may at most stay "stable" in the middle,
where the gravity forces of the around-orbiting stars cancel each other.

~~~
andygates
..and if there were inhabitants, they'd be suffering unpredictable chaotic
seasonality like the aliens in [https://en.wikipedia.org/wiki/The_Three-
Body_Problem](https://en.wikipedia.org/wiki/The_Three-Body_Problem)

~~~
chiph
Could a planet maintain an atmosphere for any length of time under such
variable attraction?

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hfsktr
I have a question that a simple search didn't answer. Do astronomers actually
measure the time it takes a star/planet/whatever to orbit? Or is it simply not
possible for things after a certain distance?

For example how did they figure out that Pluto takes 247 Earth years per
orbit? In our solar system I imagine they could measure it since it's close
enough or see it in real time but even Pluto wasn't known for 248 years to see
it go around.

With so many objects being so far away I thought it wouldn't really look like
it moved from our perspective.

~~~
gogoengie
There are different considerations for the problem: the orbiting masses
involved (what is orbiting what?), orbital period (how long for the objects to
complete an orbit around their center of mass), the distance of the objects to
our measurement device (eg, a telescope on earth), the relative luminosity of
the two objects (if they're close together, how do you resolve the darker of
the two?), etc.

From all these considerations, there are two problems: 1\. Measurement
technique (getting the data) 2\. System dynamics calculations (analyzing the
data)

For measurement, the article mentions both visual and spectroscopic
(wavelength) measurements; overviews of these and other techniques are
discussed in [1]. You are correct, distance from the measurement device makes
things interesting, especially in the presence of atmospheric distortion, but
there are lots of tricks that can improve the effective resolution (eg,
speckle imaging, which uses a bunch of images taken in rapid succession [2]).

As for the orbital period, think of it like measurements of a line. You don't
need every point, just two. If you have several measurements of a celestial
body, you can plot them on one of the (MANY) celestial coordinate systems [3]
and do some estimates. Longer periods mean more observations for better
accuracy, but you don't need to wait for a complete orbital period to get a
decent estimate.

Hope this at least gives some useful keywords for your searches!

[1]
[https://en.wikipedia.org/wiki/Binary_star#Methods_of_observa...](https://en.wikipedia.org/wiki/Binary_star#Methods_of_observation)
[2]
[https://en.wikipedia.org/wiki/Speckle_imaging](https://en.wikipedia.org/wiki/Speckle_imaging)
[3]
[http://csep10.phys.utk.edu/astr161/lect/time/coordinates.htm...](http://csep10.phys.utk.edu/astr161/lect/time/coordinates.html)

~~~
hfsktr
Awesome thanks. I figured that you didn't actually need to view the full orbit
and needed just a few points but for things so far away I thought that it
would look practically stationary. Didn't even consider atmosphere...or that
there were other factors. Of all the stuff that can be learned it probably
isn't high on the list anyways.

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vacri
Five-star systems aren't rare at all, in fact, they're problematically common.
Usually the issue is that with such a limited array of choices, people
gravitate to one star or five stars, and you don't get much meaningful
information from the resulting scores. Percentage-based systems give more
flexibility and have a wider 'active zone', without being overwhelmingly
complex - and people are much less likely to given min or max scores on such a
scale.

 _vacri whistles innocently..._

~~~
zhte415
So you're saying 5-star systems are just like a binary?

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leni536
I would rate this article five stars.

~~~
krylon
I thought this headline was pure pun-bait, I just didn't think that something
so straightforward would work. Thanks for proving me wrong! :)

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antognini
There is currently some debate about how common these very multiple systems
are. It is pretty well known now that about 40-50% of systems are single
stars, about 40-45% of systems are binary systems, about 10% are triple
systems, and around a few percent are quadruple systems. But it doesn't seem
to be well known what fraction of systems are quintuple systems or larger,
firstly because they are intrinsically rare, and secondly because the fifth
star is usually very hard to find (it tends to be both faint and distant). It
seems as though this fraction is probably around 1 percent, maybe a little
less. A few years ago it was thought that the fraction of systems with
different hierarchies followed a geometric series, but this doesn't seem to be
borne out any more as the statistics have improved.

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pluma
A five star solar system? The puns just write themselves.

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gloriousduke
[http://student.vub.ac.be/~clvidal/writings/Vidal-
Starivore-B...](http://student.vub.ac.be/~clvidal/writings/Vidal-Starivore-
Binary.pdf)

------
INTPenis
Oh my five suns. ;)

>The pair of stars orbit around a mutual centre of gravity, but are separated
by more than the distance of Pluto's orbit around the Sun.

But what could they be orbiting around?

~~~
mixmastamyk
Their center of gravity, say about here:

    
    
       o O---------------o
             ^
    

Pluto and Charon do it as well, even Earth and Moon to a small degree.

~~~
pluma
I've actually heard someone argue that because of this the moon is actually
not a moon but our sister planet.

~~~
rimunroe
All orbits go around a common center of mass which is not the center of mass
of the orbiting bodies separately. The Earth-Moon center of mass is still
located within the Earth.

[edit] I forgot to mention that the distinction between a satellite/moon and
another co-orbiting body had to do with the location of that barycenter being
within one of their bodies, if I recall correctly.

