Rare system of five stars discovered | Hacker News Reader

Rare system of five stars discovered

bbc.com

67 pointsSomeone35 comments

One more and a planet to go (https://en.wikipedia.org/wiki/Nightfall_(Asimov_short_story_..., https://www.uni.edu/morgans/astro/course/nightfall.pdf)

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...

Also, this: https://en.wikipedia.org/wiki/The_Three-Body_Problem

Yup, as soon as I saw the title I thought of Nightfall - probably my all-time favourite Asimov story :)

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?

"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.

..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

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

Dammit, I knew it sounded way too cool to be possible :(

Thanks!

Orbiting around the center of gravity only can be applied for two body systems. For many body systems it can be applied too as a careful approximation (like in the article). The movement of a small planet in the middle couldn't be described by this however, since it's an approximate three body system now (the two pairs and the planet)-

There could be stable point's in the middle, I don't know about that. Most certainly it can't be described by this approximation.

Looking into it, couldn't a planet orbit one of the two stable Lagrange points between the two groups, L4 and L5? That wouldn't be "inside" the orbit though :(

Yeah, I would think that the quasi stable orbits would be L4, L5 or orbiting around any of the pairs. If you have a symmetric situation isn't the middle point exactly L1? That would be unstable.

Edit: From Wikipedia:

>The triangular points (L4 and L5) are stable equilibria, provided that the ratio of M1/M2 is greater than 24.96.

So with a symmetric situation, where M1/M2=1 they wouldn't be stable either.

Depends on the details of the system, but in general it is highly unlikely. Usually already a binary star system does not really have nice orbits, since a too close encounter between a planet and a star basically acts like a fly-by-maneuver, so that it transfers energy to the planet and potentially kicks it out of the system.

A example for a triple star system would be Alpha Centauri and Proxima Centauri, the closest star system to the Solar System. Alpha Centauri is a double star and Proxima Centauri orbits it at a huge distance, such that the seperation between the components of Alpha Centauri is no longer important for the orbit.

So it is certainly possible to find stable orbits, even in a very unusual star system like this one. But they are ( except in very rare cases), just elliptical orbits at huge distances.

I wonder is there can be a small planet at the gravitational centre of this system, of if it's be ripped apart by the stars on each side.

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.

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... [2] https://en.wikipedia.org/wiki/Speckle_imaging [3] http://csep10.phys.utk.edu/astr161/lect/time/coordinates.htm...

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.

Objects being far away doesn't necessarily mean that our view is slowed after adjusting for our speeds, albeit delayed.

Although I doubt much at this distance is measured optically, but rather using estimates of gravitational forces, perhaps through measuring various "wobbles" within the system.

- Not a physicist.

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...

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

Would orbit again.

I would rate this article five stars.

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

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.

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

http://student.vub.ac.be/~clvidal/writings/Vidal-Starivore-B...

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?

> But what could they be orbiting around?

>> a mutual centre of gravity

The math of gravity is fascinating. Objects can orbit each other, with nothing in the middle.

You think that's good, try this: Objects can even orbit interesting points in space that are not themselves centers of gravity:

https://en.wikipedia.org/wiki/Lagrangian_point

This is linked to orbit around Lagrangian points but it's more surprising: https://en.wikipedia.org/wiki/Horseshoe_orbit

By extension it could be interesting to model the Lagrange-point space topology within this system and probe what object forms might have assembled themselves within the equivalent of L4 and L5 points. Of course, intense mutual solar winds would mitigate against that happening.

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.

http://www.abc.net.au/cm/lb/6605566/data/jupiter-and-io-take...

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

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.

Reply on news.ycombinator.com