# Trinary System Arrangment?

I'm writing a science fiction story and wondered if someone could help me with the setup of a trinary star system I have in mind. Basically I just wanted to know if:

a) a system with a central, Sun-sized star could have two smaller (orange dwarf?) stars orbiting it in a circular trojan orbit, and...

b) how much disturbance would this type of orbit cause to any planets orbiting the inner star?

This may all sound ridiculous, but any input would be greatly appreciated. Thank you.

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I'm not sure if it would be more helpful to ask at the astronomy stack exchange. Don't go cross-posting it! If it needs to be moved we'll move it. astronomy.stackexchange.com – Alan Rominger Jan 28 '12 at 5:50
Note that the Alpha Centari system is trinary. I believe it is structures as a pair that are relatively close and a very distant third component only lightly bound to the first two. – dmckee Jan 28 '12 at 17:57

## 1 Answer

a) A trojan orbit for a third star would be stable only if the ratio of the central massive star to the second star were at least 25 to 1. Moreover, the mass of the third star (in the trojan orbit) would need to be small compared the second star if the third star is to behave as a 'test particle' in the gravitational potential of the first two stars.

To ensure that hydrogen fusion can occur on the third (smallest) star, it would need to have a mass of at least 0.07 solar masses or so. It then follows from the comments above that the central star would need to have a mass that is many times the solar value. So I think it is impossible to set up a system as you've proposed if the central star is to be 'sun-like.'

b) Given the above mass discrepancy between the massive star and its satellite stars, the orbits of planets around the central massive star would only feel a weak effect from the satellite stars

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Thanks for the input! Yes, I'd like to see what the astronomy exchange has to say. If you could repost this for me, that would be great. Thanks much! – riven2000 Jan 29 '12 at 1:38