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According to Education.nationalgeographic, "A star system is a group of planets, meteors, or other objects that orbit a large star. While there are many star systems, including at least 200 billion other stars in our galaxy, there is only one solar system. That's because our sun is known by its Latin name, Sol.

So, ignoring our star system, when astronomers discovered other star systems, on what basis did they (or do they) define the boundary of a star system?

For example, when I was trying to understand what defines the boundary of a galaxy, I came across an article in Advanced Science News from last year. In that article, a team of people came up with their own explanation/definition for what defines the edge of a galaxy. They argue that the edge of a galaxy is defined as the point at which star formations inside the galaxy significantly drops.

So, in that same vein, what defines the edge/boundary of a star system?

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  • $\begingroup$ Gravity. Where the gravitational influence of one star is stronger than that of any of the stars surrounding it is part of the first star’s “system.” $\endgroup$
    – Pierre Paquette
    Commented Aug 20, 2023 at 21:47
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    $\begingroup$ I think astronomy.stackexchange.com/questions/6519/… is highly relevant to your question. There are a number of ways of defining the edge of the solar system (and by extension other star systems) But in general the edge of the solar system is the point at which the sun no longer dominates the surrounding space.... either by gravity, or by solar wind. $\endgroup$
    – James K
    Commented Aug 20, 2023 at 22:51
  • $\begingroup$ Thanks for your replies. I'm looking for a definition similar to the one I found for a galaxy (see my initial post) that nicely describes the outer boundary or edge of a star system. I've been poking around a bit but can't find one that really feels "right" or accurate. $\endgroup$
    – Hikerguy
    Commented Aug 21, 2023 at 1:06
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    $\begingroup$ Relevant xkcd, especially the alt text: "So far Voyager 1 has 'left the Solar System' by passing through the termination shock three times, the heliopause twice, and once each through the heliosheath, heliosphere, heliodrome, auroral discontinuity, Heaviside layer, trans-Neptunian panic zone, magnetogap, US Census Bureau Solar System statistical boundary, Kuiper gauntlet, Oort void, and crystal sphere holding the fixed stars." $\endgroup$
    – pela
    Commented Aug 21, 2023 at 12:36
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    $\begingroup$ Since you mentioned galaxies, I'd also like to point to Where does the Milky Way end? $\endgroup$
    – Ilmari Karonen
    Commented Aug 21, 2023 at 16:32

2 Answers 2

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I don't think there is a formal definition, but a working one would be that the system is gravitationally bound. This does impose an outer edge because of the tidal field of the Galaxy and other objects.

For example, at the position of the Sun, the tidal radius due to the Galaxy is about $1.4 (M/M_\odot)^{1/3}$ pc, which can be calculated from the measured values of the Oort constants. Anything outside this radius won't be permanently bound to a system, anything inside could be, depending on the sum of its kinetic and gravitational potential energy.

As I say, I don't think this is a formal or even rigorous condition. For example in early star formation, pairs or groups of stars are often referred to as a "system" or cluster even though they are unbound and should dissolve on a dynamical timescale.

Other than that caveat, in the context of your quote, I think this definition applies. A star system, consisting of two or more stars, or even planets, should be gravitationally bound.

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Additional, or rather complementary, to the important 'gravitationally bound' criterion which applies to solid bodies like comets, asteroids and other outer solar system small bodies, one can go a different approach via the influence of the solar wind: by definition of the heliosphere.

The heliosphere describes the bubble of influence of the Sun onto the interplanetary medium, thus where the thin gas or plasma between the major bodies is dominated by particles coming from the Sun, the solar wind. This outer edge of the heliosphere is somewhat cartographed by the Voyager probes which have passed the heliosphere some years ago. These have passed the heliopause, and are now somewhat in the bow shock region of where the interstellar gas is slowed-down by "hitting" our heliosphere. The heliosphere is not a symmetric bubble but will be compressed on one side and more extended on another side. The distance the voyager probes left the heliosphere was around 120 astronomical units. The exact distance varies as it is defined by a pressure equilibrium (e.g. Linsky et al 2022) between the flowing interstellar gas and the solar wind itself. The place of this equilibrium varies also with the activity of the Sun itself.

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  • $\begingroup$ Not disputing you could make a definition like this but there are significant problems. It would probably rule out 30-40% of binary stars as being star systems since the median separation is about 50 au (for solar-type primaries). The other problem is it isn't a very workable definition since we have little idea about the speed and density of the winds of most other stars. Finally, there are components of the Solar System that lie beyond 120 au (e.g. Sedna-like objects and wherever long-period comets come from). $\endgroup$
    – ProfRob
    Commented Aug 21, 2023 at 9:43
  • $\begingroup$ @ProfRob yes, I totally agree. It was too long for a comment, and more illustrates that the outer boundary is more a broad transition. Which somewhat stars at the outer boundary of the heliosphere and likely ends somewhere how you illustrated in your answer. Any close encounter even at cm/s speeds in the outer solar system also has the potential to eject one of the partners out of it. All this said, if hard-pressed for a single value, I'd choose the gravity criterion like or similar to how you defined. $\endgroup$
    – planetmaker
    Commented Aug 21, 2023 at 9:48

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