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In our solar system, there are 8 planets orbiting a star, the Sun. And I understand that there are about 500 confirmed solar systems out there.

But why is it always planets orbiting stars? Why can't it be several stars orbiting a planet, or a star orbiting a star? Why is a star by definition stationary as opposed to planets which are moving.

Of course, in that case, it wouldn't make much sense to call it a solar system, but still.

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  • $\begingroup$ We have Galaxies, stellar clusters, 50% of all stars are double stellar systems. There are many asteroids and comets orbiting our star. Stars are not stationary, but move around the galaxy and wobble when their planets tug on them. That is partially how we detect them. So no, your premise is wrong. It is absolutely not only planets that orbit stars. $\endgroup$ – AtmosphericPrisonEscape Dec 15 '15 at 16:15
  • $\begingroup$ "A solar system" isn't the correct term. You mean "a planetary system". The Solar System is the name of our planetary system. $\endgroup$ – Sir Cumference Dec 15 '15 at 18:29
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"Why is a star by definition stationary as opposed to planets which are moving." This just isn't true. Both star and planets orbit a point in the system known as the barycenter (a.k.a. the center of mass of the system).

Because stars are much more massive, this barycenter is much closer to the center of the star than it is to the planets. Hence it appears to the casual observer that the less massive object orbits around the more massive object. In fact both objects orbit the barycenter. Like so:

Orbit around a barycenter

The following diagram (seen on the wikipedia page above) shows the motion of the solar system barycenter with respect to the center of the Sun. Note that the barycenter is close to the center of the Sun, but often spends time outside the visible surface of the Sun. It follows this complicated path, mainly because of Jupiter but all the other planets make smaller contributions too.

Solar system barycenter

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The gravitational pull from these stars. There is so much matter compacted into these stars that the gravitational pull is strong enough to hold all these planets together.

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  • $\begingroup$ Is it instrinsically impossible for a very big star to have a very small star circle it? $\endgroup$ – user10113 Dec 15 '15 at 16:16
  • $\begingroup$ It is possible that one star could devour another, and the two mesh.. $\endgroup$ – Konnor Timmons Dec 15 '15 at 16:21
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Planets often circle stars because they have a very strong gravitational pull. Infact, our moon orbits the earth and we can look at a solar system in the same way. The earth has enough matter inside it to keep the moon in orbit, and the sun has enough matter inside it to create a gravitational pull strong enough to keep a entire solar system in its orbit.

You can't have several stars circling a planet, the pull of a single planet wouldn't be strong enough (the planet's center would be so dense nuclear reactions would start occurring and it would be a star anyway) . You can have binary star systems though, which are where 2 stars orbit around their common barycenter.

So basically, things orbit other things with a stronger gravitational pull.

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