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There is a gravitational force between every pair of objects, because of which planets revolve around the Sun in elliptical orbits.

My question is: as planets move in elliptical orbits with the Sun at a focus, do the stars in galaxies orbit as well? And if they do, then what are the foci of their orbits? Since the gravity force of black holes is very strong, are black holes at any of the focus of a star's orbit?

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    $\begingroup$ Kinda related question with a nice answer: astronomy.stackexchange.com/questions/16391/… $\endgroup$
    – userLTK
    Jan 1, 2017 at 2:26
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    $\begingroup$ Stars near our galaxy s center do orbit on nearly elliptical orbit and their orbit s loci is the center of mass of the black hole (more or less ) galacticcenter.astro.ucla.edu/animations.html $\endgroup$
    – chris
    Jul 7, 2017 at 14:58
  • $\begingroup$ I have rolled this question back to a version that is grammatically correct. If you don't like the way it's worded, fine, but move forward from this point. $\endgroup$
    – Donald.McLean
    Jul 14, 2017 at 14:48
  • $\begingroup$ I would like to clarify that the sun moves in a circular motion as it travels around the milkyway Galaxy $\endgroup$
    – Damon Syfrett
    Jul 9, 2020 at 8:58

2 Answers 2

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Planets orbit the sun in approximately elliptical orbits. The orbits of the planets are perturbed by each other, particularly by Jupiter. So the shape of orbits are very nearly, but not exactly elliptical.

A simple elliptical orbit is found when there is a single dominating mass, for example in the solar system the Sun is much more massive than the rest of the solar system put together.

Stars in galaxies do move. However, they don't move in simple elliptic orbits for the simple reason that the mass of a galaxy is not concetrated at a single point. There is a supermassive black hole at the centre of the galaxy, but although it is massive, its mass is dwarfed by the mass of the rest of the galaxy (it has a mass of about 4 million suns, the galaxy has a mass of 600000 million suns.) The stars move in orbits around the centre of mass of the galaxy, to which the black hole contributes only a very small amount. The shape of the orbit is a "wavy circle". It is not elliptical, and so doesn't have foci.

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  • $\begingroup$ A much smaller effect than the many-body gravitational perturbations is the decay of the orbits due to the emission of gravitational radiation, which in principle also makes the orbits non-elliptic, is that correct? $\endgroup$
    – Daddy Kropotkin
    Oct 23, 2021 at 11:10
  • $\begingroup$ Yes, but this would be a tiny amount. It would be like in a consideration of "What makes a car go?" mentioning the impulse produced by the rear lights, compared to the engine. $\endgroup$
    – James K
    Oct 23, 2021 at 11:22
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Since the gravity of black holes is very high

I feel there's a slight misconception floating around this expression.

Black holes have a quite ordinary gravitational fields at stellar distances. They're no different from any star in this sense. Their field is governed by their mass, which is not particularly larger than stars for typical black holes ( the majority of which are the dead remains of an even larger star ). This reputation for a very high field is only relevant when you're close to them.

The black holes at the center of many galaxies are exceptionally large and that means they are proportionately stronger, but just being a black hole doesn't make an object suddenly gain a stronger field than it's mass would dictate. In some ways it could be said to be safer - it's harder to collide with a black hole than a sun simply because it's smaller for the same mass.

The black hole thought to be at the center of our galaxy is estimated to be about 41 light-seconds across which makes it about 9 times the diameter of our Sun. But our Sun is a pretty mediocre star and there are many stars larger than that. The largest we know of are estimated to be over the 1,500 times the Sun's radius.

So as we start the New Year, can we take a vow to stop portraying harmless black holes as monsters of high gravity ? :-)

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  • $\begingroup$ Also, the tidal acceleration felt by a particle near a black hole is inversely proportional to the mass of the black hole, which is why a star passes well within the event horizon of a supermassive black hole before being tidally disrupted. $\endgroup$
    – Daddy Kropotkin
    Oct 23, 2021 at 11:08

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