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On the “sun orbits the galaxy”… I’ve read that the galaxy isn’t bound together by some massive black hole but instead by dark matter. The black hole in the cent

On the “sun orbits the galaxy”… I’ve read that the galaxy isn’t bound together by some massive black hole but instead by dark matter. The black hole in the center doesn’t have nearly enough mass for things like the sun to orbit it. So what is the sun and all its neighbors actually orbiting around?

sandworm101 5y ago
They orbit each other. It isnt a bunch of stuff in orbit around something, It is a mass of stuff spining in a big homegeneous blob. Our sun feels far more pull from neighbouring stars than it does anything near the center of our galaxy.
mannykannot 5y ago
As big as that black hole is, its gravitational field is nowhere near large enough to hold the galaxy together. What everything is orbiting around is the center of mass of the whole system. [1] 'Everything', here, includes that black hole, all the planets of all the stars (which orbit along with their suns as a solar system), and also the dark matter, if there is any. Dark matter has been hypothesized to explain why galaxies generally seem to be rotating too fast to hold together, based on the mass of the matter that we have observed, and it does so by proposing there is more mass than we thought. There is another hypothesis (MOND) that posits that the anomaly is on account of gravity being slightly different at long ranges. In this view, too, everything is still orbiting around the center of mass - what's different is the size of the gravitational force each body is subject to. Note that, in general, you cannot calculate the velocities within a galaxy simply by assuming a point mass, equal to that of the whole galaxy, at its center. In a spherically-symmetric situation, only the mass inside of a body's radius counts, as that outside of it pulls in all directions in a way that cancels out (AFAIK, this is so for symmetric disks as well. [2]) In practice, the motion of any body close to our galaxy's center will be dominated by the central black hole, as it is located close to the galactic center. [1] https://en.wikipedia.org/wiki/Barycenter https://en.wikipedia.org/wiki/Barycenter [2] https://physics.stackexchange.com/questions/277820/could-a-disc-have-a-gravitational-pull#547568 https://physics.stackexchange.com/questions/277820/could-a-d...

sandworm101 5y ago

They orbit each other. It isnt a bunch of stuff in orbit around something, It is a mass of stuff spining in a big homegeneous blob. Our sun feels far more pull from neighbouring stars than it does anything near the center of our galaxy.

mannykannot 5y ago

As big as that black hole is, its gravitational field is nowhere near large enough to hold the galaxy together. What everything is orbiting around is the center of mass of the whole system. [1] 'Everything', here, includes that black hole, all the planets of all the stars (which orbit along with their suns as a solar system), and also the dark matter, if there is any. Dark matter has been hypothesized to explain why galaxies generally seem to be rotating too fast to hold together, based on the mass of the matter that we have observed, and it does so by proposing there is more mass than we thought. There is another hypothesis (MOND) that posits that the anomaly is on account of gravity being slightly different at long ranges. In this view, too, everything is still orbiting around the center of mass - what's different is the size of the gravitational force each body is subject to. Note that, in general, you cannot calculate the velocities within a galaxy simply by assuming a point mass, equal to that of the whole galaxy, at its center. In a spherically-symmetric situation, only the mass inside of a body's radius counts, as that outside of it pulls in all directions in a way that cancels out (AFAIK, this is so for symmetric disks as well. [2]) In practice, the motion of any body close to our galaxy's center will be dominated by the central black hole, as it is located close to the galactic center. [1] https://en.wikipedia.org/wiki/Barycenter https://en.wikipedia.org/wiki/Barycenter [2] https://physics.stackexchange.com/questions/277820/could-a-disc-have-a-gravitational-pull#547568 https://physics.stackexchange.com/questions/277820/could-a-d...