# Orbital speed of stars within galaxies?

I have read some information, from various sources, pertaining to the orbital speed of stars about the center of galaxies, and some of it appears to be conflicting.

Some sources claim that the stars closer to the galactic center orbit faster, while those father out, orbit slower. This of course seems to follow traditional laws of gravitation.

However, I have also read that it has been documented that all the stars in galaxies seem to orbit the center at the same speed, regardless of their distance from the center. This so baffled scientists, that it was one of the reasons for postulating the existence of so-called "dark matter" to account for "missing mass".

So, my question is, does anyone have any definitive answer to which is correct?

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mgb_phys
Homework Helper
Stars orbit with the normal laws of gravitation.
But that law has a term for the mass of the galaxy ( in this case the mass within the radius at that stars distance)
If the mass of the galaxy was only due to visible stars the outer stars would be going more slowly - the speed would fall off as you expect.
But they aren't which suggests that there is extra mass we can't see (or the law of gravity is wrong!)

DaveC426913
Gold Member
Stars orbit with the normal laws of gravitation.
But that law has a term for the mass of the galaxy ( in this case the mass within the radius at that stars distance)
If the mass of the galaxy was only due to visible stars the outer stars would be going more slowly - the speed would fall off as you expect.
But they aren't which suggests that there is extra mass we can't see (or the law of gravity is wrong!)
This is something I've often wondered about.

Missing matter is only one possible way to reconcile this paradox, correct?

There could be other explanations (for example, that gravity behaves differently than we expect) - it's just that the "dark matter" explanation is the one that least disrupts our current understadning of physics (for example, having to throw out almost everything we thought we knew about gravity).

Are there independent, compelling observations that corroborate the dark matter explanation while weakening other explanations?

Stars orbit with the normal laws of gravitation.
But that law has a term for the mass of the galaxy ( in this case the mass within the radius at that stars distance)
If the mass of the galaxy was only due to visible stars the outer stars would be going more slowly - the speed would fall off as you expect.
But they aren't which suggests that there is extra mass we can't see (or the law of gravity is wrong!)
So you are in agreement that "outer" stars are orbiting the galactic center faster than they should be, IF the mass of the galaxy is made up only from visible stars.

mgb_phys
Homework Helper
There could be other explanations (for example, that gravity behaves differently than we expect)
A galaxy is pretty much the largest thing you can make direct measuremnts of gravitational effects with.
So to say GR is true based on experiments of the scale of Sun-Mercury and assume thet must be true at the scale of a galaxy - and then say in that case there must be some extra type of matter seems to be pushing it a bit - but thats true of a lot of cosmology!

DaveC426913
Gold Member
A galaxy is pretty much the largest thing you can make direct measuremnts of gravitational effects with.
So to say GR is true based on experiments of the scale of Sun-Mercury and assume thet must be true at the scale of a galaxy - and then say in that case there must be some extra type of matter seems to be pushing it a bit - but thats true of a lot of cosmology!
I'm not sure I'm reading you right, but it sounds like you're promoting the idea that gravity might not be constant at all scales, and that this potential fact alone would be enough to explain the paradox we see on galactic scales - no extra or missing matter needed.

Am I correct?

mgb_phys
Homework Helper
I really don't know if DM is real or not - I'm not an expert.
But ...
100 years ago somebody decided that because of a small difference in the orbit of mercury all our laws of gravity for the last 300 years must be wrong. It was one of the big pluses of science - we were prepared to throw out everything that was obviously true (absolute space/time/mass etc) because of a tiny experimental difference.

Now we are saying that our laws of gravity can't possibly be wrong - so we must invent this bizarre mysterious matter to explain away this large experimental effect.

"100 years ago somebody decided that because of a small difference in the orbit of mercury all our laws of gravity for the last 300 years must be wrong."

Thanks for saying this. It seems that there is exponential "building" of theories on theories on theories these days in science. But when it really counts, we still use Newton's science to launch rockets to Mars and beyond. Did Einstein start a new type of theory physics, the one that goes "way out there" knowing the theory can never be proven wrong? (Because it will never be possible to conduct the related experiment.)

Given the distances to other stars in other galaxies, and the many distorting variables in their observations and observations of their orbits, could the "mass differences" be attributed to an appropriate margin of error?

Doesn't it lead us back to the F factor, where F = faith, NOT science?

dark matter ... the F factor to explain gravity variations
inflationary expansion ... the F factor to fit universe temperatures related to the big bang.
actually you can say the big bang is another F factor, it will always be a theory.
Or my favorite ... regarding the LHC ... we will duplicate the conditions 1 / billionth of a second AFTER the Big Bang, HA!
(are you sure it won't be 2 / billionths of a second AFTER the Big Bang, that happened "14 Billion years ago"
higgs bison ... the F factor to make particle physics work
worm holes ... the F factor for hope of traveling to another earth that is light years away

Sorry, I love physics, but I never knew there was SO many F factors.

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mgb_phys
Homework Helper
we still use Newton's science to launch rockets to Mars and beyond. Did Einstein start a new type of theory physics, the one that goes "way out there" knowing the theory can never be proven wrong? (Because it will never be possible to conduct the related experiment.)
Any new theory has to give the same answers as existing theories. You can't have a new theory of gravity that means that a football thrown today doesn't go the same way as one thrown yesterday.
General relativity (Einstein's theory of gravity) has been tested experimentally, back in 1919 and every time you use a GPS. But that doesn't mean it is correct on the scale of a Galaxy.

If you have a theory that doesn't explain a new experimental result then either your theory is wrong (at least in those circumstances) or there is some other experimental factor you don't know about (in this case dark matter).
But to make up some new matter with properties hand picked to make your existing theory work and changing those properties for each new observation raises eyebrows.

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I completely agree with the latter part of your post mgb, but regarding General relativity ...

and the referral to Eddington and Dyson in 1919. Their experiment and data was "plagued by systematic error and possibly confirmation bias", wanting to align properly with Einstein. I believe someone previously (another solar eclipse) had data stating Einstein was wrong, but was afraid of stating such. And I have problems with the GPS thing. I've heard that in the scheme of all variations and adjustments to correct GPS clocks, that GR's part is very LOW on the list and almost non existent, and certainly within the margin of error, (when talking about nanoseconds).

And the Hafele–Keating experiment? Why do we still reference a 37 year old experiment that can easily and more dramatically AND more accurately be redone today? I'm just asking, because "Louis Essen, the inventor of the atomic clock, published an article in which he discussed the (in his opinion) inadequate accuracy of the experiment." And I don't see any data published for the 25th anniversary duplication of this experiment?

We can fly higher, faster, with more accurate clocks and more accurate instruments. So let's do it already.

But I take us off track of the original question.

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