# Does the Milky Way move through space?

Does our galaxy moves through space? Or does it stay in a single location? If it does move, what causes it to move?

• Check out the articles about Galaxy Clusters and the Great Attractor, they may be of interest to you. Sep 12, 2017 at 20:41
• Only if you want to use something besides our galaxy's origin as your frame of reference. You could structure any maths to accurately describe our galaxy as the ONLY stationary one. It just isn't very modest to do so. Sep 12, 2017 at 21:07
• "Through space" is not a thing. Space doesn't have locations. Motion is always relative to something else, like another galaxy, or the Cosmic Microwave Background, or whatever. You've already received some answers for the relative motion, see below. Sep 12, 2017 at 23:30
• I have a term that you might be interested in to Google: laniakea Sep 13, 2017 at 7:22
• In Soviet Union, space moves through Milky Way! Sep 13, 2017 at 7:48

Does the Milky Way move through space?

Yes it does.

I'm very fascinated with space, although I don't have a degree or any formal education, I'm still very in love with everything about it and want to learn constantly.

Good man Mike.

One thing I ask myself is if our galaxy moves through space?

It does. When we look at the Cosmic Microwave Background Radiation we see a "dipole anisotropy" due to the motion of the Earth relative to it:

Image courtesy of William H. Kinney's Cosmology, inflation, and the physics of nothing

See Wikipedia for more:

"From the CMB data it is seen that the Local Group (the galaxy group that includes the Milky Way galaxy) appears to be moving at 627±22 km/s relative to the reference frame of the CMB (also called the CMB rest frame, or the frame of reference in which there is no motion through the CMB) in the direction of galactic longitude l = 276°±3°, b = 30°±3°.[82][83] This motion results in an anisotropy of the data (CMB appearing slightly warmer in the direction of movement than in the opposite direction).[84]"

627 km/s is quite fast. See this article, which says it's 1.3 million miles an hour. The speed of light is just under 300,000 km/s or 670 million miles per hour, so the Milky Way is moving through the Universe at circa 0.2% of the speed of light. Also see the CMBR physics answer by ghoppe which talks about the CMBR reference frame, which is in effect the reference frame of the universe.

Or does it stay in a single location? If it does move, what causes it to move?

I'm afraid I don't know why it's moving. Perhaps it's because the Universe is full of things moving in fairly random directions. Like a gas.

Hopefully the question makes sense, if not I can elaborate.

It certainly makes sense to me!

Edit 13/09/2017 : as Dave points out in the comments, there are other motions, including the motion of the solar system around the galaxy, which is circa 514,000 mph. (See the Wikipedia Galactic Year article). And the motion of galaxies isn't neat and tidy either.

• Awesome response! There's a ton for me to look-up and read about. I really appreciate your time/explanation.
– Mike
Sep 12, 2017 at 19:29
• "Why is it moving?" It would be an incredible coincidence if it were perfectly still. There are uncountable numbers of objects in the universe exerting gravitational attractions in different directions. They would have to exactly balance out the for the galaxy not to move. Sep 12, 2017 at 20:59
• @Barmar : good point. Why didn't I think to say that? Duh! Sep 12, 2017 at 21:13
• Well, at least everything continues to move (how it all started to is still up for debate I suppose ;) because stuff is still hot. When everything gets cold, absolutely nothing will move anymore.... supposedly: heat death of the universe. To put it simply as I understand it, eventually once all the atoms in the universe equalize in temperature, no work can be done anymore because there will be no potential energy left anywhere to do any work. Or something like that. Sep 12, 2017 at 23:15
• @Barmar: From "A brief history of time" : Newton realized that, according to his theory of gravity, the stars should attract each other, so it seemed they could not remain essentially motionless. Would they not all fall together at some point? and We now know it is impossible to have an infinite static model of the universe in which gravity is always attractive. Sep 13, 2017 at 9:58

Galaxies move through space with velocities of the order of a several 100 km per second; small velocities for small groups (~100 km/s; e.g Carlberg et al. 2000) and large velocities for rich clusters (~1000 km/s; e.g Girardi et al. 1993).

In addition to this so-called "peculiar velocity", galaxies also also carried away from each other due to the expansion of the Universe, at a velocity proportional to the distance from each other (the "Hubble flow"). But this is not a motion through space; rather it is space itself that is expanding (and hence the velocity may exceed the speed of light for sufficiently large distances).

One may define a "global reference frame" with respect to which velocities are measured. Any reference is valid, but it makes sense to use the frame in which all galaxies are, on average, in rest (when the Hubble flow is subtracted)$$^\dagger$$. In this frame, the Local Group that the Milky Way is a part of moves with some $$620\, \mathrm{km \, s}^{-1}$$ (as noted in John Duffield's answer above), whereas the center of the Milky Way has a velocity$$^\ddagger$$ of $$\mathbf{565 \pm 5 \, km \, s^{-1}}$$ (Planck Collaboration et al. 2020).

What causes this movement of galaxies? Galaxies that are not too far from each (i.e. closer), "feel" each others mutual gravitational forces. A galaxy in a group or cluster moves around in the common gravitational field, but for galaxies that are farther away, the Hubble flow carries them away from each other too fast for them to attract each other.

This movement can be traced back to the tiny quantum mechanical fluctuations in the primordial soup of particles during cosmic inflation, i.e. less than $$\sim10^{-32}\,\mathrm{s}$$ after the Big Bang. As time went by, ever-so-slight overdensities grew in amplitude, until they collapsed to form the structure we see in the Universe today. During this collapse, matter grew turbulent, whirling clumps around that eventually became the galaxies that orbit each other.

$$^\dagger$$Formally, one uses the frame in which the cosmic microwave background is isotropic.

$$^\ddagger$$Taking into account our motion around the Galactic center, our Sun (currently) moves through space at $$369.82\pm0.11\,\mathrm{km\, s^{-1}}$$.

• I'm in awe by the intelligence of you guys. I had to read your response a few times to barely grasp what you are saying. But none nonetheless, I'm going to be researching/learning about everything you are talking about. (which I'm sure that will raise more questions, lol)
– Mike
Sep 12, 2017 at 19:33
• @Mike It's less a question of intelligence and more one of experience and study. Most things in life are like that. Sep 13, 2017 at 0:20
• I second that, @jpmc26!
– pela
Sep 13, 2017 at 6:31

Simply put: The important thing about the motion of our Milky Way in the Universe is simply gravitational attraction among galaxies and clusters of galaxies that causes movement. At a small scale we are attracted to the Andromeda galaxy and are on a “collision” course with it (Of course we won’t collide because galaxies are way too diffuse so to speak). The most important gravitational attractions that cause our Galaxy to move even faster is the Great Attractor (a large collection of galaxies) and the much more awe-inspiring Shapley Supercluster of galaxies. It looks like these are the major clusters of galaxies to which our Milky Way Galaxy is gravitationally attracted, and it causes our Galaxy to move at around 600 km/sec with reference to the microwave background radiation. This background radiation or CMB (cosmic microwave background) is the absolute reference against which to measure velocities in our Universe. The CMB is the remnant of and the proof of the Big Bang event.

And as was mentioned before, the space expands in our universe producing the effect of galaxies (that are far away for each other) to look like they run away from each other very fast. It holds only for galaxies that are very far away in space. That’s because space expands like a rubber band with most distant points moving away from each other faster. In other words, galaxies that are extremely far away from each other, well, they run away from each other almost with the speed of light or even faster! No, they are not moving faster than the speed of light – it’s just that space in the universe expands very fast between two very distant points, even faster than the speed of light! It is just like the further apart the two points are on the elastic rubber band the faster they will move apart when we extend the band. The difference here is that our three-dimensional space acts like a rubber band if it is a three-dimensional space we live in after all (here's a lot to be discovered in the future, I guess)?!

• I really love this response, the analogies really helped... thank you!
– Mike
Sep 13, 2017 at 3:52
– user18491
Sep 13, 2017 at 17:26

NO, if don't want to consider it to be doing so.

Everything in the universe only "moves" when considered from a different frame of reference. From the observational frame of reference of the Milky Way galaxy, every other object in the universe is moving, and the Milky Way is stationary.

When I walk to work, what I am actually doing is dragging the surface of the Earth, and everything on it, towards me with my feet, until my office arrives.

• man, no wonder I need coffee when i get to the office.. Sep 13, 2017 at 11:42
• What about someone else walking to work from the opposite direction? Don't the two of you cancel each other out? Sep 13, 2017 at 19:17
• That's certainly how it feels today. Maybe I need to reset my reference frame. Or find a better office! Sep 13, 2017 at 21:31
• @Barmar: Both people think that they're stationary, that the universe is moving towards them at 5km/h and the other person is walking towards them at 10km/h. Sep 14, 2017 at 7:58
• Einstein is reputed to have asked a ticket inspector, "Does Crewe stop at this train?". Sep 14, 2017 at 8:12

Monty Python does not seem very accurate in scientific issues, but this time it does it's job quite well in the Galaxy Song also mentioning the milky ways movement.

Just remember that you're standing on a planet that's evolving
And revolving at nine hundred miles an hour,
That's orbiting at nineteen miles a second, so it's reckoned,
A sun that is the source of all our power.
The sun and you and me and all the stars that we can see
Are moving at a million miles a day
In an outer spiral arm, at forty thousand miles an hour,
Of the galaxy we call the 'Milky Way'.
Our galaxy itself contains a hundred billion stars.
It's a hundred thousand light years side to side.
It bulges in the middle, sixteen thousand light years thick,
But out by us, it's just three thousand light years wide.
We're thirty thousand light years from galactic central point.
We go 'round every two hundred million years,
And our galaxy is only one of millions of billions
In this amazing and expanding universe.

The universe itself keeps on expanding and expanding
In all of the directions it can whizz
As fast as it can go, at the speed of light, you know,
Twelve million miles a minute, and that's the fastest speed there is.
So remember, when you're feeling very small and insecure,
How amazingly unlikely is your birth,
And pray that there's intelligent life somewhere up in space,
'Cause there's bugger all down here on Earth.


And yes, these figures are accurate: (at the time of publish) https://en.wikipedia.org/wiki/Galaxy_Song#Accuracy_of_astronomical_figures

• But, just to be a killjoy, it doesn't answer the question. Sep 13, 2017 at 14:14
• "We're thirty thousand light years from galactic central point.We go 'round every two hundred million years," is a clear "YES" IMHO for the "Milky Way is moving" topic Sep 13, 2017 at 14:39
• Of course it isn't. It is a clear statement that the Milky Way is a rotating disc of which we are a part. Rotation and translation (moving thru space) are two entirely different things. Sep 13, 2017 at 15:51
• True. All the motions mentioned in the song are rotations, except for the expansion of the universe. And giving a single speed for the latter is wrong. Sep 14, 2017 at 15:00