Why does the Milky Way rotate?

February 11, 2015 by Elizabeth Howell, Universe Today
The Milky Way is a spiral galaxy with several prominent arms containing stellar nurseries swathed in pink clouds of hydrogen gas. The sun is shown near the bottom in the Orion Spur. Credit: NASA

We live in a galaxy that is called the Milky Way. It's called a barred spiral galaxy, which means that it has a spiral shape with a bar of stars across its middle. The galaxy is rather huge—at least 100,000 light-years in diameter, making it the second-biggest in our Local Group of galaxies.

More mind-blowing is that this mass of stars, gas, planets and other objects are all spinning. Just like a pinwheel. It's spinning at 270 kilometers per second (168 miles per second) and takes about 200 million years to complete one rotation, according to the National Radio Astronomy Observatory. But why? More details below.

It's worth taking a quick detour to talk about how long it takes the Solar System to move around the center of the galaxy. According to National Geographic, that's about 225 million years. Dinosaurs were starting to arise the last time we were in the position we are today.

Scientists have mapped the spin using the Very Large Baseline Array, a set of radio telescopes. They examined spots where stars were forming and paid particular attention to those areas where enhance radio emission, according to the National Radio Astronomy Observatory. Dubbed "cosmic masers", these areas shine brightly in radio waves.

As Earth moves in its orbit, the shift of these molecules can be mapped against more distant objects. Measuring this shift shows how the entire galaxy rotates—and can even provide information about the mass of the Milky Way. So that's all very neat, but why is it rotating in the first place?

If we think back to the early Universe, there are two big assumptions astronomers make, according to How Stuff Works: there was a lot of hydrogen and helium, with some parts denser than other areas. In the denser areas, gas clumped together in protogalactic clouds; the thickest areas collapsed into stars.

The rising Milky Way at Sentosa Island in Singapore. Credit and copyright: Justin Ng.

"These stars burned out quickly and became globular clusters, but gravity continued to collapse the clouds," How Stuff Works wrote. "As the clouds collapsed, they formed rotating disks. The rotating disks attracted more gas and dust with gravity and formed galactic disks. Inside the galactic disk, new formed. What remained on the outskirts of the original cloud were and the halo composed of gas, dust and dark matter."

A simpler way to think about this is if you're creating a pizza by tossing a ball of dough into the air. The spin of the dough creates a flat disc—just like what you observe in more complicated form in the Milky Way, not to mention other galaxies.

In this image from the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile young stars huddle together against a backdrop of clouds of glowing gas and lanes of dust. The star cluster, known as NGC 3293, would have been just a cloud of gas and dust itself about ten million years ago, but as stars began to form it became the bright group we see here. Clusters like this are celestial laboratories that allow astronomers to learn more about how stars evolve. Credit: ESO/G. Beccari

Explore further: Image: A Hubble sweep of the dust filaments of NGC 4217

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29 comments

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Tuxford
1 / 5 (6) Feb 11, 2015
Because it is actually a pinwheel. The core rotates and spits gas and stars toward the outskirts. And there is inconvenient data supporting this model. Just check the data.

http://phys.org/n...ays.html

jazzy_j_man
5 / 5 (3) Feb 11, 2015
"How Stuff Works wrote. "As the clouds collapsed, they formed rotating disks. "

THAT'S your answer??? This article reads like a high school report.
ombones1
5 / 5 (1) Feb 11, 2015
Jazzy, I agree with you. What kind of answer is that? They might just as well have said, "it rotates because it rotates."
Bob one
2.3 / 5 (3) Feb 11, 2015
Based on the known mass and velocities, the galaxies must be very young or they would have wound up on themselves.
altizar
not rated yet Feb 11, 2015
Same reason the earth rotates, and the same reason the sun rotates . . .
ztb
not rated yet Feb 11, 2015
"Just like a pinwheel. It's spinning at 270 kilometers per second (168 miles per second) ..."
Supposedly, this is the speed of Earth (and of our solar system) around the center of our galaxy. The speed of other star systems must be slower or faster depending on their distance from the core. Obviously, the speed of rotation at the core is almost zero if one rotation takes 200 million years. (Or have I missed something?)
IMP-9
4.5 / 5 (8) Feb 11, 2015
Based on the known mass and velocities, the galaxies must be very young or they would have wound up on themselves.


No. You run on the assumption that the arms rotate with the galaxy and that they were formed when the galaxy was. Spiral density waves explain how spirals can be stable over long periods. It's also backed observationally by pattern speed measurements.
cantdrive85
1.4 / 5 (9) Feb 12, 2015
Why does the Milky Way rotate?


Because the universe is pervaded by electrons and ions flowing in galactic sized Birkeland currents controlled by electromagnetic processes, rotation is a natural occurrence.

http://www.holosc...alaxies/

Same reason the earth rotates, and the same reason the sun rotates . . .


This is correct, as both the Sun and Earth are connected to these same electric Birkeland currents!
Stylz
not rated yet Feb 12, 2015
Totally unsatisfying answer. I had a guess, did a quick search and turns out my guess was right. If you are looking for the answer, it most certainly isn't in this "article".
abecedarian
5 / 5 (1) Feb 12, 2015
Why rotate? Same reason my kids spun around: it made things interesting and they couldn't help themselves.

What makes that possible is another discussion.

But apparently "How Stuff Works" thinks "A simpler way to think about this is if you're creating a pizza by tossing a ball of dough into the air. The spin of the dough creates a flat disc...."
Actually, making a pizza involves:
- pounding the dough flat on a table, then slapping the dough between your hands as you slowly rotate it so it stretches out, and there's often little holes that rip in random places,
- or, flatten the dough and shape what becomes the outer edges of the crust, then stretch it across your hands, spinning it occasionally so hopefully you get an even surface,
- or, roll it out in the shape of a disc, hoping the yeast doesn't make odd bubbles,
- or buy one already made.

But it's a humorous analogy suggesting the mass at the outer edges is spreading its insides outwards, which isn't happening.
Bob Osaka
not rated yet Feb 12, 2015
No one has mentioned the spin of the SMBH in the center? Gravitational interactions absolutely influence the distribution of objects with mass but the first question remains. Why does it spin?
antialias_physorg
3.3 / 5 (7) Feb 12, 2015
Why does it spin?

Conservation of angular momentum. Two galaxies rip past each other (or into each other) and you're going to get a spinning galaxy as a result.
ROBTHEGOB
not rated yet Feb 12, 2015
Interesting question. Here is another thought: everything we can see is rotating around the center of something - Earth, our sun, the galaxy, clusters of galaxies etc. Does this go on forever in space? It is a conundrum - if everything has a center of rotation, then does that logically imply that there is a center of the universe, though we cannot "see" it? And if that is so, then that would imply that the universe is of limited size, not infinite. I find that a bit of a troubling thought, though many people are OK with such a notion (the finite size of the universe). I will not lose any sleep over it, but it is a challenge to the human intellect, for sure.
antialias_physorg
4.3 / 5 (6) Feb 12, 2015
if everything has a center of rotation, then does that logically imply that there is a center of the universe

If space had one definite center then we'd see the afterglow of the Big Bang come from that direction, only. But we don't
The universe HAS a center and we're it (and also every other point in the universe). That's why we see the cosmic microwave background coming from every direction and not just from one direction. Space expanded. Matter didn't "explode" into an already existing space.
DavidTheShepherd
2.3 / 5 (6) Feb 12, 2015
"we'd see the afterglow of the Big Bang come from that direction, only. But we don't"

Well, this then also raises the issue as to whether the Big Bang is in fact a valid model, doesn't it?

Consider the fact that there is currently no explanation as to how exactly the first stars could form all by themselves.

In every current explanation for the formation of stars, an already existing star or stars is required.

There is no observational support for the theory that the first stars could or did form out of the hydrogen and helium gases that pervaded the "early" universe.

This then makes the whole Big Bang theory of really tenuous quality since it cannot even begin to explain the existence of the most basic of cosmological elements - the star.
Please don't flame me for this - I'm merely pointing out the current real and valid limitations of the BB model at present.Also, note that bringing dark matter and dark energy into doesn't add any value.
antialias_physorg
5 / 5 (5) Feb 12, 2015
Consider the fact that there is currently no explanation as to how exactly the first stars could form all by themselves.

Gravity? Seems to do the trick now. Why do you think it didn't do the trick back then?
In every current explanation for the formation of stars, an already existing star or stars is required.

I don't know where you get your 'current explanations'. But where I get them (science papers) that's not something I have ever heard. Supenovae CAN cause density changes in interstellar gas that push regions over the edge into collapsing. But the structure of the CMBR tells us that even in the very early stage the universe wasn't completely homogeneous - so collapse of regions was possible even back then.
DavidTheShepherd
3 / 5 (4) Feb 12, 2015
Anti-Alias,
Remember we're talking about gas here. So at what point does gravity lead to such high densities that it provides ignition?
What happens to a gas that is compressed, especially if there is no containing/constraining exterior force?

There is currently no explanation for that critical point - spontaneous ignition of a gas compressed by gravity alone.
Think of the problems people are experiencing with the fusion reactors at the moment and you'll begin to see the problem with gas being spontaneously ignited in empty space.
In your reading of science papers - did they not explain that supernovae are the result of existing stars? Where did the first star come from?
TopCat22
not rated yet Feb 12, 2015
This rotational observation would indicate that the Milky Way Galaxy has complete less than 67 rotations since the Big Bang 13.5 billion years ago based on the one rotation taking 200 million years.
antialias_physorg
5 / 5 (5) Feb 12, 2015
Remember we're talking about gas here. So at what point does gravity lead to such high densities that it provides ignition?

When the density is such that the gravitation force overcomes the electrostatic repulsion. This doesn't have to happen all at once (even with Supernovae shockwaves it's not like the shockwave itself pushes gas beyond this density).
It's completely adequate that a gas rich region will have some non-uniformity in it. Gravity does the entire rest with accretion over time once uniformity is disturbed.

What happens to a gas that is compressed, especially if there is no containing/constraining exterior force?

Gravity is a constraining force.
Notice how stuff like the air you breathe hangs around this planet? If gravity didn't keep it on it would dissipate.
antialias_physorg
5 / 5 (5) Feb 12, 2015
In your reading of science papers - did they not explain that supernovae are the result of existing stars? Where did the first star come from?

For the hard of reading: There are more mechanisms than just supernovae that cause fluctuations in density of a gas cloud. Looking at the CMBR it looks like the quantum fluctuations in the very early universe (right when inflation kicked in) did the trick.

Even if that weren't the case the finite extent of any gas cloud (positing that there are a finite number of atoms in the universe) would cause collapse (though complete, isotropic collapse in that case which would just create one supermassive black hole.)
reset
1.8 / 5 (5) Feb 12, 2015
Remember we're talking about gas here. So at what point does gravity lead to such high densities that it provides ignition?

When the density is such that the gravitation force overcomes the electrostatic repulsion. This doesn't have to happen all at once (even with Supernovae shockwaves it's not like the shockwave itself pushes gas beyond this density).
It's completely adequate that a gas rich region will have some non-uniformity in it. Gravity does the entire rest with accretion over time once uniformity is disturbed.

Gravity is a constraining force.
Notice how stuff like the air you breathe hangs around this planet? If gravity didn't keep it on it would dissipate.


Somebody needs a course on the properties of Hydrogen. Even in earths gravity look where it is and it's density profile. You didn't answer his question, you tried to dodge it with your unconfirmed theory.
It's hydrogens thermally conductive properties that sink your (Jeans) theory.
reset
2.1 / 5 (7) Feb 12, 2015
THe notion that a hydrogen nebula can overcome the temperature increase experienced during compression, so that it continues to increase in density until fusion is reached, using only it's own mass as the compression mechanism, is more of a fairy tale than the other 3 (DM,DE,BH's). Nobody who has ever worked with Hydrogen would agree this is possible, only someone who thinks math is science can make this claim and believe it. The hydrogen MUST arrive as ions in order to get enough of it to make a star without thermal repulsion forcing it all away from itself.
TechnoCreed
4.3 / 5 (6) Feb 12, 2015
The notion that a hydrogen nebula can overcome the temperature increase experienced during compression, so that it continues to increase in density until fusion is reached, using only it's own mass as the compression mechanism, is more of a fairy tale...

That is where you get it wrong because as temperature increases (as you rightly said) the law of entropy sets in. How entropy is going to operate on a collapsing nebula? Conduction?...no. Convection?...of course not. RADIATION? Yes, it is going to emit photons in the infrared spectrum (black body radiation) and is going to lose energy doing so. So it does not matter how much the piston exert pressure on the gas (the piston is gravity in this case) the equation of state will be kept at equilibrium (PV=nRT) since the temperature of the universe is 2.73 K the nebula is going to collapse until the pressure is high enough for nuclear fusion to take effect.
Hat1208
4.3 / 5 (6) Feb 12, 2015
@TechnoCreed

Well said. That is one of the most elegant statements of early star formation I have ever read
reset
1 / 5 (4) Feb 12, 2015
Technocreed - Same to you regarding the properties of Hydrogen, learn them. It is the most thermally conductive gas, so you have a hot dense portion where you believe it is hot enough and dense enough to initiate fusion surrounded by what? A slightly less dense cloud of a gas that absorbs and transmits heat to the atom next to it better than any other element...which makes it spread out. You have to think about what you are saying. It can only lose energy to the atom next to it, which heats it, which makes them move apart. There is no piston...the claim is that it can do this by itself with it's own mass....NOT PHYSICALLY POSSIBLE.
TechnoCreed
5 / 5 (3) Feb 12, 2015
You have to think about what you are saying. It can only lose energy to the atom next to it...
Sorry I could not make it any simpler; this is really undergrad basic physics. If you do not understand thermal radiation, or do not want to understand it, I cannot help you any further. https://www.youtu...iZyoYT80
cantdrive85
1 / 5 (4) Feb 13, 2015
You have to think about what you are saying. It can only lose energy to the atom next to it...
Sorry I could not make it any simpler; this is really undergrad basic physics. If you do not understand thermal radiation, or do not want to understand it, I cannot help you any further. https://www.youtu...iZyoYT80

Swell cartoon... When you can demonstrate in the lab hydrogen collapsing into fusion merely under the presence of gravity to create fusion then you will have a solid hypothesis, until then all you have is theoretical mumbo jumbo as usual.
Bob Osaka
5 / 5 (1) Feb 13, 2015
Dear antialias--physorg
Conservation of angular momentum. Two galaxies rip past each other (or into each other) and you're going to get a spinning galaxy as a result.

Not a very satisfying answer. Globular clusters thought to be remnants of past collisions have all but lost angular momentum. To bring the example closer to home, the direction of the orbits of planets in our solar system is governed by the spin of the central mass. Unless you're willing to argue the rotation of the planets around the sun is due to a collision with another solar system, your explanation will not hold water.
The spin of the SMBH at the center of the Milky Way is the central cause of its rotation.
adam_russell_9615
not rated yet Feb 14, 2015
The spiral pattern seems to suggest that the outer edges take longer to rotate than the center. Is that correct? Then saying it takes 200 million years to rotate is an incomplete statement.

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