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According to the Wikipedia page on Galaxy Types, there are four main kinds of galaxies:

  • Spirals - as the name implies, these look like huge spinning spirals with curved "arms" branching out
  • Ellipticals - look like a big disk of stars and other matter
  • Lenticulars - those that are somewhere in between the above two
  • Irregulars - galaxies that lack any sort of defined shape or form; pretty much everything else

Now, from what I can tell, these all appear to be 2D, that is, each galaxy's shape appears to be confined within some sort of invisible plane. But why couldn't a galaxy take a more 3D form?

So why aren't there spherical galaxies (ie: the stars and other objects are distributed within a 3D sphere, more or less even across all axes)? Or if there are, why aren't they more common?

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marked as duplicate by BMS, Kyle Kanos, Prahar, WetSavannaAnimal, JamalS Nov 26 '14 at 7:42

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ Is this a better fit for Astronomy.SE ? $\endgroup$ – paisanco Nov 23 '14 at 17:39
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    $\begingroup$ Related: physics.stackexchange.com/q/12140/2451 , physics.stackexchange.com/q/93830/2451 , and links therein. $\endgroup$ – Qmechanic Nov 23 '14 at 17:55
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    $\begingroup$ Possible duplicate of physics.stackexchange.com/q/93830 $\endgroup$ – Kitchi Nov 24 '14 at 5:37
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    $\begingroup$ Just a reminder to everyone (e.g. @paisanco) that astronomy questions are on topic here as stated in the help center. $\endgroup$ – David Z Nov 25 '14 at 15:33
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    $\begingroup$ The "duplicate question" asks how is it possible to get disk galaxies? This question asks why aren't there spherical galaxies? The answer there (which I linked to) is an excellent summary of how disk galaxies form. It doesn't provide any sort of answer to this question. So why mark it as a duplicate? $\endgroup$ – Rob Jeffries Nov 26 '14 at 10:27
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All matter in the galaxy has to rotate (not necessarily in the same direction) so that a centrifugal force acts. Without the centrifugal force, all matter contained in the galaxy will collapse into the center of the galaxy due to gravitation. The rotation happens about an axis, a line about which all matter revolves in the galaxy. Now, the manner in which all the matter revolves around that axis is planar. Why is it planar and why does it have to rotate about an axis only? The answer to this question will decisively clear that doubt.

But how does the planar galaxy continue to retain planarity for billions of years?

Let's imagine that a planar galaxy has a few bodies which don't revolve around the central axis and have their own axis of rotation. In any direction perpendicular to that axis, centrifugal force keeps the body from collapsing into the center of the galaxy. In any direction parallel to that axis however, there is no such centrifugal force; but there is a component of the gravitational force from the matter contained in the planar galaxy below. This component of gravitational force keeps pulling the body toward the plane, and there is no force to stop it. Thus, even this body will eventually join the galactic plane. All such fringe bodies which do not obey to the galactic plane will be attracted by gravity to eventually join the plane. Therefore the galaxy manages to maintain planarity.

As Rob Jeffries pointed out, there are galaxies that are of spherical and other three-dimensional shapes. There, however, since there is no pre-existing plane of rotation, nothing is causing the matter to collapse into a plane. Therefore, those galaxies retain their three-dimensional shape.

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    $\begingroup$ In principle, zero total angular momentum doesn't mean that all the matter just moves radially. Different bodies may compensate angular momenta of others, so the system could be (somewhat) stable. $\endgroup$ – Ruslan Nov 23 '14 at 19:07
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    $\begingroup$ This answer is just plain incorrect, on an individual star basis it's true that each star needs some angular momentum, but the galaxy as a whole does not need a net angular momentum to be stable. In reality every galaxy, even those that appear nearly spherical, have some amount of angular momentum, but some galaxies can be really darn close to spherical. $\endgroup$ – Guillochon Nov 23 '14 at 20:52
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    $\begingroup$ Collision rate per star is number density * cross section * velocity, with typical numbers we have 1 pc^-3 * r_sun^2 * 200 km/s ~ 10^-9 per star over the age of the universe. So only one billionth of stars have a collision in a typical galaxy. This has no meaningful effect. $\endgroup$ – Guillochon Nov 24 '14 at 3:48
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    $\begingroup$ @Ruslan: The OOrt cloud is such an example. $\endgroup$ – dotancohen Nov 24 '14 at 8:31
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    $\begingroup$ This answer fails to correct the OP's misconception that there are no spherical galaxies. $\endgroup$ – Rob Jeffries Nov 24 '14 at 9:10
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This whole question is a mistaken premise. There are spherical (or at least nearly spherical) galaxies! They fall into two basic categories - those elliptical galaxies that are pseudo-spherical in shape and the much smaller, so-called "dwarf spheroidal galaxies" that are found associated with our own Galaxy and other large galaxies in the "Local Group".

Of course when you look at a galaxy on the sky it is just a two dimensional projection of the true distribution, but one can still deduce (approximate) sphericity from the surface brightness distribution and large line of sight velocity distribution for many ellipticals and dwarf spheroidals.

Dwarf spheroidal galaxies may actually be the most common type of galaxy in the universe.

These galaxies are roughly spherical because the stars move in orbits with quite random orientations, many on almost radial (highly eccentric) orbits with no strongly preferred axes. The velocity dispersion is usually much bigger than any rotation signature.

There is an excellent answer to a related question at Why the galaxies form 2D planes (or spiral-like) instead of 3D balls (or spherical-like)?

Pretty pictures: UK Schmidt picture of the Sculptor dwarf spheroidal galaxy (credit: David Malin, AAO)

UK Schmidt picture of the Sculptor dwarf spheroidal galaxy (credit: David Malin, AAO)

The E0 elliptical galaxy M89 (credit Sloan Digitized Sky Survey).

E0 galaxy M89. (Credit SDSS)

Details: I have found a couple of papers that put some more flesh onto the argument that many elliptical galaxies are close-to spherical. These papers are by Rodriquez & Padilla (2013) and Weijmans et al. (2014). Both of these papers look at the distribution of apparent ellipticities of galaxies in the "Galaxy Zoo" and the Sloan Digitized Sky Surveys respectively. Then, with a statistical model and with various assumptions (including that galaxies are randomly oriented), they invert this distribution to obtain the distribution of true ellipticity $\epsilon = 1- B/A$ and an oblate/prolate parameter $\gamma = C/A$, where the three axes of the ellipsoid are $A\geq B \geq C$. i.e. It is impossible to say whether a circular looking individual galaxy seen in projection is spherical, but you can say something about the distribution of 3D shapes if you have a large sample.

Rodriguez & Padilla conclude that the mean value of $\epsilon$ is 0.12 with a dispersion of about 0.1 (see picture below), whilst $\gamma$ has a mean of 0.58 with a broader (Gaussian) dispersion of 0.16, covering the whole range from zero to 1. Given that $C/A$ must be less than $B/A$ by definition, this means many ellipticals must be very close to spherical (you cannot say anything is exactly spherical), though the "average elliptical" galaxy is of course not.

This picture shows the observed distribution of 2D ellipticities for a large sample of spiral and elliptical galaxies. The lines are what you would predict to observe from the 3D shape distributions found in the paper.

Distribution of observed ellipticities of spirals and ellipticals.

This picture from Rodriguez and Padilla show the deduced true distributions of $\epsilon$ and $\gamma$. The solid red line represents ellipticals. Means of the distributions are shown with vertical lines. Note how the dotted line for spirals has a much smaller $\gamma$ value - because they are flattened. Distributions of gamma and epsilon

Weijmans et al. (2014) perform similar analyses, but they split their elliptical sample into those that have evidence for significant systematic rotation and those that don't. As you might expect, the rotating ones look more flattened and "oblate". The slow-rotating ones can also be modelled as oblate galaxies, though are more likely to be "tri-axial". The slow rotators have an average $\epsilon$ of about 0.15 and average $\gamma$ of about 0.6 (in good agreement with Rodriguez & Padilla), but the samples are much smaller.

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    $\begingroup$ Congrats on your populist badge for this excellent answer, @RobJeffries. $\endgroup$ – David Hammen Jan 24 '15 at 16:31
  • $\begingroup$ There are also some older papers in the 1990ies which deduce the intrinsic shape distribution of ellipticals from their projected shapes. As far as I can remember, these studies easily rule out very flattened (=2D) shapes but require some degree of triaxility. $\endgroup$ – Walter May 31 '15 at 17:17
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Actually, there are parts of a galaxy that extend beyond the galactic plane:

  • Galactic halo: This is actually the primary part of a galaxy that is not in the main galactic disk. It's made up of multiple sections, and is composed or an array of objects.

    • Dark matter halo: This is a section of the galaxy's dark matter that exists in a semi-spherical shape. We can figure out the size and shape of the halo (though it's typically spherical) through its effects on the large-scale motion of stars.

    • Galactic spheroid: This is a region near the center of the galaxy made up of stars with odd orbits. I think of them as sort of like the comets in the Kuiper Belt - following odd, 3D orbits. The stars might have been perturbed by the central black hole in the galaxy - in our case, Sagittarius A*.

    • Galactic corona: Bits of gas and dust that follow irregular paths across the galaxy. They interact with matter inside the galactic disk and thereby oscillate around.

  • Galactic bulge: This is the central part of the galaxy, around the central supermassive black hole. They're composed of gas, stars and dust.
  • Stellar stream: A series of stars that have interacted with another object gravitationally. They may be the remnants of a dwarf galaxy.

I list these as examples to show that not all objects stay in the galactic plane. The other answers should give you an idea of why most objects do stay in the plane.

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It is due to the combined effect of rotation and "dissipation". A rotating cloud of gas consists of particles which interact strongly with each other (colliding physically) on relatively short timescales can radiate away some of their energy and momentum by emitting photons. For both of these reasons, a dense cloud of rotating gas will collapse to form a rotating disk. But there are some star systems that do remain pretty spherical, they called globular clusters.

On the other hand, if the gas in a cloud forms stars very quickly, so that the particles in it are stars rather than atoms, then these stellar "particles" do not interact strongly on short timescales (for instance the time between direct collisions for a star in a globular cluster is > $10^{10}$ years, and globular clusters are pretty much spherical) can not radiate away their energy and momentum by emitting photons; they can emit gravitational radiation, but that's not as effective

For these reasons, a spherical cluster of stars will remain spherical for very long periods of time; much longer than the current age of the universe.

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  • $\begingroup$ The only practical form of “radiating” dynamical energy out a (wide) system of stars is ejection of members. $\endgroup$ – Incnis Mrsi Nov 24 '14 at 17:00
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You mentioned elliptical galaxies, which the other answers haven't touched upon.

Contrary to your statement about the galaxies being 2D, elliptical galaxies are "3 dimensional" in the sense that the stars are not confined to one plane; You could think of them as being "egg shaped".

So why are elliptical galaxies not confined to a plane? Mostly because they (usually) have low angular momentum, i.e., they aren't spinning too fast about any axis, so the reasoning in Simha's answer no longer applies.

Also, it's worth noting that this doesn't mean that there is no spinning motion. The galaxy as a whole need not be rotating about an axis, but the stars within the galaxy will be. The stars will all be moving around fairly randomly within the elliptical galaxy, so the net angular momentum is near zero.

In contrast to a spiral galaxy, the galaxy as a whole has a very definite angular momentum, so in addition to the stars own velocity (often called it's peculiar velocity) the star will be rotating about the galactic centre in the same direction as it's neighbours, and indeed every other star in the galaxy.

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