Starship Asterisk*

Universe Today wrote:
“The Sombrero is more complex than previously thought,” said Dimitri Gadotti of the European Southern Observatory in Chile and lead author of the report. “The only way to understand all we know about this galaxy is to think of it as two galaxies, one inside the other.”

Although it might seem that the Sombrero is the result of a collision between two separate galaxies, that’s actually not thought to be the case. Such an event would have destroyed the disk structure that’s seen today; instead, it’s thought that the Sombrero accumulated a lot of extra gas billions of years ago when the Universe was populated with large clouds of gas and dust. The extra gas fell into orbit around the galaxy, eventually spinning into a flattened disk and forming new stars.

Nitpicker wrote:Is the term "egg-like" used because describing the shape of an elliptical galaxy as "elliptical" is self-referential?

What's wrong with "oval"?

Nobody mention footballs, please.

DesertNative wrote:Over time, will the random movement of stars in an elliptical galaxy ever resolve into a common movement like spiral galaxies? If not, why not?

Chris Peterson wrote:

Nitpicker wrote:

Chris Peterson wrote:Maybe the best way to think of all galaxies is as "oddly and unevenly squished spheroids".

I'm certainly not thinking a perfect oblate spheroid, but I think a perfect oblate spheroid would probably enclose it. I'm failing to imagine the actual sectional shape when "orbits with small galactic radiuses [radii] will show more range than those with large ones". I don't quite get your meaning there.

Well, take a spiral galaxy as an extreme case. It's basically an intersection of a sphere and a disk. There are components that are in orbits in the disk structure, and components in orbit in the sphere structure. In sagittal section, this is obviously not elliptical. I think that an elliptical galaxy may maintain some of that discontinuity, with large radius orbits being slightly flatter than you'd expect for an elliptical cross-section, and inner orbits being somewhat more inclined.

Ann wrote:

Chris wrote:
As I hope I made clear, I only use "disk" loosely to describe a shape that is circular on one axis and extended along the perpendicular- either a small amount, meaning that the object is quite disk-like (producing a cigar shaped galaxy when viewed at an angle) to spherical (producing a circular galaxy when viewed from any angle). An elliptical galaxy is very similar to the bulge of a spiral galaxy. Over time, any flat galaxy will evolve to a spherical shape, since that's what large, gravitationally bound particle systems do.

Is the argument that I've highlighted in color the same as saying that every circle is an ellipse, only a special case of it? In other words, are all stable galaxies disk galaxies, where the perfectly spherical ones are just special cases of disks?

Nitpicker wrote:

Chris Peterson wrote:Maybe the best way to think of all galaxies is as "oddly and unevenly squished spheroids".

I'm certainly not thinking a perfect oblate spheroid, but I think a perfect oblate spheroid would probably enclose it. I'm failing to imagine the actual sectional shape when "orbits with small galactic radiuses [radii] will show more range than those with large ones". I don't quite get your meaning there.

Ann wrote:

Chris wrote:
I only use "disk" loosely to describe a shape that is circular on one axis and extended along the perpendicular- either a small amount, meaning that the object is quite disk-like (producing a cigar shaped galaxy when viewed at an angle) to spherical (producing a circular galaxy when viewed from any angle).

Is the argument that I've highlighted in color the same as saying that every circle is an ellipse, only a special case of it?

Chris wrote:
As I hope I made clear, I only use "disk" loosely to describe a shape that is circular on one axis and extended along the perpendicular- either a small amount, meaning that the object is quite disk-like (producing a cigar shaped galaxy when viewed at an angle) to spherical (producing a circular galaxy when viewed from any angle). An elliptical galaxy is very similar to the bulge of a spiral galaxy. Over time, any flat galaxy will evolve to a spherical shape, since that's what large, gravitationally bound particle systems do.

Chris Peterson wrote:Maybe the best way to think of all galaxies is as "oddly and unevenly squished spheroids".

Nitpicker wrote:It makes me think that galaxies have perhaps been over-classified, or at least that their classifications are over-emphasised.

From now on, I'm thinking of all of them as oblate spheroids of some kind. It works for me.

There are four distinct cases of which one is degenerate:

a>b>c — tri-axial or (rarely) scalene ellipsoid;
a=b>c — oblate ellipsoid of revolution (oblate spheroid);
a=b<c — prolate ellipsoid of revolution (prolate spheroid);
a=b=c — the degenerate case of a sphere;

Mathematical literature often uses 'ellipsoid' in place of 'tri-axial ellipsoid'. Scientific literature (particularly geodesy) often uses 'ellipsoid' in place of 'ellipsoid of revolution' and only applies the adjective 'tri-axial' when treating the general case. Older literature uses 'spheroid' in place of 'ellipsoid of revolution'.

Any planar cross section passing through the center of an ellipsoid forms an ellipse on its surface: this degenerates to a circle for sections normal to the symmetry axis of an ellipsoid of revolution (or all sections when the ellipsoid degenerates to a sphere.)

Nitpicker wrote:

Chris Peterson wrote:Ellipsoids are characterized by having every cross-section through their center creating an ellipse.

That is only the case for scalene (or triaxial) ellipsoids.

Ann wrote:

Chris wrote:
Neither is it clear that any elliptical galaxies are ellipsoidal. They are called "elliptical" because they appear, in two dimensions, similar to ellipses. In fact, most are disks, which may be nearly flat or could be expanded out nearly to spheres. That does not produce an ellipsoidal shape.

Interesting, Chris. It is just possible that I'm more of a galaxy nerd than you are, and I haven't heard the claim that most elliptical galaxies are disks.

My impression is that when I look at galaxy clusters full of elliptical galaxies, the largest galaxies always appear to be (fairly) round. If some of them are disk galaxies, we ought to see some of the largest ones as elongated disks. Yet we don't, in my opinion. So may I inquire about your sources that say that most elliptical galaxies are disks?

Chris Peterson wrote:Ellipsoids are characterized by having every cross-section through their center creating an ellipse.

Nitpicker wrote:I use the term "ovoid" as the 3-D version of "oval". No implication of a taper, but no implication that there isn't, either. I maintain this is the more common (and certainly the more general) definition.

Ellipsoids may be scalene, oblate, prolate, or spherical. It is only a scalene ellipsoid that always yields non-circular axial sections. The others can all yield circular sections. Based on other definitions I've just read of elliptical galaxies, it seems to me that they typically form something roughly approximating an oblate ellipsoid (a.k.a. an oblate spheroid).

Chris wrote:
Neither is it clear that any elliptical galaxies are ellipsoidal. They are called "elliptical" because they appear, in two dimensions, similar to ellipses. In fact, most are disks, which may be nearly flat or could be expanded out nearly to spheres. That does not produce an ellipsoidal shape.