Astronomy · Comparing galaxies

Galaxy shapes and structural components

Compare structural classifications while testing how projection changes the appearance of the same ideal disk, without turning a classification diagram into an evolutionary sequence.

Continue in the approved chapter order. Necessary vocabulary, quantities and assumptions are explained in the lesson.

Back to the chapter

After reading, you can

Describe structural features used to distinguish broad galaxy morphologies.
Calculate a projected axis ratio under a declared thin circular-disk model.
Separate morphology, integrated colour and central activity as different observational descriptions.

Begin with what the image measures

Viewing another galaxy from outside gives access to patterns that are difficult to reconstruct within the Milky Way. The image is still a projection through a particular observing band and angular response. It does not directly show the three-dimensional distribution or all of the gravitating material.

Morphology describes observed structure: a flattened disk, a central concentration, an elongated bar, spiral patterns, a smooth light distribution or less regular features. Classification makes such comparisons manageable. Its labels organize observations without requiring every galaxy to fall on an exact physical boundary.

The image’s depth and resolution affect which features are recognized. A compact centre can blend with its surroundings, while faint extended light can disappear into background uncertainty. Comparing classifications therefore requires knowing whether the observations could reveal the same kinds of structure.

The broad types below are a starting vocabulary. A useful description can add several measured properties, and an uncertain or intermediate classification is a legitimate result rather than a reason to force a single label.

Broad types describe combinations of features

A spiral galaxy has a disk with recognizable spiral structure, often with a central bulge; some also have a bar. Bright young stars and gas can emphasize arms, but a spiral galaxy also contains older stars. An arm pattern is not the entire stellar population confined to one age.

A lenticular, or S0, galaxy retains a disk without obvious spiral arms. The prominence of its bulge, its dust and its stellar populations need their own descriptions. Treating every disk as a visibly armed spiral loses that distinction.

An elliptical classification concerns the form of the projected light distribution, generally smoother than obvious spiral structure. Its apparent outline can be round or elongated. Neither the label nor a smooth image establishes that every member star has a random orbit, that the galaxy has no rotation or that its intrinsic shape is uniquely known.

Irregular is used for systems that do not fit the simple regular shape classes. Interactions can produce irregular features, but irregular appearance is not proof of one recent merger. Faint systems and other varieties also broaden the classification scheme.

Change the viewpoint while holding the object fixed

For an original geometry example, take one infinitesimally thin circular outline of radius R. View it by orthographic projection, so all viewing rays are parallel. Define inclination i as the angle between the disk normal and the line of sight, with i = 0° face-on. There is no thickness, bulge, intrinsic oval, dust or instrumental blur in this construction.

Choose projected coordinates X = R cos θ and Y = R cos(i) sin θ, where θ labels position around the original circle. For 0° ≤ i < 90°, the projected semimajor axis is R, the semiminor axis is R cos(i), and their ratio is q = cos(i). The area enclosed by the projected outline is πR² cos(i).

At i = 0°, q = 1 and the enclosed projected area is πR². At i = 60°, q = 1/2 and that area is πR²/2. The physical disk has not evolved between these two hypothetical views. No surface-brightness or radiative-transfer law is supplied by the outline calculation.

The result does not make every projected ellipse an elliptical galaxy or give a unique inclination to an arbitrary observed system. Thickness, intrinsic shape, bars, dust and projection can complicate an actual deprojection. A round outline alone can also belong to a face-on disk whose other features need further evidence.

Original orthographic projections of the same thin circular outline. R is unchanged; i is measured from the disk normal to the line of sight. Area refers to the enclosed projected outline, not a change in physical disk area or emitted luminosity.
Inclination i	Semimajor axis/R	Semiminor axis/R	Axis ratio q	Enclosed projected area/(πR²)
0°	1	1	1	1
60°	1	1/2	1/2	1/2

One disk, two projected outlines

Scroll sideways to see the full diagram. Its explanation is below.

Original orthographic geometry, not galaxy observations or an evolutionary sequence. The same infinitesimally thin circular disk has radius R. Inclination i is the angle between its normal and the line of sight, where zero degrees is face-on. Its projected outline has semi-major axis a = R and semi-minor axis b = R cos(i). The left solid blue outline, with circle markers at four cardinal points, is face-on at i = 0 degrees: a/R = 1, b/R = 1, and q = b/a = 1. The right orange outline, with square markers at corresponding cardinal points, is at i = 60 degrees: a/R = 1, b/R = 1/2, and q = 1/2. Both main outlines share the same length scale. Dashed radial guides and dimension lines identify a and b. The lower edge-on reference views define the inclination angle: each line through the centre represents the disk plane. In the tilted side view the vertical dashed arrow points along the line of sight, and the green arrow gives the disk normal. These side views use their own separate common scale. In the face-on side view the normal and line of sight coincide in one green arrow. In the tilted side view, the angle from the line of sight to the normal is exactly 60 degrees and the disk line is perpendicular to the normal. The drawing has no disk thickness, bulge, intrinsic oval shape, dust, finite resolution or perspective. A projected ellipse does not by itself identify an elliptical galaxy, a unique inclination or a change of galaxy type. No surface-brightness, luminosity or galaxy-history inference is made.

A structural type is not a complete physical diagnosis

An integrated colour compares received light through specified bands. It is affected by the stellar mixture, attenuation and possible nonstellar contributions. A redder measured colour therefore does not uniquely identify an elliptical structure or an exact population age.

Skibba and collaborators’ Galaxy Zoo study distinguishes morphology from colour when comparing environments. Such work includes the possibility of red spiral systems and tests how a relation changes when another measured property is held fixed. A strong population correlation need not be a one-to-one classification rule.

Central activity supplies another descriptive axis. Compact emission associated with an active nucleus can occur within different host morphologies. A bright unresolved centre should be separated observationally and modelled before it is treated as the form or luminosity of the whole host.

The active-nucleus physics is developed later. Here the useful distinction is that host structure, colour and nuclear activity answer different observational questions. None of those labels alone supplies a unique black-hole mass, stellar formation history or complete account of the galaxy.

Compare galaxies without imposing one mandatory path

Historical early-type and late-type terminology is still used for broad morphological classes. Early and late in this vocabulary are not measured ages. The arrangement of classes in a classification diagram is not a required route from a round galaxy to a spiral.

Galaxies can change structure through several processes, and more than one history can produce similar present appearance. Conversely, different observing bands or viewpoints can make the same system look different. A proposed evolutionary interpretation needs evidence beyond placement in a shape scheme.

A careful comparison states the imaging band, depth, resolution, orientation information and classification rule, then adds independent colour, spectral and motion measurements. The result is a set of constrained properties with uncertainties, rather than an age assigned from a silhouette.

The next lesson examines the mixed stellar light that contributes to those colours and images. It shows why the parts contributing most luminosity need not contain most of the current stellar mass.

Worked examples

Apply the stated inclination convention

What axis ratio does the ideal circular outline have at i = 60°, and what has physically changed?

The stipulated orthographic thin-circle geometry gives q = cos(i).
At 60°, q = 1/2; the enclosed projected area is also one half of πR².
Only the viewing direction differs between the two constructions; the physical outline and radius are fixed.

The projected axis ratio is 1/2. No physical evolution, luminosity change or galaxy-type transformation follows.

Ask which evidence a round image lacks

Does a round projected outline by itself establish an elliptical galaxy rather than a face-on disk?

The same ideal circular disk is round when viewed face-on.
An actual classification also examines disks, spiral patterns, bars and the light distribution at adequate depth and resolution.
Intrinsic shape and orientation can remain uncertain even after a structural classification.

No. The outline alone is insufficient; additional structural and, where useful, kinematic evidence is needed.

Try the reasoning

Does early-type mean that a galaxy’s age has been measured to be younger than every late-type galaxy?
Show the answer and reasoning
No.
These are historical structural labels, not a universal age ordering.
Does recognizable spiral structure imply that the whole galaxy contains only young stars?
Show the answer and reasoning
No.
Young luminous tracers can emphasize arms while older stars also populate the system.
Does a red integrated colour uniquely determine morphological type?
Show the answer and reasoning
No.
Population mixtures, attenuation and other emission affect colour; colour and morphology are distinct descriptions.
Is irregular appearance alone proof of one recent merger?
Show the answer and reasoning
No.
Several structures, histories and observing limitations can contribute, so an interaction interpretation needs further evidence.

What to carry forward

Morphology organizes observed structural features under particular observing conditions.
Projection can alter the same object’s outline without changing its physical state.
Shape, colour, central activity and evolutionary history require distinct descriptions and evidence.

References

Teaching resources and research supporting this lesson’s explanations and examples.