Learn · Other galaxies
Galaxy types — shape and viewing angle
A galaxy is not a single nebula but a system of a hundred billion stars, of gas, dust and dark matter. A photograph superimposes two things: its spatial structure and the angle from which we happen to see it. Separating the two lets you read far more from an image — and claim less that is wrong.
Hubble's tuning fork
In 1926 Edwin Hubble sorted galaxies by appearance: on the left the smooth ellipticals from round (E0) to strongly flattened (E7), on the right the spirals, split into two prongs — ordinary spirals (Sa to Sc) and barred spirals (SBa to SBc). At the junction sit the lenticular S0 galaxies: a disk without recognisable arms. Whatever fits no scheme is called irregular. From Sa to Sc the central region shrinks while the arms open up and become knottier. Gérard de Vaucouleurs later refined the scheme with intermediate stages (SAB for weak bars, rings, the classes Sd and Sm). Hubble (1926): Extra-galactic nebulae
What lies behind the shapes
Behind the classification stand two different ways in which stars hold their galaxy together. In a disk all stars move in the same direction on nearly circular orbits — rotation supports the system, just as it does for the planets around the Sun. That is why a disk is thin: its stars have little motion perpendicular to the plane. In an elliptical galaxy, by contrast, the stars fly every which way, each on its own, often highly eccentric orbit. The system is held up against its own gravity not by rotation but by this disordered motion — much as gas pressure supports a star. Such systems are round or ellipsoidal, but never flat like a disk.
What the galaxies contain follows from this. Disks have cold gas and dust from which stars keep forming; their arms therefore glow blue and are dotted with red H II regions. Elliptical galaxies have largely lost or used up their cold gas. They consist mostly of old stars and glow yellowish to reddish — not because they are cool, but because the hot, blue, short-lived stars burnt out long ago. How these two blueprints arise is the subject of the article on galaxy evolution.
Bars: the rule, not the exception
A bar is no foreign body but an instability of the disk itself: when enough stars travel on elongated orbits that reinforce one another, a rotating rod of stars forms through the centre. In the near infrared, where dust hardly interferes, about two thirds of all spiral galaxies show a bar — our own Milky Way is a barred spiral of type SBbc. Bars matter because they redistribute gas: gas loses angular momentum along the flanks of the bar and streams inward, where it feeds rings of star formation or a central black hole. That is exactly what is being studied in NGC 6951, whose small nuclear ring of young stars lies only a few hundred parsecs from the centre. In our photograph that ring is not resolved; the bar itself shows as the elongated bright middle from which the arms take off. ESA/Hubble: NGC 6951
Perspective changes the portrait
A disk looks completely different depending on its tilt. Seen face-on it is a circle, at an angle an ellipse, edge-on a streak. Because a stellar disk is very thin compared to its diameter, the axis ratio of the ellipse gives away the inclination almost directly: the short axis divided by the long axis is roughly the cosine of the inclination. A galaxy that appears half as wide as it is long is tilted by about 60 degrees. Only very close to edge-on does the rule break down, because the disk then shows its own thickness — in NGC 891 we see exactly that thickness, complete with the dust lane that splits the disk lengthwise.
Inclination also decides what becomes visible in the image at all. At low inclination the arms lie spread out before us, but the dust lane hardly stands out because we look through the thin disk. At high inclination the arms pile on top of one another and become impossible to untangle — but the dust of the disk pushes itself in front of the starlight as a dark band, because we now look lengthwise through thousands of light-years of disk. NGC 891 is therefore regarded as the picture our Milky Way would present from outside: similar size, similar type, edge-on. That the Milky Way itself runs across our sky as a band with dark rifts is the same geometry seen from within. Hubble: NGC 891 edge-on
What colour reveals — and what it does not
Bright blue areas in the arms come from young, massive stars that live only a few million years; where they shine, star formation is under way. Pink knots are H II regions — hydrogen made to glow by such stars. A yellowish central region contains mostly older stars, from which the blue ones are long gone. So far the rule of thumb from the article on stellar lives holds. But a photograph is not a measurement: dust reddens the light of the stars behind it, filters and processing shift colours, and a broadband image cannot tell a star of ten million years from one of a hundred million. Age maps and star-formation rates need spectra and multi-colour photometry. The image shows where something is happening — not how much. Hubble: Galaxies
AI-generated illustration: from left, spiral galaxy, barred spiral, lenticular and elliptical galaxy. The forms show neither a size sequence nor an evolutionary timeline. Irregular galaxies are not shown here. This image is AI-generated — not a photograph but a visualisation.
A few things worth knowing:
- “Early” and “late” types are a historical error. Ellipticals are still called “early type” and spirals “late type” because it was once believed that galaxies drift from left to right through the tuning fork as they age. If anything the opposite is true: when spirals merge, an elliptical is often the result.
- Dwarfs are the majority. The conspicuous large spirals and ellipticals are a numerical minority. Most galaxies are faint dwarf galaxies — in the Local Group, three large spirals are accompanied by dozens of dwarfs such as M 32 and M 110.
- Classification is a matter of judgement. Assigning Sa, Sb or Sc is done by eye and depends on viewing angle, exposure and wavelength. The citizen-science project Galaxy Zoo therefore had hundreds of thousands of galaxies classified independently by many people. Galaxy Zoo
- The Milky Way is a barred spiral. Type SBbc, with a bar some tens of thousands of light-years long that we can detect from the Sun only indirectly — through the motions and distribution of stars in the centre.
- Lenticular galaxies are not “spirals without arms”. They have a disk but hardly any gas left. Many of them are probably spirals that lost their gas in galaxy clusters — more on that in the article on galaxy groupings.
In my own sky
The gallery by now covers a good part of the tuning fork — and, above all, almost every inclination:
