Modern Description of Spiral Structure

Possibly related to the “luminosity class” for galaxies is the fact that some galaxies have well defined arms, while other spirals have poorly defined or even disorganized arms. Galaxies with well defined and highly organized spiral structure are referred to as “grand design” galaxies. At the other extreme are the “flocculent” galaxies, which clearly look like spirals but which have only short stubby segments of spiral features. “Flocculent” refers to the “fleecy” appearance of the spiral-like structures. “Multiple arm” galaxies are, perhaps, intermediate and show characteristics of both grand design and flocculent types.

grand design
galaxies with long, symetric, continuous arms
short stubby asymmetric pieces that merely give the illusion of a spiral pattern
multiple arms
two symmetrical arms in the inner part of the disk; outer regions highly branched with many segments and many parts of arms

The occurance of these various types of arm structures seems to be independent of the Hubble type and various examples can be found throughout the Hubble spiral sequence. However, grand design galaxies tend to have either nearby companions or bars. When galaxies have both companions and bars over 90% are grand design. Recall that the presence of bars seems to be correlated with the presence of nearby companions.

Isolated spiral galaxies tend to be flocculent. Spirals in clusters, especially those in dense clusters with increased opportunities for interactions, tend to be grand design.


Descriptions of Spiral A

Van der Berg has attached several descriptive terms to the spiral arms of galaxies. There are “normal” arms and there are…

nebulous arms
patchy arms
distorted arms

The second and third columns refer to moderate and then extreme examples of the described characteristic. These are suffixes that can be attached to the normal Hubble spiral classifications.


Wave Density

Classically, the notion of density waves has been used to attempt to explain the existence of spiral arms. This notion suggests that a wave, much like a sound wave or other form of compressional wave, propigates through a galaxy and generates spiral structure. The physical nature of such a wave and the medium through which it propigates has not been successfully identified. However, the theory can indeed produce spiral patterns comparable to the observed patterns. Note that while star formation occurs in the arms, the mean stellar density remains the same within the arms and in the inter-arm regions.


Spiral Sequences

Why are there two sequences of spirals?

While the reason for the existence of barred spirals is still highly uncertain, it appears the appearance of these objects may be related to the absence of a massive halo of faint stars or dark matter. Linear radial structures like a bar cannot be stable structures in rotating systems. Since the rotation rate will tend to vary with radial distance from the center, bar-like structures should be disrupted as soon as they are formed. Yet, too many barred systems are known for this to be a statistical anomaly. Apparently bar-like structures may develop and remain stable only in the absence of a massive halo. This also implies that the galactic material orbits the nucleus like a solid body in which all the material has the same angular velocity. (As it turns out, even ordinary spirals like the Milky Way exhibit remarkably constant angular velocities in their inner regions.) A massive halo seems to prevent the development of bar-like structures. NGC 1300 Barred Spiral Galaxy
NASA/ESA Hubble Space Telescope, Hubble Heritage Team

Why should some galaxies have massive halos and other galaxies not have these structures? What form do these massive halos take? That is, what kind of matter comprises the halos? The matter cannot be “normal” stars or we would see the halos in visible light. The matter cannot be dust, for this would obscure the underlying galaxy. This is one aspect of the mystery of dark matter or the “missing mass.”

Are there other sequences of spirals?

It has been suggested that two additional sequences of spirals may exist. These are sometimes considered to be varieties of the two main families of S (or SA) and SB spirals. These varieties distinguish between ring shaped structures near the nucleus (denoted r) versus “S” shaped structures (denoted s). In this system, at each stage along the sequence of spirals there could be 4 varieties: SA(r), SA(s), SB(r), and SB(s).

There could also be intermediate varieties: SA(rs) and SB(rs). These intermediate varieties exhibit both ring and “S” shapes in their nuclei.

While it is certainly true that there are galaxies with ring-shaped structures in the nucleus, while other neuclei appear “S” shaped, this rather complex classification system has not been widely adopted. For example, most (if not all!) SBa galaxies seem to have ring-like structures present, while numerous Sa galaxies are known which do not possess a central ring. That is, it seems there are at least some classifications (or pigeon-holes) that do not appear to have any known examples. Thus, the system may not be related to any significant physical characteristic of galaxies. As a compromise, a suffix (r) or (s) may be added to the designation to indicate the appearance of the region surrounding the nucleus.



For the barred spirals the bar extends from over one-third to one-half the disk diameter for an SBa, while an SBc has a bar which only extends from one-fifth to one-third the diameter of the disk. In the early types the arms originate tangentially from a continuous ring which has the bar as a diameter. In the later types the arms start at right angles from the ends of the bar.

Since the bars clearly seem to retain their identity and structure for several rotations of the galaxy, the rotation must take place as in a solid body. That is, the angular velocity of rotation is the same throughout the galaxy and does not vary with distance from the center. This is unlike the revolution of planets in the solar system, for example, where the angular rotation speed increases as one approaches the center. Such motion is termed Keplerian and galaxies display decidedly non-Keplerian motion.

The variation in morphology as one moves along the spiral sequence from early to late types is apparently related to the orbital speed of the stars in the disk. Early type spirals have higher orbital speeds and late type spirals have lower orbital speeds. Sa types have orbital speeds near 300 km/sec while Sc types have orbital speeds near 100 km/sec.


Components of Spiral Galaxies


Spiral galaxies show three distinct regions: a central bulge, a nearly spherical halo and a flattened disk. The relative extent of the bulge varies from early to late spirals. However, the bulge is basically round and consists of relatively low mass, cool, red stars. In this sense the bulge of a spiral galaxy resembles an elliptical galaxy. However, the bulge contains stars of varying metal content, some of them having metallicity higher than solar, others having lower metal content (in an astronomical context, “metal” refers to any element heavier than helium, i.e., those elements, at least from carbon onward, produced inside stars by nuclear fusion). This suggests multiple epochs of star formation for the bulge.

The halo of a spiral galaxy is distinct from both the bulge and the disk. It contains old stars, similar to those in the bulge, though of generally uniformly low metallicity . Most of the stars in the halo are concentrated in the globular clusters, but it appears there is a thin distribution of single stars as well. Stars in the globular clusters and in the halo were probably the first stars to be formed in any galaxy.

The bulge and the halo are sometimes said to be “pressure supported” because their stars move on elliptical orbits that are random in their orientation and ellipticity. This is a bit misleading because the stars in a galaxy do not exert much pressure on each other. Perhaps a better way to think of the motions in the bulge and halo is that they are dissipationless: the halo and bulge stars are stuck on their original random orbits because they cannot easily exchange energy with each other, sort of the opposite of the way we think of pressure working in a gas. As a result, the bulge and halo cannot change from the original roughly shperical shape that characterized the protoglactic cloud from which the galaxy formed.

M104 "Sombrero Galaxy"
This image of the Sombrero galaxy (M104) clearly shows the dust lanes in the disk, the small galactic bulge, and the glow of the halo.

Globular Star Cluster NGC 6093
Globular Star Cluster in Milky Way halo

Both Images: NASA/ESA Hubble Space Telescope, Hubble Heritage Team


The disk lies within the halo and contains most of the stars in a galaxy. The fact that the disk is flat gives us clues about how it must have formed; unlike the halo and bulge, the disk must have collapsed as gas, and only later started to form stars. Unlike stars, gas can suffer collisions and dissipate energy; because the spin of the original gas cloud prevented it from collapsing uniformly in all directions, the collapsing cloud formed a rotating disk with its spin axis aligned perpendicular to the sense of the largest rotational motions within the original cloud. Thus we often say that the disk is rotationally supported.

The spiral patterns themselves consist of large luminous clouds of gas (HII regions like the Orion nebula) plus dark dust clouds and concentrations of luminous massive young stars. The actual density of stars varies little from the arms to the inter-arm regions, although the arms do contain higher concentrations of massive, young, luminous blue stars. It is these stars that illuminate the gas and dust clouds in the arms. The arms are also the regions where active star formation is occuring. As a result, supernovae always occur in the disk, usually within the arms of spiral galaxies. The progenitors of many supernovae are massive young stars. Supernova are never observed in elliptical galaxies where star formation does not take place, they are only seen in spiral or irregular galaxies where stars are forming or have recently been forming.