University of California, San Diego
Center for Astrophysics & Space Sciences

The Spiral Nebulae

	In 1755 Immanuel Kant first speculated on philosophical grounds that there might exist "island universes" of stars like the Milky Way. In 1845, William Parsons, third Earl of Rosse, using a 72-inch home-built telescope at Birr Castle in Ireland, nicknamed the Leviathan of Parsonstown , determined that some of the nebulae have Spiral Structure and adopted Kant's term for them.
The Leviathan of Parsonstown

The nature of the Spiral Nebulae was debated both literally and figuratively for the next several decades. A summation of the arguments was made in the Shapley-Curtis Debate (1920), between two eminent Astronomers, Harlow Shapley, then at Mt. Wilson and H. D. Curtis of Lick Observatory. Hubble detected Cepheid variables in Messier 31 - the Andromeda Galaxy - and other Local Group galaxies, establishing distances of hundreds of thousands to millions of light years.

Hubble Classification

Hubble also developed a classification scheme for galaxies which, with minor revisions remains in use today. Hubble divided galaxies into two principal catagories Elliptical and Spiral, with a third "Irregular" category left to catch those galaxies which defied regulae classification. Elliptical galaxies, which essentially consist of only a nuclear bulge component are subdivided among seven ellipticity classes from E0 (circular) to E7 (cigar shaped). Numerically the ellipticity is given by

Spiral galaxies are subdivided among three classes Sa, Sb, Sc, with a parallel sequence for Barred Spirals SBa, SBb, SBc . (More modern classifications add a class Sd and subdivide among the classes Sab, Sbc, Scd.) The three criteria are:

• Size of nuclear bulge (Sa=large; Sc=v. small)
• Openness of spiral pattern (Sa=tightly wound; Sc=v. open)
• Resolution of arms into supergiant stars and HII regions (Sa=smooth, few small HII regions; Sc=clumpy, lots of bright supergiants & HII regions).

Tradionally the classification scheme is arranged in a "tuning fork diagram":

Virtually all of the observable characteristics of galaxies are directly related to their Hubble Classification as shown below:

Galaxy Characteristics are Uniquely Related to Classification
	E0-E7	S0	Sa	Sb	Sc	Irr
Nuclear Bulge	"All Bulge" No disk	Bulge & Disk	Large		Small	None
Spiral Arms	None	None	Tight/Smooth		Open/Clumpy	Occasional traces
Gas	Almost none	Almost none	~1%	2-5%	5-10%	10-50%
Young Stars HII Regions	None	None	Traces		Lots	Dominates Appearance
Stars	All Old (~ 1010yr)	Old	Some young			Mostly(?) young (but some v. old)
Spectral Type	G-K	G-K	G-K	F-K	A-F	A-F
Color	Red	Red				Blue
Mass (M)	108-1013	(More)  1012-109    (Less)				108-1011
Luminosity (L)	106-1011	(More)  1011-108    (Less)				108-1011

Here's a tuning fork composed of real galaxy images; click on the image to see a larger version, click on the name & Hubble Class to see more information about the galaxy from the SEDS Messier Database.

M87 - E0

M110 - E5

M86 - S0

M94 - Sa M95 - SBa

M81 - Sb M91 - SBb

M101 - Sc M61 - SBc

Irregular galaxies come in two types:

• Irr I which are in some sense a logical extension of the Hubble tuning fork, having characteristics "beyond" those of class Sc - high gas content, dominant presence of a young population. Irr I galaxies may show bar-like structures and incipient spiral structure like the Large Magellanic Cloud, below. Such galaxies are sometimes referred to as "Magellanic Irregular" galaxies.

• Irr II which are galaxies which defy classification because of some form of disturbance. M82, shown below, is undergoing an intense period of star-formation.

LMC - Irr I

M82 - Irr II

The unique relationship between the Hubble Class and other properties, most especially stellar populations suggests that the Hubble Class is fundamentally related to the way in which galaxies form and evolve. Originally Astronomers thought that the Hubble sequence might be an evolutionary sequence with galaxies evolving from right-to-left (or vice versa) along the tuning fork. This cannot be correct because to the best of our knowledge

It is clear, however that the Hubble sequence is a sequence in evolutionary state, however. For some reason Elliptical galaxies formed all their stars a long time ago, using up all their gas, so that new stars are no longer forming, there is virtually no young stellar population nor gas nor dust. Spirals, on the other hand, have retained much of their gas and are continuing to form stars. The answer is reminiscent of the Nature vs. Nurture debate occurring among biologists, psychologists & sociologists about human behaviour:

• Nature - Rotation/Angular Momentum: The rotational characteristics of Spiral & Elliptical galaxies are different as shown below; Spiral galaxy disks have well organized rotational dtructure whereas elliptical galaxies have random orbits. A rotating cloud of protogalactic gas would certainly contract more slowly than one without rotation and could only contract along the rotation axes, forming a disk as we see in spiral galaxies. It is plausible to suppose that a non-rotating, freely collapsing protogalaxy would reach higher densities at earlier times, using up its gas rapidly in a time of order a billion years. As the rotating protogalaxy contracts more slowly to a disk shape, it would not reach such high densities, star-formation would proceed more slowly, preserving gas for future epochs of star formation

  Circular Orbits in a Spiral Disk

  Random Orbits in an Elliptical (or Spiral Bulge)

• Nurture - Physical Environment: The environments of differing types of galaxies are also distinct. Most rich clusters contain only E & S0 galaxies, whereas Spirals and Irregular galaxies are found principally in small groups. It has become apparent that collisions, both between galaxies themselves and between the fragments that conglomerated together to build galaxies, are important in the formation/evolution of galaxies. As we have discussed collisions between galaxies have a much more dramatic effect upon the gas than upon the stars because of the great interstellar distances. Collisions and mergers of galaxies can stimulate intense bursts of star formation as molecular clouds collide. Collisions can also strip the gas out of galaxies.

It is quite likely that both effects play a role in determining the nature of galaxies. Certainly collisions and mergers must play a role in the formation of cluster elliptical galaxies. It seems almost certain, for example, that the giant elliptical galaxies seen at the centers of rich clusters formed as galaxies gradually accumulated at the center of the cluster's strong gravitational field. While rotation probably also plays a role, especially for disk (spiral and S0) galaxies, the history of the Milky Way suggests that the contraction of the Galaxy to a disk was slower than had previously been thought with accumulation of 10⁶ - 10⁸M fragments forming first the bulge, then the halo and gradually settling down into a disk due to angular momentum. The idea of this "bottom-up" accumulation of larger galaxies from smaller fragments is also supported by current calculations of the formation of structure in the early Universe. Our picture of the formation of galaxies remains hazy, however, and much remains to be understood.

Galaxy Links

• Try your hand at classifying galaxies here
• Messier Database with information & images about galaxies in the Messier Catalog of 100+ galaxies, clusters & nebulae.
• National Observatories Galaxies Page
• Bill Keel's Messier Image Gallery at the University of Alabama.
• Princeton Galaxy Catalog

Spiral Structure

Clusters & Dark Matter   The Distance Scale   Education & Outreach   CASS Home  

• M94 & M101 - Dr. Bill Keel, University of Alabama Messier Image Gallery
• M87, M95, & LMC - Dr. David Malin, Anglo Australian Observatory.
• M31 & M61 - John P. Gleason, Celestial Images
• M82 - S. Kohle & T. Credner, Univ. Bonn
• M86 - Limber Observatory
• M81 - Grove Creek Observatory, NSW, Australia
• M110 = NGC 205 - Dr. Andjelko Glivar