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

Gene Smith's Astronomy Tutorial

Galaxies - The Spiral Nebulae

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


where a is the length of the major axis and b is the length of the minor axis. Of course, the Hubble Classification does not tell us the true shape of the galaxy (e.g. an E0 could be a "cigar" seen down its barrell). Statistical arguments suggest that the distribution of galaxies among the ellipticities is roughly uniform.

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:

Upon completion of the scheme Hubble realized that he needed to allow for an intermediate classification between Elliptical & Spiral S0, SB0

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
Stars All Old
(~ 1010yr)
Old Some young Mostly(?) young
(but some v. old)
Color Red Red Blue
108-1013 (More)  1012-109    (Less) 108-1011
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:

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

All galaxies are about the same age ~ 15 billion years old

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:

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

Spiral Structure

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

Galaxy image credits & copyright: Several of the avove "deep-sky" photographs were obtained by amateur astronomers with modest sized telescopes.
Comments? Gene Smith
Conducted by:
Prof. H. E. (Gene) Smith
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Last updated: 6 March 2000