Here are some of the more important properties of galaxies (by type). Remember, the more important question to ask yourself is not "what are the properties?", but "why are the properties?". You should be able to compare/contrast some of the major observational properties, especially between spiral and elliptical galaxies - but I don't suggest memorizing every little detail! Instead, look for patterns and find cause & effect. Can you tell a story? Note: this set of notes has several built in web links (mostly images of galaxies) that of course you cannot see when you print this page. You may want to look through this page at least once while on-line, so that you can click on those links. You might want to refer to this page, nicely showing the "Hubble Tuning Fork Diagram of Galaxy Classification".  Astronomers want to know: how and why is it that galaxies differ?

1) Spiral Galaxies (don't worry about the differences between "barred" and regular spiral galaxies)

3 major types of spiral galaxies and their disk properties: Do you see a pattern between the presence of (cold) gas, the present star formation rate, and the average stellar spectral type (and so color and contrast of the disk/spiral arms) amongst these 3 types of spiral galaxies? Does that pattern make sense to you? Please also take note that the average stellar spectral type of just the bulge of any spiral galaxy is that of G-K stars, as expected for a very old population of stars.

2) Elliptical Galaxies
3) Irregular Galaxies (excluding the "galactic wrecks")
4) S0 (or lenticular) Galaxies: disk galaxies without spiral arms - very little gas; intermediate to old stellar populations. When they are observed to have gas and dust, they are confined to the central regions of the disk.  Let's not worry about these too much - though hopefully you can see the correlations between the lack of cold gas, the lack of any significant recent star formation, and the lack of spiral arms within a disk. Lack of recent star formation also impacts the types of stars present, their typical ages and the average spectral type of the galaxy. How?

But why? (if you don't understand the following, ask me!)

As we discussed in class, observations tell us that the bulk of the stars presently found in the bulges and halos of spiral galaxies formed before the remaining available gas could settle into an orderly rotating disk to form stars there. Ok, then why the differences amongst the spiral galaxy types? Apparently, the Sa type spirals formed within environments that allowed them to efficiently convert most of their available supply of cold gas into stars billions of years ago, first in the sphere and then in the disk component4, and so most of their light is presently dominated by long-lived low mass stars with spectral types G-K (including lower mass giant stars). With relatively less cold gas in their disks at the present time, the star formation rate there is relatively slow. As a consequence, the disks of Sa spirals generally have relatively fewer of the higher mass, hot (blue), very luminous main sequence stars (because their lifespans are short), and the spiral arms in Sa type spiral galaxies do not stand out as in brightness in contrast to the light emitted by stars in the disk outside the spiral arms.  Note too that their stellar bulges are large in comparison to their disks - as you would expect if vigorous star formation occurred very early on for these types of spirals.

Just the opposite is true for the Sc type spirals, which apparently formed within environments that were not as efficient in converting gas into stars early in their evolution. So relatively more gas ended up in their disks (and resulting in smaller stellar bulges). The gas that fell into and formed their disks may have had a lower peak star formation efficiency in comparison to the Sa type spirals, leaving behind more gas in the disk - therefore potentially available to form future stars. In any case, this significant remaining cold gas reservoir within the disks of Sc type spiral galaxies continues forming stars at a significant rate even today. Consequently, their arms are very brightly lit by the hotter (bluer), more luminous, shorter-lived higher mass stars. Compare the color of the starlight in the linked examples of the Sa vs. Sc types, above. Note the colors of the bulges in all types (about the same yellowish/orange). The Sb type spirals (like the Milky Way and the Andromeda galaxies) have properties lying in between the Sa and Sc types.

Elliptical galaxies have properties very similar to (though not always precisely the same as) the bulges of spiral galaxies. Apparently, these objects (especially the regular/giant varieties) formed within dense environments that allowed the rapid and efficient conversion of their available gas into stars very early on in their histories, well before any remaining gas could form an orderly rotating flattened disk. So star formation in these galaxies ended a long time ago, and their light is completely dominated by long-lived lower mass stars (giants as well as main sequence stars) that are also cooler.

Galactic Wrecks
It has recently become apparent that the merging of two colliding spiral (or disk) galaxies may ultimately result in an elliptical galaxy.  In the collision, gravitational forces accelerate the stars in each of the two disks, throwing them this way and that, with the result that the stars' motions become largely randomized - just like in elliptical galaxies. While virtually none of the stars actually physically collide during this collosal "train wreck", the giant molecular clouds often do. This compresses them, setting off a firestorm of star formation - and largely emptying the "gas reservoirs" of the merging pair. This scenario is expected as long as the gas content of the two merging spirals is relatively small. This strange giant elliptical galaxy is almost certainly the result of such a collision.  On the other hand, direct collisions between two gas rich disk galaxies may result in a spiral disk galaxy with a big central bulge. This particular scenario likely played out early on in the history of galaxy formation (i.e., 10 billion years ago or so).  However, observations and theory seem to indicate that relatively few of today's giant ellipticals had their origins in the "recent" merger between two star-rich spiral galaxies.

Observations of distant galaxies tell us that the star formation rates in spirals and especially ellipticals were much higher in the distant past than they are at present.

The generally low mass irregular galaxies apparently never went through "big booms" in their star formation rates, and so even at present have large reservoirs of gas available for star formation. S0 or lenticular galaxies appear to be disk galaxies stripped of much of their gas and dust due to their passing through the relatively dense intergalactic gas filling the volume within large galaxy clusters, and so star formation has been halted ever since the stripping took place. They are found in near the centers of dense galaxy clusters, whereas spiral galaxies are almost always found in their outskirts or in small galaxy groups (like the Milky Way). While we won't worry much about irregular and lenticular galaxies, you should be able to see the relationship between the presence of (cold) gas clouds, star formation (on-going? lots of it?), and the general appearance and structure of the galaxy (e.g., what sorts of stars emit most of the light? how do the stars move? is it an elliptical or spiral?).

Why the star forming histories and efficiencies differed amongst the resulting galaxy types is another question that we won't pursue here. Things are almost certainly more complex than I've outlined above, and there are still many questions that do not have satisfactory answers yet. Finally, I note that the above general properties apply strictly to relatively nearby galaxies only, say within a few billion light years.

1Excludes dark matter; includes only stars, gas, dust. The category "stars" includes normal stars, stellar remnants (e.g., white dwarfs), and brown dwarfs.
2Old means age > 10 billion years, intermediate age means 1-10 billion years, young means age < 1 billion years.
3Our Sun's spectral type is G2, main sequence. Its main sequence life span is about 10 billion years, and will last some 12 billion years in total before dying, leaving behind a white dwarf. The average spectral type refers to what sorts of stars produce the majority of the light in the galaxy.
4More recent observations and galaxy dynamics theory suggest that some portion of the bulges of especially Sc type spiral galaxies may have been formed during the early evolutionary stages of the disk. We won't worry about this, for the present time.

Kirk T. Korista
Professor of Astronomy
Department of Physics
Western Michigan University