Clearly, galaxies are discussed a lot around here as we spend our time studying them. There’s a lot of terminology that can be confusing at first. Of course, it becomes second nature after a while working with galaxies, so it’s hard to avoid using. This post is a basic introduction to galaxies and some of the common terminology – it’ll be a good reference for future posts that talk about specifics of galaxies.
The Hubble Tuning Fork
The Hubble tuning fork (pictured below) is the main method used for classifying galaxies. This sequence divides galaxies into two main types – spiral and elliptical – and then subdivides those further based on their appearance. Elliptical galaxies have smooth light profiles and, not surprisingly, are elliptical in shape. They are further classified by a number (0 to 7) that describes how elliptical they are: E0 galaxies appear circular and E7 are the most elongated. The differences in the shapes of various elliptical galaxies is mostly a result of viewing the galaxies in different alignments. Sometimes we see the end of an elliptical galaxy and it appears more circular; other times, we see the long side of the galaxy facing us. Spiral galaxies have a central bulge and spiral arms. The split in the tuning fork is the split between spirals and barred spiral galaxies, which have a bar connecting the bulge to the spiral arms. The classifications from Sa to Sc describe how prominent the bulge/spiral arms are. Sa galaxies have a dominant bulge and small, tightly wound spiral arms. Sc galaxies have a small, faint bulge and loose spiral arms. The S0 galaxy is a transition from elliptical to spiral. This galaxy type has a faint disk and massive bulge, but no spiral structure.
Early Type vs. Late Type
The terminology early and late type to describe galaxies is frequently used. Elliptical galaxies are early-type: they formed their stars early in the universe and have a remaining old (and red) stellar population. They’re often referred to as “red and dead”. Spiral galaxies are late-type: they have current star formation and have young stellar populations that include bright, blue stars. One way to remember the early- vs. late-type distinction is to think of the Hubble tuning fork as an evolutionary sequence. Originally, Hubble thought his classification of galaxies presented a life sequence of galaxies, from elliptical evolving to a tight spiral to a loose spiral. Now, we know this isn’t true (in fact, we believe spirals turn into ellipticals when processes stop star formation), but it is one of the origins of the ellipticals as “early” galaxies and the spirals as “late” galaxies.
To make things more confusing, the classifications of early- and late-type can also be applied within the spiral galaxy category. S0 and Sa galaxies can be referred to as early-type spirals. This is because they have a strong bulge component, which contains old, red stars like elliptical galaxies. Sc galaxies are dominated by the spiral disk, which is full of young, blue stars, so they are late-type spirals.
In terms of ALFALFA, one of the important distinctions between a late- and early-type galaxy is gas content. Late-type galaxies have lots of neutral hydrogen gas that serves a reservoir for the ongoing star formation. Early-type galaxies have little to no gas, so they have no material out of which to form stars.
There are two other main classes of galaxies not shown on the tuning fork above: irregulars and dwarfs. Irregular galaxies are exactly what they sound like – irregular in shape. Typically, these galaxies are the results of interactions and mergers that we happen to be seeing while they are still disturbed. Dwarf galaxies are galaxies that are much less massive and bright than the spirals and ellipticals and are often found as satellites around the larger galaxies. The dwarf galaxies follow a similar scheme to the larger galaxies. There are dwarf elliptical (dE) galaxies which have old, red stellar populations. Dwarf irregular (dI) galaxies are late-type dwarf galaxies: they are gas rich and have ongoing star formation.