The Cosmic Tuning Fork: Hubble Galaxy Classification Quiz

Just like ordinary spirals, barred spirals are graded (a, b, c) based on bulge size and arm tightness. There is also a barred version of the lenticular galaxy, known as SB0, which has a bar and a disk but no arms.

The number in elliptical classification is 10 times the eccentricity (1 - b/a). It is a mathematical way to describe the ratio of the short axis to the long axis, helping astronomers quantify exactly how "flat" an elliptical galaxy appears.

This describes a Lenticular (S0) galaxy. They are placed at the "junction" of the tuning fork because they share the spherical bulge of an elliptical but possess the flat disk structure of a spiral.

The system is a morphological classification. While it often correlates with a galaxy's star formation rate and gas content, it was designed specifically to categorize galaxies based on their physical appearance in photographic plates.

While brilliant, the system is a 2D snapshot. It doesn't easily categorize the tiny dwarf galaxies that are most common in the universe, nor does it explicitly show how the invisible dark matter halo or violent mergers change a galaxy's type over time.

Peculiar galaxies (marked with a 'p') are those that generally fit a Hubble type but show significant distortions, like tails or bridges of stars. These are often the "smoking guns" of recent gravitational interactions between two or more galaxies.

Elliptical galaxies are composed mostly of old, cool stars. Because they lack the hot, blue, young stars found in the arms of spirals, their overall light is shifted toward the red end of the spectrum, indicating an older stellar population.

Modern mapping has confirmed that our galaxy has a central bar. This places us on the "Barred Spiral" prong of the Hubble Tuning Fork, with arms that are moderately wound and a medium-sized central bulge.

Galaxy types are not permanent. A spiral can lose its gas and become a lenticular (S0), or two spirals can collide and merge to become a giant elliptical (E). These transformations are driven by the galaxy's environment and its history of interactions.

De Vaucouleurs introduced a more complex "classification volume" that added details like rings (r) and intermediate stages between ordinary and barred spirals (SAB). This allows astronomers to give a much more precise "address" to a galaxy based on its specific features.

The Hubble Sequence is often called the "Tuning Fork" diagram. It starts with a handle of elliptical galaxies that splits into two prongs: one for ordinary spiral galaxies and one for barred spiral galaxies, representing the diversity of galactic shapes.

In Hubble's system, the number represents the degree of ellipticity. An E0 galaxy is perfectly spherical (circular in appearance), while an E7 is the most elongated or flattened of the elliptical types.

The main structure of the Tuning Fork consists of Ellipticals as the handle, with Spirals and Barred Spirals forming the two parallel prongs. While Irregulars exist, they were originally placed outside the main fork because they lack a symmetrical structure.

The "B" in "SB" stands for "Barred." These galaxies differ from ordinary spirals (S) because their spiral arms originate from the ends of a distinct central bar rather than directly from the circular galactic bulge.

In spiral classification, 'a' types have a large bulge and tightly wound arms. As you move to 'c', the central bulge becomes smaller and the spiral arms become much more loose and "patchy" or open.

S0 or Lenticular galaxies are considered the bridge between ellipticals and spirals. They have a central bulge and a distinct disk, but they have used up their gas and no longer have the active star-forming arms that define true spirals.

Moving from Sa to Sc, galaxies generally become richer in interstellar gas and dust. This leads to higher rates of star formation in the looser arms, whereas Sa galaxies are more dominated by the older stellar population of the large central bulge.

Irregular galaxies (Irr) lack a distinct symmetry or central nucleus. They are often the result of gravitational collisions or "near misses" between larger galaxies that have pulled the stars and gas into a chaotic, non-repeating pattern.

Hubble originally suspected that galaxies started as simple ellipticals and "evolved" into complex spirals. We now know this is incorrect—galaxies often evolve from spirals into ellipticals through mergers—but the classification system remains the standard for describing galaxy shapes.

Elliptical galaxies (E) are generally "red and dead," meaning they have very little cold gas for star formation. "Sc" spirals are gas-rich and contain many young, blue stars, making them much more active than any elliptical type.