Is Hubble’s Tuning Fork Diagram ‘misleading’?

Misleading visualization results in misrepresenting of information

Hubble’s tuning fork diagram, published in 1936 by US astronomer Edwin Hubble, is the earliest method for classification of galaxies. It classifies galaxies based on their apparent structure – into three main categories – elliptical, spiral and barred spiral.

Edwin Hubble’s program of observing distant galaxies, during 1920s, collected 100s of high-quality images of galaxies. He devised a classification scheme based on the structural variations in the family of galaxies.

At first, in a 1926 paper, he divided galaxies into three main classes according to their appearance:

1. Ellipticals
2. Spirals
3. Irragulars

The “Irragulars” was created for anything that didn’t fall into the other two. They are frequently the product of two galaxies colliding with each other, or at least affecting each other through the force of gravity.

Spirals were again divided into two subfamilies – Normal Spiral ‘S’ whose spiral arms emerged from the centeral core of the galaxy, and the Barred Spiral ‘SB’, whose spiral arms began at the two end of the bar like feature running through the center.

Each of these subfamilies were given linear development so-called ‘early’ to ‘late’ spiral types. ‘Early’ is where spiral arms were close to the galactic centre and ‘Late’ is where spiral arms spread much far away from the centre and more numerous and less well defined. Normal Spirals form early to late were designated as Sa, Sb and  Sc and Barred Spirals as SBa, SBb and SBc. Similarly, Ellipticals were also given early to late types, starting from E0 to E7 (number after the ‘E’ being the galaxy’s ellipticity – the ratio of the ellipse’s major axis to minor axis).

Hubble later included a ‘Lenticular’ category. They appeared like they were in transition between the elliptical and spiral galaxies. These were labeled S0. Lenticular galaxies have a central bulge and a disk but no spiral arms.

Hubble’s misleading diagram?

“Early” and “Late” notations indicate that Hubble believed that galaxies started at the left end of the diagram and evolved to the right. He meant that a E0 elliptical can evolve into a Sb Spiral. We now know that he was wrong: galaxies do not move down the forks of the diagram as they evolve. Spiral galaxies rotate quickly (on an astronomical scale), while elliptical galaxies do not. There is no way that an elliptical galaxy could spontaneously begin rotating, so there is no way an elliptical galaxy could turn into a spiral galaxy.

In fact, Spirals do evolve into barred spiral. Barred Spiral Galaxies are “Latecomers” to the Universe. Bars form when stellar orbits in a spiral galaxy become unstable and deviate from a circular path. Bars are a sign of galaxies reaching full maturity as the “formative years” end. – Credit: NASA, ESA, and Z. Levay (STScI).

Read More: Barred Spiral Galaxies Are Latecomers to the Universe

Conclusion

Although Hubble was wrong about his theory of galaxy evolution, his diagram provides a useful way to classify galaxies. In fact, astronomers today still use his terminology: elliptical galaxies are still referred to as “early galaxies” and spirals as “late galaxies.”

Reference – Cosmic Imagery: Key Images in the History of Science (Affiliate Link)

~AK