Question

The Hubble Tuning Fork diagram proves that galaxies gradually move from being elliptical galaxies to spiral galaxies over time.

Answer 1

I assume you want an explanation for the same. THe hubble tunig fork diagram is more of a model or a theory built to explain the evolution of galaxies from their infacny stage. The model is divided into two parts, first the elliptical galaxies (ellipticals) and second the spiral galaxies (spirals). Hubble gave the ellipticals numbers from zero to seven, which characterize the ellipticity of the galaxy - "E0" is almost round, "E7" is very elliptical. However the spiral part of the galaxy is divided in to sub parts, and hence the tuning fork shape. The evolution of the galaxies depends into either of the arm is based on several factors such as mass, compostion etc. They are subdivided in normal spirals and barred spirals. The spirals are assigned letters from "a" to "c," which characterize the tightness of their spiral arms. "Sa" spirals, for example, are tightly wound whereas "Sc" spirals are more loosely wound. Also it is worth noting that the sizes of the round central regions in spirals, called bulges, increase in size the more tightly the spiral arms are wound. The most important difference between these two groups is the bar of stars that runs through the central bulge in barred spirals. The spiral arms in barred spirals usually start at the end of the bar instead of from the bulge. Barred spirals have a "B" in their classification. An "SBc" is thus a loosely wound barred spiral galaxy.