Classifying spiral galaxies: citizen scientists question the Hubble model

An image to illustrate spiral galaxies
© X-ray: NASA/CXC/SAO; Optical: Detlef Hartmann; Infrared: NASA/JPL-Caltech

The Hubble model of classifying spiral galaxies is widely used in astronomy. However, citizen scientists have questioned the authoritative classification based on new evidence.

The key observation made by the astronomer Edwin Hubble was that galaxies with larger bulges tend to have more tightly wound spiral arms, which supports the ‘density wave’ model of spiral arm formation. However, the new findings on spiral galaxies contradict Hubble’s model.

The new research found that there was no correlation found between the sizes of the galaxy bulges and how tightly wound the spirals are. This finding could mean that most spirals are not in fact static density waves, as Hubble’s theory suggest.

The Hubble model for formation of spiral galaxies

Galaxy Zoo Project Scientist and the first author of the new work, Professor Karen Masters from Haverford College, US, explained: “This non-detection was a big surprise, because this correlation is discussed in basically all astronomy textbooks – it forms the basis of the spiral sequence described by Hubble.”

Masters added: “We always thought that the bulge size and winding of the spiral arms were connected. The new results suggest otherwise, and that has a big impact on our understanding of how galaxies develop their structure.”

The limitations of the Hubble theory

While Hubble was limited by the technology of the time, and could only observe the brightest nearby galaxies. The new work is based on a sample 15 times larger from the Galaxy Zoo project, where members of the public assess images of galaxies taken by telescopes around the world, identifying key features to help scientists to follow up and analyse in more detail.

Future research

Brooke Simmons, Deputy Project Scientist for the Galaxy Zoo project, concluded: “It’s clear that there is still lots of work to do to understand these objects, and it’s great to have new eyes involved in the process. These results demonstrate that, over 170 years after spiral structure was first observed in external galaxies, we still don’t fully understand what causes these beautiful features.”

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