Hubble’s Legacy: Unraveling the Spiral Wonders of the Universe

This Hubble Space Telescope image presents a detailed look at NGC 3430, a spiral galaxy located in the constellation Leo Minor, 100 million light-years away. Credit: ESA/Hubble & NASA, C. Kilpatrick

NGC 3430, a spiral galaxy in Leo Minor, is spotlighted in this New Hubble Image

In this stunning Hubble Space Telescope image, we are treated to a wonderfully detailed snapshot of NGC 3430. A spiral galaxy, it lies 100 million light-years from Earth in the constellation Leo Minor. Several other galaxies are located relatively near to this one, just out of frame. In fact, oone is close enough that gravitational interaction drives some star formation in NGC 3430.

Hubble’s Galactic Classification

That NGC 3430 is such a fine example of a galactic spiral may be why it ended up as part of the sample that Edwin Hubble used to define his classification of galaxies. Namesake of the Hubble Space Telescope, in 1926 he authored a paper that classified some four hundred galaxies by their appearance — as either spiral, barred spiral, lenticular, elliptical, or irregular. This straightforward typology proved immensely influential, and the modern, more detailed schemes that astronomers use today are still based on it. NGC 3430 itself is an SAc galaxy, a spiral lacking a central bar with open, clearly defined arms.

Evolution of Galaxy Terminology

At the time of Hubble’s paper, the study of galaxies in their own right was in its infancy. With the benefit of Henrietta Leavitt’s work on Cepheid variable stars, Hubble had only a couple of years before settled the debate about whether these ‘nebulae’, as they were called then, were situated within our galaxy or were distant and independent. He himself referred to ‘extragalactic nebulae’ in his paper, indicating that they lay beyond the Milky Way galaxy. Once it became clear that these distant objects were very different from actual nebulae, the favored term for a while was the quite poetic ‘island universe’. While NGC 3430 may look as if it still deserves this moniker, today we simply call it and the objects like it a ‘galaxy’.