Astrophysicist Summer Ash on galaxy formation

‘Stephan’s quintet’ blue elliptical and red spiral galaxies interacting. Photo by NASA/ESA/Hubble SM4 ERO Team. Source:

Astrophysicist Summer Ash writes this article for Slate entitled, “Galaxy formation: Dark matter, ellipticals, spirals, harassment, stripping, strangulation, and cannibalism,” that describes what post Hubble, electromagnetic spectrum measuring telescopes are teaching us about galaxies.

She begins by stating that we live in the Milky Way galaxy, one of tons of galaxies that through, ‘augmented eyes’ of modern telescopes can be seen ‘everywhere we look’. Summer describes the early work of Edwin Hubble:

Edwin Hubble, of the eponymous telescope, was the first to devise a classification scheme to attempt to make sense of the variety of nearby galaxies. In the 1920s, Hubble spent night after night taking pictures of the heavens with the 100-inch telescope atMount Wilson Observatory near Pasadena, Calif. In Hubble’s day, the notion that other galaxies existed was debatable. But some astronomers theorized that the fuzzy objects they observed were just that. Hubble grouped his observations of what were later recognized to be galaxies according to their shape: elliptical or spiral.

For a long time Hubble’s system seemed to cover all the bases: Galaxies seemed to slot easily into each category and even into each subcategory. So easily, in fact, that we thought we had it all figured out. But with more and more observations, we find there are numerous galaxies that seem to defy characterization. Hubble somewhat anticipated this by adding a category for “irregulars.” We now know that the majority of irregular galaxies are actually interacting galaxies, gravitationally pulling each other apart into all sorts of odd shapes.

With the benefit of telescopes that are able to measure more than just ‘visible light’, astrophysicists are now  seeing galaxies in much greater detail. Ash explains[Emphasis is mine]:

Until the middle of the 20th century, telescopes that detected visible light were the only game in town. With data from modern telescopes that collect light across the electromagnetic spectrum, we now know that in addition to stars, galaxies contain varying amounts of gas and dust, not to mention plasma, black holes, superheated jets, shock waves, and much more. The relative amounts of stars, gas, and dust revealed by infrared,ultraviolet, and X-ray light provide more detailed systems for classifying galaxies.

One such system is based on a galaxy’s color. Galaxies with lots of gas appear blue because they are dominated by regions of active star formation and hot, young stars. Galaxies with very little gas and filled with older, cooler stellar populations appear “red and dead” (yes, that’s the technical term). Most blue galaxies look like spirals and most red galaxies are elliptical in shape, therefore spirals are young galaxies and ellipticals are old: Spirals must evolve into ellipticals, right?

Maybe. Astronomers are studying how spiral galaxies could run out of gas, literally and figuratively, and turn into ellipticals. Exactly how this transformation takes place is still a mystery.

To read the full article, click here.


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