A Spiral Galaxy with Attitude

By Doug on September 30, 2006 at 8:09 pm | In Blog Posts |

No doubt you have seen beautiful images of spiral galaxies many times. Those systems with very pronounced arms tend to be imaged repeatedly. In 1925, Hubble established that they were “island universes” - stars and gas grouped together in space at great distances from our system. He did this by recognizing Cepheid variables in M31, and soon thereafter in other Local Group galaxies.

However, there was a question that persisted many years after that revelation - do spiral galaxies rotate with the ends of their arms “leading” or “trailing”? The answer turns out to be “trailing”. But the evidence for that fact was a long time coming.

“Why?”, you may well ask. “Couldn’t they just use a spectrograph and measure the Doppler shift?”. Yes, measuring the Doppler shift is possible and it does tell you whether or not part of the image is heading towards you or away from you. However, what is generally difficult to determine is which side of the galaxy is closest to you!

Find a picture of M83 - a face-on spiral, and M104 (”The Sombrero”) which is close to edge-on. First, look at M83. You’ll agree it is a beautiful thing. Look at it carefully - there is no clue as to which side is closest. Now look at M104. It is abundantly obvious which side is closest because of the obscuring dust lane. Unfortunately, its disk is inclined so close to edge-on that the direction of its spiral structure is undetermined! That, in a nutshell, was the observational situation for a long time. You could either see the pattern, or know which side was closest, but not both at the same time.

Hubble finally found a galaxy where you could have your cake and eat it, too. In 1943, he wrote a paper entitled “The Direction of Rotation in Spiral Nebulae” in the Astrophysical Journal. The galaxy which finally yielded this secret was NGC 4216 - a largei, well-inclined spiral in the Virgo Cluster that is easily seen in amateur telescopes. It is inclined just enough for a prominent dust lane to reveal the near-side and little enough for the spiral direction to be clearly discerned.

It can be located on any clear April evening. Even though it does not have a Messier number or a fancy name, it still played an important role in allowing us to understand how real galaxies behave.

[I wrote this piece many years ago for the newsletter of the Hamilton Amateur Astronomers. It seemed like the sort of thing which might interest SG readers!]