Cosmic Illusions: Hubble Spots a Spiral Galaxy in Disguise

This Hubble Space Telescope image features NGC 4423, a galaxy in the constellation of Virgo. It appears irregular at first glance but is actually a spiral galaxy. Its true shape, characterized by a dense central bulge and spiral arms, is obscured due to our fixed viewing angle from Earth.  Credit: ESA/Hubble & NASA, M. Sun

NGC 4423, located in Virgo, misleadingly appears as an irregular galaxy but is truly a spiral galaxy. Our fixed perspective from Earth limits our view, and despite the movement of celestial objects, we are unlikely to see a change in our view of NGC 4423 due to the immense distances in space.

Here we see NGC 4423, a galaxy that lies about 55 million light-years away in the constellation Virgo. In this image NGC 4423 appears to have quite an irregular, tubular form, so it might be surprising to find out that it is in fact a spiral galaxy. Knowing this, we can make out the denser central bulge of the galaxy, and the less crowded surrounding disc (the part that comprises the spiral arms).

If NGC 4423 were viewed face-on it would resemble the shape that we most associate with spiral galaxies: the spectacular curving arms sweeping out from a bright center, interspersed with dimmer, darker, less populated regions. But when observing the skies we are constrained by the relative alignments between Earth and the objects that we are observing: we cannot simply reposition Earth so that we can get a better face-on view of NGC 4423!

Of course, celestial objects do not remain sedentary in space, but often move at extremely rapid velocities relative to one another. This might suggest that, should a galaxy be moving in a fortuitous direction relative to Earth, we might be able to view it from a substantially different perspective once it has moved far enough. This is theoretically possible, but the reality is that the distances in space are simply far too big, and human lifetimes far too short, for a noticeable difference in relative alignment to occur. In other words, this is more-or-less the view of NGC 4423 that we will always have!