This NASA/ESA Hubble Space Telescope image shows the galaxy NGC 4694. Most galaxies fall into one of two main types. Spiral galaxies are young and energetic; It’s full of the gas needed to form new stars and sports spiral arms populated by these hot, bright youngsters.

Elliptical galaxies have a much more pedestrian appearance, and their light comes from a homogeneous population of older, redder stars. But some galaxies require a closer look to classify their types: This is the case of NGC 4694, a galaxy located 54 million light-years from Earth in the Virgo galaxy cluster.

NGC 4694 has a smooth-looking, armless disk with almost no star formation, like an elliptical galaxy. But its stellar population is still relatively young, and new stars are actively forming in its core, feeding its bright center and giving it a star profile quite different from the classical elliptical.

Although elliptical galaxies often contain significant amounts of dust, they generally do not contain the fuel needed to form new stars. NGC 4694 is filled with hydrogen gas and dust, normally seen in a young, bright spiral, and a huge cloud of invisible hydrogen gas surrounds the galaxy.

As this Hubble image shows, the dust in NGC 4694 forms chaotic structures that suggest some form of confusion. It turns out that the cloud of hydrogen gas around NGC 4694 forms a long bridge to a nearby faint dwarf galaxy called VCC 2062. The two galaxies collided violently, and the larger NGC 4694 is collecting gas from the smaller galaxy.

This collision helped give NGC 4694 its distinctive shape and star-forming activity that classifies it as a lenticular galaxy. Lenticular galaxies do not have obvious spiral branches but still have a central bulge and disk. They also contain more star-forming gas than an elliptical galaxy.

Some galaxies, such as NGC 4694, cannot be classified so easily. Their true nature requires some further investigation, and thanks to Hubble we have the opportunity to unravel their secrets.