Messier 101, also known as the Pinwheel galaxy, is a spiral galaxy located about 27 million light-years in the constellation Ursa Major, having a diameter about twice that of the Milky Way and containing vast amounts of high-density hydrogen gas, which gives it a fluffy-looking appearance. According to some estimations, it measures up to one trillion solar masses and is seems it is asymmetrical to one side, suggesting that in the past it interacted gravitationally with another galaxy passing by it.

At the same time, the Pinwheel galaxy possesses one of the highest known gradients of metals, having a high concentration of heavy elements in its core, which gradually drops towards the outer edges. In infrared light this distribution of matter is easily highlighted through a variation in the emitted wavelengths of light. While the core of the galaxy appears blue, the outer edges have a distinctive red color.

A new study published recently and based on observations made with the help of NASA's Spitzer Infrared Space Telescope now shows that, although organic molecules, such as polycyclic aromatic hydrocarbons, are found throughout most of the galaxy, in this particular red colored outer area they are completely absent.

"If you were going [to] look for life in Messier 101, you would not want to look at its edges. The organics can't survive in these regions, most likely because of high amounts of harsh radiation," said Karl Gordon of the Space Telescope Science Institute in Baltimore.

Additionally, the team found that, just like the concentration of metals, the amount of polycyclic aromatic hydrocarbons decreases in concentration from the center towards the outer edges, although in the outskirts they disappear completely and can no longer be detected. "There's a threshold at the rim of this galaxy, where the organic material is getting destroyed," Gordon said.

In the early days of the universe, metals as well as polycyclic aromatic hydrocarbons were scarce, thus the Pinwheel galaxy offers the possibility of understanding how the first stars and galaxies formed, as it allows a preview of how a distant galaxy would look like today.