The galaxy Messier 100, or M100, shows its swirling spiral in this infrared image from NASAs Spitzer Space Telescope. The arcing spiral arms of dust and gas that harbor starforming regions glow vividly when seen in the infrared.
M100 is a classic example of a grand design spiral galaxy, with prominent and well-defined spiral arms winding from the hot center, out to the cooler edges of the galaxy. It is located about 55 million light years away from Earth, in the little-known constellation of Coma Berenices, near to the more recognizable Leo.
In the center, we can see a prominent ring of hot, bright dust surrounding the inner galactic core. Moving further out, the spiral arms peter out towards the edges of the galaxy, where thick webs of dust dominate. Beyond the edges of the dust clouds, a faint blue glow of stars extends to the edge of the galaxys disk.
Two small companion galaxies, known as NGC 4323 and NGC 4328, appear as fuzzy blue blobs on the upper side of M100. These so-called lenticular galaxies are virtually clear of any dust, so they lack any of the red/green glow seen in their bigger neighbor. The shape of M100 is probably being perturbed by the gravity of these galaxies.
This, then, is the power of infrared telescopes like Spitzer, their ability to see through the dust, which scatters visible light quite well, but is transparent to infrared, and show us the activity inside of a galaxy’s spiral arms.