The Small Magellanic Cloud galaxy here seen in infrared light, but it looks different when viewed at other wavelengths. ESA/NASA/JPL-Caltech/STScI
We are bathed in starlight. During the day we see the Sun, light reflected off the surface of the Earth and blue sunlight scattered by the air. At night we see the stars, as well as sunlight reflected off the Moon and the planets.
But there are more ways of seeing the universe. Beyond visible light there are gamma rays, X-rays, ultraviolet light, infrared light, and radio waves. They provide us with new ways of appreciating the universe.
Have you looked at the Moon during the daytime? You will see part of the Moon bathed in sunlight and the Earth’s blue sky in front of the Moon.
The Moon behind a blue sky. Flickr/Ed Dunens, CC BY
Now put on your X-ray specs, courtesy of the ROSAT satellite, and you will see something intriguing.
The Sun emits X-rays, so you can see the daytime side of the Moon easily enough. But the night time side of the Moon is silhouetted against the X-ray sky. The X-ray sky is behind the Moon!
The Moon seen in X-rays by ROSAT. The night side of the Moon is silhouetted against the X-ray background. DARA, ESA, MPE, NASA, J.H.M.M. Schmitt
Just what is the X-ray sky? Well, X-rays are more energetic than visible light photons, so X-rays often come from the hottest and most violent celestial objects. Much of the X-ray sky is produced by active galactic nuclei, which are powered by matter falling towards black holes.
In X-rays, the Moon is silhouetted against many millions of celestial sources, powered by black holes, scattered across billions of light years of space.
If you’re in the southern sky and away from light pollution (including the Moon), then you can see the Small Magellanic Cloud. This is a companion galaxy to our own Milky Way. With the unaided eye it looks like a diffuse cloud, but what we are actually seeing is the combined light of millions of distant stars.
Visible light images of the Small Magellanic Cloud are dominated by starlight. ESA/Hubble and Digitized Sky Survey/Davide De Martin
Radio waves provide a very different view of the Small Magellanic Cloud. Using the Australian Square Kilometre Array Pathfinder, tuned to 1,420.4MHz, we no longer see stars but instead see atomic hydrogen gas.
Radio waves can trace the hydrogen gas in the Small Magellanic Cloud. ANU and CSIRO
The hydrogen gas is cold enough that the atoms hang onto their electrons (unlike ionised hydrogen). It can also cool further and collapse (under the force of gravity) to produce clouds of molecular hydrogen gas and eventually new stars.
Radio waves thus allow us to see the fuel for star formation, and the Small Magellanic Cloud is indeed producing new stars right now.
If the universe were infinitely large and infinitely old, then presumably every direction would eventually lead the surface of a star. This would lead to a rather bright night sky. The German astronomer Heinrich Olbers, among others, recognised this “paradox” centuries ago.