We often think of the vast areas of space between the stars as being completely empty. However, this is not really true. Much of the space between the stars is filled with atomic and molecular gas (primarily hydrogen and helium), and tiny pieces of solid particles or dust (composed mainly of carbon, silicon and oxygen).

A surprise discovery from the IRAS mission was that space is filled with faint wisps of dust which cannot be seen in visible light. This has been given the name "infrared cirrus" because it resembles the cirrus clouds in the Earth's atmosphere. Infrared cirrus is very cold (15-30 K or -433 to -406 F) and can only be detected in the infrared. Its temperature is due to dust grains being slightly heated by starlight.

This image shows cirrus as seen by the Midcourse Space Experiment (MSX) at 8.28 µm. The dying star Eta Carinae is about one-quarter of the way across this image from the left, and about halfway down. This image covers about 5 x 10 degrees in the sky.

Image credits: MSX/IPAC/NASA/J. Keller