A color-coded image of the Boomerang Nebula, as taken by the Hubble Space Telescope. Image credit: NASA.

Colder than empty space? How the Boomerang Nebula does it.

In the depths of intergalactic space, the Big Bang’s leftover glow is just 2.73 K. But this spot in our own galaxy is even colder.

“If you want your boomerang to come back, first you’ve got to throw it.”
-Steven Hall

Anywhere you go in the Universe, there are heat sources to contend with. The farther away you are from all of them, the colder it gets. At a distance of 93 million miles from the Sun, Earth is kept at a modest ~300 K, a temperature that would be nearly 50º cooler if it weren’t for our atmosphere. Move farther out, and the Sun becomes progressively less and less able to heat things up. Pluto, for instance, is just 44 K: cold enough that liquid nitrogen freezes. And we can go to an even more isolated place, like interstellar space, where the nearest stars are light years away.

The dark nebula Barnard 68, now known to be a molecular cloud called a Bok globule, has a temperature of less than 20 K. Image credit: ESO, via http://www.eso.org/public/images/eso0102a/.

The cold molecular clouds that roam, isolated, throughout the galaxy are even colder, just 10 K to 20 K above absolute zero. As stars, supernovae, cosmic rays, stellar winds and more all provide energy to the galaxy as a whole, it’s hard to get much cooler…