Most of space is extremely cold. Although the universe is filled with countless stars that give off heat, the sheer distance between them means that temperatures in space are constantly freezing. Interestingly, there are some locations in space that are actually colder than empty space. To date, the coldest known place in the universe is the Boomerang Nebula, located 5,000 light years away in the constellation Centaurus.
The Boomerang Nebula
Temperatures in the Boomerang Nebula average at minus 458 degrees Fahrenheit (minus 272 degrees Celsius). The coldest possible temperature in physics is called absolute zero, which is equivalent to minus 460 degrees Fahrenheit (minus 273 degrees Celsius). That means that the Boomerang Nebula is only a single degree higher than the coldest possible temperature of our universe. The Boomerang Nebula is even colder than the Cosmic Microwave Background Radiation (CMBR), which is a form of microwave radiation left over from the Big Bang. Light from the CMBR has been traversing the universe for over 13-billion years, and so its temperature has dropped significantly since the Big Bang. As of yet, the Boomerang Nebula is the only known object in the universe that has a temperature lower than the CMBR. How exactly did a nebula get to be this cold?
A Cosmic Refrigerator
The Boomerang Nebula contains a red giant star that is currently in the final stages of its life. The nebula itself is defined as a protoplanetary nebula, and is actually composed of the ejected material from its central star. One day, the red giant star will become a white dwarf and the Boomerang Nebula will become a true planetary nebula. It may seem strange then that the Boomerang Nebula is so cold since it contains a red giant. Interestingly, the reason why this particular nebula is so cold is similar in principle to how a refrigerator works. A refrigerator uses expanding gas to keep itself cold, and within the Boomerang Nebula, gas from the red giant star is expanding outwards at an extremely fast rate. This causes the entire structure to become exceedingly cold, and so the Boomerang Nebula can be thought of as a kind of cosmic refrigerator.