The Milky Way is Inside a Massive Hot Bubble of Gas

Illustration from NASA -- but see text

Observations from the Chandra X-ray Observatory, the  European Space Agency's XMM-Newton space observatory, and Japan's Suzaku satellite, suggest that the Milky Way is shrouded in a huge halo of hot gas -- and "hot" here means hot, between 1 and 2.5 million degrees Kelvin. The halo (which is a bubble -- astronomers use "halo" to mean "bubble") is perhaps 300,000 light years in radius or more (the graphic at right shows the Milky Way misleadingly small -- it actually has a diameter of about 100,000 light years, so it should be 1/3 the bubble's radius). The mass of the bubble was estimated by using data from Chandra to measure x-ray absorption by oxygen and data from XMM-Newton and Suzaka to measure total x-ray emission by the bubble and then, based on estimates as to the amount of oxygen in the bubble, calculating the mass the bubble would have.  If this plays out, it will help answer the "missing baryon" problem, which is that a lot more matter was hypothetically created by the Big Bang than is currently observed.