Taotao Fang

A distant city seen through a fogged-up window: bright lights blur, details are lost. That's roughly how astronomers see gas with magnesium ions (magnesium) in our galaxy — the strongest spectral lines "peg the needle," preventing us from knowing how much material is really there.

Around our Galaxy, like an invisible crown, stretches a tenuous cloud of gas heated to millions of degrees. It is called the galactic halo. Seeing this glow directly is difficult: it emits soft X-rays that are absorbed by Earth's atmosphere.

The Andromeda galaxy is our nearest large neighbor, and like the Milky Way, it has an invisible gaseous shroud. This shroud of extremely hot, rarefied gas extends for hundreds of thousands of light-years. Seeing it directly is nearly impossible — the gas is too faint.