Can there be wind where there is no air?
The Discovery of Vast Tail on Dying Star Mira, as reported widely across various news sources, piqued my curiosity. Marc Kaufman at Washington Post wrote
The 13-light-year-long tail is made up of molecules of oxygen, carbon and nitrogen shed by the slowly dying but very fast-moving star as it speeds through the Milky Way at almost 300,000 miles per hour. … Because of its great size, the gravitational force keeping it together is weak at the edges, allowing the winds created by the fast-moving star to pull matter off and send it back into the tail.
…
The scientists likened the star’s movement through space to a bullet traveling at supersonic speed through air. The front end of the star creates a shock wave of sorts as it pushes forward, and that shock causes hot gas to fly back. Cool winds coming off the star mix with the hot gases to form the tail and make it glow, or fluoresce, with ultraviolet light. The process is similar to a speeding boat leaving a choppy wake or a steam train producing a trail of smoke, they said.
The images make sense when you envision the movements of a bullet, boat, or train on earth, as they travel against the air, which we assume is stationary. But what is Mira moving against in the vast space? Ether? Will whatever is in the dark vacuum give enough resistance to Mira?
Of course, with a gassy star like Mira you don’t have to have resistance to make a tail — it Mira is accelerating, it would throw its outter shell behind. But an ever-accelerating star would be more remarkable than a star with a tail, I’d think.
Anyhow, my earth-bound mind can’t understand heavenly objects.
[Update, same day:
It is starting to make sense after reading the source from Galex.
While most stars travel along together around the disk of our Milky Way, Mira is charging through it. Because Mira is not moving with the "pack," it is moving much faster relative to the ambient gas in our section of the Milky Way. It is zipping along at 130 kilometers per second, or 291,000 miles per hour, relative to this gas.Mira’s breakneck speed together with its outflow of material are responsible for its unique glowing tail. Images from the Galaxy Evolution Explorer show a large build-up of gas, or bow shock, in front of the star, similar to water piling up in front of a speeding boat. Scientists now know that hot gas in this bow shock mixes with the cooler, hydrogen gas being shed from Mira, causing it to heat up as it swirls back into a turbulent wake. As the hydrogen gas loses energy, it fluoresces with ultraviolet light, which the Galaxy Evolution Explorer can detect.
So there is air (gas), and Mira is giant enough and fast enough that the thin ambient air makes a difference.
Cool. So physics — as I know it — still stands. It was just my ignorance of astronomical facts.
Not to shift the blame, but does the Post writer simply assume his readers know there is air (gas) in the sapce? Or did he paraphrased the above Galex news release, not knowing he left an important premise behind? Apparently, the "train" analogy was the creation of the Post writer - the Galex piece mentioned both the "bullet" and the "boat". But the train leaving a smook trail is quite a different mechanism from the supersonic bullet or the water piling up in front of a boat. The latter two have to do with the shock, and the train can never create a shock wave, at least not as of this year. I am not sure if he understood the differences.
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