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Planetary nebulae are among my favorite objects in the sky. When a star a bit more massive than the Sun starts to die, it blows off a super solar wind of gas. As it ages more, this wind it blows speeds up, slamming into the stuff previously ejected, carving it into weird and amazing shapes. Eventually, the entire outer layers of the star blow off, exposing the starâs hot, dense core. This floods the surrounding gas with ultraviolet light, causing it to glow.
Once it starts to emit light, the gas cloud becomes visible to us on Earth, and we can see the weird forms it can take. This structure can be quite fantastic, depending on how exactly the star was spinning as it blew off those winds, what angle we see this at, and the chemical composition of the gas.
In the case of the planetary nebula Sh2-68, though, we have an added factor: motion. The star at the heart of this nebula is moving rapidly through space, and it happens to be in a location in our galaxy where there is more gas and dust between stars than usual. So as it moves, the gas it blows off is itself blown back, like a dogâs hair is blown back when it sticks its head out of a car window.
The image above, taken by my friend Travis Rector using the KPNO 4-meter telescope, shows this in detail. The blue gas is oxygen (which is slightly false-color here; this flavor of oxygen is actually more greenish), and the red is hydrogen. The star itself is the blue one right in the center of the blue gas.
I could go into great detail about the overall shape of the gas, the cavities in it, the actually quite interesting physics of how the gas interacts with the interstellar gas â¦ but câmon. Seriously.
The nebula looks like a giant screaming head with its hair aflame streaking across the galaxy!
That is just way too cool. I used to read the Ghost Rider comic books when I was a kid, and when I look at Sh2-68, itâs hard to shake the image of Johnny Blaze on his motorcycle, his flaming skull grinning maniacally as he wreaks vengeance upon evildoers.
Or maybe thatâs just me. Either way, this is no illusion; we are seeing the gas blowing back due to what is essentially wind, with the parallel tendrils and streamers of gas flowing downwind adding to the effect. The red color Travis used doesnât hurt, either.