Heart and Skull nebula

The entire universe in blog form
April 7 2011 7:00 AM

Heart and Skull nebula

I've been an astronomer a long, long time. Even so, I still sometimes get surprised at how different the same object can look when imaged in different ways. I just saw an excellent example of this... W5, aka the Soul Nebula:

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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[Click to ennebulanate.]

Pretty, isn't it? It was taken by César Cantú, an amateur astronomer in Mexico. It's not a true color picture. Not even close! For one thing, he used three filters which let through extremely narrow wavelengths of light (that is, the filters reject all light except for a very thin range of wavelengths; I've written about them before). Our eyes see broad ranges of colors, so immediately these filters change the very nature of the picture. Different atoms in space emit at different colors, and the filters he chose select for hydrogen, oxygen, and sulfur, which tend to emit light very strongly in gas clouds.

Not only that, he mixed and matched the colors. The hydrogen filter lets through red light, but he colored it green in the picture; oxygen is usually green but he made it blue*; and sulfur is red which he actually did color red. This throws off my usual sense of what I'm seeing in a picture (I really am used to hydrogen being red and oxygen green) so it forces me to re-evaluate how I see this gas cloud. But more than that, when César said this was the Soul nebula I almost didn't believe him! I've seen this nebula any number of times, including most famously as seen in the infrared picture here from the Spitzer Space Telescope (go to that link to see a much larger version). It's tilted the other way, but if you look carefully you'll see this is in fact the same gas cloud; look closely at the cusp at the top of the nebula in particular.

And I can't help but laugh at the funny irony here: in the Spitzer image, W5 looks like a classic valentine, a heart. But in César's picture it looks like a skull! It even has glowing eyes, like something right out of Halloween. Talk about contrast. The more I look at it, the creepier it gets. Yikes.

Of course, there's science here. Stars are forming in this region of space, and the glowing eyes of the skull are actually where newborn stars are blowing strong winds and emitting fierce amounts of ultraviolet light. That acts to carve cavities in the gas, making what look like the eye sockets. In fact, the whole outline of the nebula is shaped by the winds from those stars.

So I guess there are many ways to see this same object: different filters, different colors for those filters, different wavelengths, different body parts as interpretations of the shape, and thinking scientifically instead of pareidoliacally. Which I think isn't a word, but maybe is now.

Anyway, if you like this picture, go take a look César's website where he has beautiful images of galaxies, planets, and other astronomical objects. And when you do, keep your eyes and your brain open. Who knows what you'll see!



* For the pedants out there, the color emitted by an atom depends on other factors as well, like the temperature, the density, and how ionized the atom is. Oxygen missing one electron tends to emit in the blue, but an oxygen atom with two missing electrons emits in the green. It's pretty complicated in reality, as the Universe is sometimes, so I'm simplifying here.



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