Feel Me Through The Darkness

A thousand live bats fluttered from the walls and ceiling while a thousand more swooped over the tables in low black clouds, making the candles in the pumpkin stutter.

The Lagoon Nebula in Hydrogen, Sulfur, and Oxygen

A Blue Bubble That Isn’t

Blue bubble in Carina The lovely gas cloud surrounding the star WR 31a. I strongly urge you to click to ennebulenate. ESA/Hubble & NASA/Judy Schmidt

Gaze upon this gorgeous Hubble image of the nebula surrounding the star WR 31a but be warned: What you’re seeing is not what you get!

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

First things first. The star WR 31a is what’s called a Wolf-Rayet (or WR) star. These come in a variety of flavors, but most of them are extremely massive, hot, and luminous. And I’m not fooling around here: They start at 20 or more times the mass of the Sun, are five to 40 times hotter, and can be millions of times more luminous.

These are stars nearing the ends of their lives. They’ve already run out of hydrogen in their cores to fuse into helium, and are now fusing helium (or heavier elements) to create energy. Between those two stages in a massive star’s life it becomes a red supergiant, and a natural consequence of them blasting out so much energy is that they’ve shed their outer layers; octillions of tons of hydrogen blown away into space by the vast energies brewing underneath.

This exposes the hotter and brighter layers that used to be deep inside the star. This can illuminate the expelled gas, creating a beautiful nebula around the star. This is similar to a planetary nebula, when a star with lower mass blows off its outer layers, exposing the core, which excites the expelled gas, causing it to glow literally like a neon sign. For WR stars, the gas isn’t glowing but reflecting the star’s light, making them what are called protoplanetary nebulae.

That’s the case in WR 31a. The beautiful wisps and tendrils you see used to be a part of the star but have been thrown off in a dense stellar wind. The gas is being lit by the star itself at the center. And just to give you a sense of scale, that structure is about nine light-years across: 90 trillion kilometers. On this scale, our entire solar system would be invisibly small.

But have a care here! The nebula is being called a “blue bubble,” but it’s not really blue. Kinda.

This image was taken using two different filters on Hubble Advanced Camera for Surveys. What’s colored red in the image is actually from the near-infrared, light our eyes don’t see, but to which ACS is sensitive. What you see colored blue in the image is actually from a filter that lets through light from the green part of the spectrum out into the red. The color used to display the light from that filer in the photo is more of an artistic choice than a physical one; they could’ve chosen green or orange or yellow. That’s arbitrary.

The funny thing is, in this case that filter actually blocks blue light. In other words, of all the colors the human eye can see that this gas is sending toward us, blue is the only one not really shown in the image! And it does reflect blue light; in fact, many such reflection nebulae appear blue to the eye.