I love planetary nebulae; they are among my favorite objects in the sky. First, of course, they are beautiful: eerie rings, ellipses, flowing gas, symmetry. Second, they are poetic: the dying gasps of a sun-like star, ejecting septillions of tons of gas that glows and forms these dramatic objects. And they all have a tale to tell. One of the best stories belongs to the Helix Nebula. And it's telling it loud.
Oh yes, you want to click that to embiggen it. That picture is in fact the Helix: a star that was once not unlike the Sun, now undergoing paroxysms, blasting out a super stellar wind of gas. The naked core of the star is white hot -- 120,000 Celsius, 25 times hotter than the Sun! -- flooding the gas with ultraviolet light, causing it to fluoresce like a neon sign (in fact, neon can be seen in the spectrum of such nebulae). This image was taken using the 2.2-metre Max-Planck Society/European Southern Observatory telescope at the La Silla observatory in Chile. This is an incredibly wide field shot; it spans about half the width of the Moon in the sky. The Helix is huge, more than 2 light years (20 trillion km or 12 trillion miles) across, and close to us: only about 700 light years away. That's practically in our laps on a galactic scale. Most planetary nebulae are not spherical shells, but elliptical in shape. If the star is spinning rapidly, the gas it throws off will not be spherical but will be oblate or prolate -- like a ball being sat on and squished a bit. The Helix is like that, and we're seeing it roughly down the pole, so it looks pretty round. If we saw it from the side, it would definitely look squashed.
The overall shape is interesting, and I could write a thousand words on it, but instead I want you to take a look in the center of the nebula, in the region surrounding the star that is at its heart. It's the star at the center of the image here. Take a good look; the Sun will look pretty much like this in just a bit more than 7.5 billion years from now. The star that spawned the Helix was more massive than the Sun -- we probably won't make a big planetary like the Helix -- so the remaining ember we see here is more massive and hotter than the Sun will be as well. But even that's not as interesting to me as the simple fact that in that image, you can see distant galaxies right through the nebula itself! I've marked a few in the image, and if you look at the original hi-res image you can see dozens of them scattered behind the nebula. Nebulae like this look like gigantic solid objects, but in fact a dense one would still be considered a hard vacuum in the laboratory. There might be something like 10,000 or even 100,000 atoms in a cubic centimeter of nebular gas, but compare that to the 10,000,000,000,000,000,000 atoms in air at sea level! That's why you can see through them. Across the several light year diameter of the nebula there are enough atoms all together to see the glow of the gas, but it's still so thin it might as well hardly be there at all. And still, that gas has much to tell us. The spectrum reveals what gas is in there -- hydrogen (red), oxygen (blue as well as green), sulfur (very red) -- and also how hot it is, how dense it is, how fast it's moving... and the shape of the nebula itself tells us how the star lived, and ultimately how it died as well. All that, from gas so thin it's barely distinguishable from space itself. And yet, still very, very beautiful. Do I have to say it? Science. I love this stuff!
