Blogs / Bad Astronomy

Planetary nebulae are too cool.

When a star like the Sun dies, it goes through a series of episodes where it blows off dense winds, vast volumes of gas which expand out from the star in exotic shapes. This is caused by paroxysms in the star’s core; at its advanced age, fusion of one element into another is unstable, and sometimes huge amounts of energy are suddenly dumped into the star’s outer layers. These outer layers respond by swelling and shrinking, and this in turn is reflected in the winds the star blows.

NGC 2371, seen here in a new Hubble Space Telescope picture, is just such a nebula. The winds from the star have slammed into each other, creating the odd puffy shape. As the star sheds its overcoat of material, the hot, dense core is exposed — you can see it as the pinkish-white dot in the center. That color isn’t real; in fact the star, now called a white dwarf, would be bluish or intensely white. But it’s hot, no doubt: it’s over 130,000 degrees Celsius — and that’s not even the hottest one known, which is well over 200,000 degrees!

At that temperature, the star floods the gas with ultraviolet light, which ionizes the material and makes it glow in the same way as a neon sign. In this particular image, sulfur and nitrogen glow red, hydrogen is green, and oxygen is blue. The colors aren’t real; they were just chosen for aesthetics. In general, hydrogen is reddish and oxygen is green.

I was intrigued by the two pink stubs you can in the nebula, on opposite sides of the central star. Those are called FLIERs, for Fast Low-Ionization Emission Regions (I have details on what they are at that link). Their exact formation mechanism isn’t well-understood, but they always appear like that, on opposite sides of the star, so some symmetric shaping force is at work.

I had to laugh when I saw them; they looked like the electrical studs in the neck of the classic Frankenstein’s monster. Too bad I don’t get to name nebulae! I guess, though, after a second look the studs are too high. They look like ears, maybe, or antennae. There was a robot in an old movie or a book cover; I can’t remember, but it had little antennae sticking out of its head just like this. Anyone remember what I’m talking about? Stuff like that makes me crazy when I can’t remember it. Like an itch you can’t scratch.

Anyway, if you like planetary nebulae, then search the blog here for more; I’ve written about them quite bit, since I studied them for both my Masters and PhD. The Hubble website has dozens and dozens of them, too.

The picture above shows a cosmic bulls-eye of epic alignment. But before I can tell you about it, I have to tell you about how the dart got thrown.

One of the more amazing aspects of looking into deep, deep space is that the path there is tortured and twisted. Space itself can be distorted by mass; it gets bent, like a road curves as it goes around a hill. And like a truck that must follow that road and steer around the hill, a photon must follow the curve of space.

Imagine a distant galaxy, billions of light years away. It emits light in all directions. One particular photon happens to be emitted almost — but not quite — in our direction. Left on its own, we’d never see it because it would miss the Earth by thousands or millions of light years.

But on its travels, it passes by another massive galaxy. This galaxy warps space, and the photon does what it must do: it follows that curve in pace, and changes direction… and it just so happens that the curve is just right to send it our way.

The intervening galaxy is essentially acting like a lens, bending the light. If the more distant galaxy is exactly behind the lensing galaxy, we see the light from that more distant galaxy distorted into a perfect ring, a circle of light surrounding the lens. We call this an Einstein Ring. If the farther galaxy is off to the side a bit, we see an arc instead of a complete ring. Gravitationally lensed arcs and rings are seen all over the sky, and they can be used to determine the mass of the intervening galaxy! The more mass, the more distorted the light from the farther galaxy. So the Universe has given us a nice method to let us weigh it.

In a surprising twist, astronomers have found a new type of lensed galaxy: a double ring! In a rare alignment, there are two distant galaxies aligned behind an intervening lensing galaxy. They’re like beads on a wire, lined up just right such that both more distant galaxies are lensed by the nearer one. In this case, the lens is about 3 billion light years away, and the other two are 6 and 11 billion light years away, an incredible distance.

This image is amazing, but it is also a powerful scientific tool. It allows us to measure not just the mass of the lensing galaxy, but also the amount of mysterious dark matter nearby. We cannot see the dark matter, but it too bends light, and contributes to the lensings. By observing lenses like this, we can take a sample of dark matter in the Universe, and that’s a crucial first step in understanding it. Even better, these double rings allows us to measure the amount of total mass not just in the nearest galaxy, as is usual, but also in the middle galaxy as well, since it distorts the light from the galaxy behind it (turns out it’s a rather lightweight one billion solar masses; our own Galaxy has more than 100 times that mass, so the middle galaxy is considered a dwarf).

This is a beautiful happenstance; it gives us a measure of the Universe at two points, with one being for free. In fact, Tommaso Treu, the astronomer at U.C. Santa Barbara who investigated this lens, points out that if we can find as few as 50 of these double rings, we can get a much better idea of the distribution of not just dark matter, but also the even more mysterious dark energy in the Universe. That’s one of the biggest goals of modern astronomy… and we may get a handle on it due to a coincidental ring toss.

M81 and M82 are bright nearby galaxies; you can spot them with binoculars easily in the northern sky, and they are a mere 12 million light years from us (for comparison, the Milky Way Galaxy is 100,000 light years across, so if you think of the Milky Way as a DVD, M81 and M82 would be about 14 meters away). These two galaxies interacted a couple of hundred million years ago, and the gravitational interaction drew out long tendrils of gas (which is very common in colliding galaxies).

Astronomers examined this bridge of material using Hubble, and found clusters of stars in it. That was totally unexpected; the gas was thought to be too thin to form stars! Amazingly, many of the stars are blue, indicating they are young (blue stars burn through their fuel much more quickly than redder stars. This means that the gas is still forming stars, even 200 million years after the collision!

In the image below, almost all the stars you see are young blue stars formed in the aftermath of that titanic collision. The reddish stars are stars in our galaxy, and the bigger objects are distant background galaxies.

Most likely, the stars formed when turbulence in the tendril caused local regions of denser gas, which could collapse to form stars. Before these observations, it wasn’t really thought it was possible to form stars in the regions between galaxies, so this is an interesting new find.