Bad Astronomy

A Dying Star Shoots Out a Cosmic Enigma

Oh, my, but do I have a treat for you today. Feast your eyes on the spectacularness of NGC 5189, a dying star seen by the Hubble Space Telescope:

Hubble picture of nebula NGC 5189
Hubble image of NGC 5189. Click to ennebulenate.

Holy Haleakala! You absolutely must grab the embiggened version, since I shrank it considerably to fit the width of the blog here. There’s also a 7Mb version at 4,000 x 4200 pixels that will have your jaw hitting the floor. Honestly, this is one of the most spectacular Hubble images I’ve ever seen, and I’ve seen a lot.

[If you like this picture, the folks at Hubble Heritage are hosting a contest about it, asking if it reminds you of some other object like a dragon. I see a dog’s face looking obliquely left. See it?]

Now that your eyes have had their fill, it’s time to feast your brain. What are you seeing here?

This object is an expanding cloud of gas rushing away from a dying star. Right in the very center you can see the star itself, a tiny blue dot whose appearance belies its power. Once a star like the Sun, the central star of NGC 5189 is now a dense, extremely hot cinder called a white dwarf. It’s probably only the size of the Earth but is 100,000 times denser than our planet.

A few thousand years ago this star was dying. It had swollen into a red giant, a huge, bloated thing that was expelling a strong, thick wind of gas into space. Over time the star shrank and heated up, turning bluish and starting to blow a thinner but much faster wind. The fast wind caught up with and slammed into the older, slower, thicker wind, carving out a cavity in it. We call these kinds of clouds planetary nebulae, becasue through small telescopes some of them look round and green, like planets.

So what’s with the huge backwards S-shape?

To be honest, it’s not completely understood. However, the most likely cause is a jet of material beaming away from the poles of the central star, powered by the intense heat and magnetic fields of the fierce white dwarf. These types of jets are common in situations like this.

If the star were just sitting there, the jets would plunge up and down, slamming into the material, punching through it along the star’s axis of rotation. However, something is causing the white dwarf to wobble. Perhaps there is a second star we can’t see tugging on it, or there could be planets in the doomed system whose gravity is affecting the white dwarf. But instead of simply rotating, the star is precessing, that is, wobbling like a top that’s spinning down. This changes the direction of the jets over time, sweeping out a huge circle above and below the star.

The S is actually where the jet has rammed into the material surrounding the star, expelled in its earlier winds. You can see streamers of material blowing away, outward, where the jet has hit denser stuff clumped around the star. In fact, this tells us the orientation of the entire structure: Since the nebula is expanding, material moving toward us must be on the near side, and stuff moving away must be on the far side. The picture above uses this information to map the nebula: Parts colored red are moving away from us and therefore on the far side; the blue material is heading toward us and therefore closer.

As you can see, the top of the nebula is tipped away from us. Although I can’t be sure, the current orientation of the jets of material blasting from the star is probably from the upper left to the lower right, where the S shape suddenly stops. As the jet sweeps around, the ends of the S will continue on in those directions.

Velocity map of NGC 5189.
Map of NGC 5189: Red is material on the far side; blue on the near side.

Image credit: Sabin et al.

But it’s actually pretty difficult to tell. It’s not at all clear what’s going on here—and I’ll note I spent a few years studying these types of objects. Their structure is complex and tangled, making interpretation a nightmare. In the near side/far side map, you can see there is also a ring of material around the star like the waist of an hourglass. This type of feature is again common in such nebulae, but the arcs of the S overlap it so it’s fiendishly hard to figure out what’s what here.

So it’s a puzzle! And the only way to solve it is to take more detailed observations, use more careful analysis, and apply the math and physics we’ve developed over the past couple of centuries. It’s only this way that we can properly learn about what we’re seeing here.

That’s one of my favorite things about astronomy, in fact. It really is a treat for both our eyes and our brain. Such devastating beauty is supported, magnified, amplified by the awe we feel when we truly understand it.