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SYFY WIRE Bad Astronomy

Hubble gets crabby for its 29th anniversary

By Phil Plait
The Southern Crab nebula, a huge structure caused by the winds of a dying star. Credit: NASA, ESA, and STScI

Next week marks the 29th anniversary of the launch of the Hubble Space Telescope into orbit. On April 24, 1990, the Space Shuttle Discovery roared into space, and just one day later, astronaut — and, fittingly, astronomer — Steve Hawley used the Canadarm to grasp Hubble, pull it from the payload bay, and deploy it into its own orbit around Earth.

To say it revolutionized astronomy is to ridiculously understate the case. And to this day, nearly three decades later, I still wonder if its biggest impact is on the public perception of astronomy.

After all, you don't have to understand much about science to gasp out loud at an image like this:

The Southern Crab nebula, a huge structure caused by the winds of a dying star. Credit: NASA, ESA, and STScI

That is the Southern Crab, aka Henize 2-104, a nebula something like 12,000 light years away that is flamboyantly announcing the location of a star on the thin hairy edge of dying… and it's previously demised companion, which plays an important role, too.

First off, the nickname "Southern Crab" is an homage to the Crab Nebula, one of the most famous objects in the sky… that also happens to look nothing like a crab. At least this one really does look like one! Kinda. I mean, the big arcs resemble crab legs. Sorta. Really it's more like an hourglass.

But what is it?

Although the details are hard to pin down, there's a general idea of how this works. In the very center of this object is a pair of stars. Both of them were once very much like the Sun, but not any more. One is a white dwarf, the remnants of a Sun-like star that used up all its nuclear fuel, puffed up into a red giant, blew off its outer layers, and exposed its hot, dense, tiny core to space. It plays a key role here as the engine that made this nebula… but the other star was the gas tank. Literally.

The other star is a red giant, a star that is almost done using up its nuclear fuel. Complicated reactions in its core have bloated it up to something like 500 times the Sun's size, and it's blasting out radiation at a rate thousands of times the Sun's, too. If it ever had any planets, they're either cooked by its heat or consumed by its growth.

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It's also blowing out a tremendous wind of dusty material, choked with grains of silicates (rock) and carbon. This material is so thick and opaque it blocks our view of the star almost entirely. A lot (but not all) of it winds up circling the white dwarf, forming a flattened disk called an accretion disk. As it whirls around the dwarf it gets very hot and very bright. This disk is small, probably less than a million kilometers across.

The two stars orbit each other with a period of roughly 30 years, putting them a couple of billion kilometers apart. As material flows out from the red giant, the centrifugal force from the orbital motions of the two stars means this material tends to get ejected in the plane of the orbit. This forms a dense, thick disk around both of them (though in detail probably more like a torus or donut shape), so it's billions of kilometers across.

But then deep inside the red giant, disaster. The physics is quite complicated (if you're curious, I have a detailed description in my book "Death from the Skies!" in the chapter "The Death of the Sun"), but the amount of energy generated in the red giant's core is extremely sensitive to temperature. Any hiccup inside winds up creating chaos and vast amounts of energy very rapidly. This is called a thermal pulse: Something tips the scales, and a huge blast of energy erupts from the red giant.

This paroxysm makes its way to the surface, causing a huge belch of gas and dust from the star. This is like turning a hose spigot all the way up: Material starts to flow down onto the white dwarf's accretion disk a lot more rapidly. The disk gets so hot and energetic that it can reverse the flow, blasting this material away at high speed.

This ejected stuff wants to expand away in all directions, but can't because of that thick disk surrounding both stars. So it goes up and down, away from the equatorial plane, two colossal bubbles expanding outward. As it slams into material already there it breaks up into knots and clumps (due to the Rayleigh-Taylor instability, which happens a lot in expanding gas in space). That is what formed those big outer arcs of the hourglass; we're seeing the edges of the two bubbles. We don't see the outer edge possibly because that part has become disrupted entirely (causing the fringes at the outer edge).

But there's more. See those two blobs at opposite ends of the bubbles? Those may be winds that were focused into beams (astronomers call them jets) by the strong magnetic field of the white dwarf's accretion disk. This is known to occur in other systems, so it makes sense as part of the picture here. They tend to be faster than other material, so they're farther out.

Eventually, perhaps after a century or three, things calm down (although the timing of this is somewhat arguable). The winds from the red giant slow, the garden hose of material aimed at the white dwarf slows, and the system settles. But there's still a wind from the giant onto the dwarf, which still has a hot accretion disk. So some of the wind material is still ejected, but much more slowly. It also forms an hourglass figure, but smaller and brighter due to being thicker. If you look closely at the inner hourglass you'll see some circular features along the edge; those may be where sudden small outbursts ejected a bit more material for a brief time.

When did all this happen? Well, the big hourglass is probably about 9 light years end-to-end (which is crushingly enormous). Looking at how rapidly the gas is expanding, astronomers put a date on all this of about 5,700 years ago! That's very recent, astronomically speaking.

Happy anniversary, Hubble! Credit: NASA / mi_brami, Open Clip Art Library

Which brings up an important point: This structure won't last long. As huge and bright as it is, it's gossamer and ephemeral. In a few thousand more years it'll expand away, fading as it does, and it will disappear.

… but maybe only to be resurrected. Stars like the red giant typically undergo several thermal pulses, each one blasting off a significant part of the star's outer layers. This may be the first in that series, or in any case not the last one. If so, in a hundred thousand years it may all happen again.

But at some point the red giant will run out of stuff. It'll blow its entire outer shell into space, revealing its core: A white dwarf, much like its companion. At that point, the two dead stars will circle one another, cool off after a trillion years or so, and, well that'll be that*.

So take a good look at this image while you can. By Hubble's 29,000th anniversary, it'll be gone.

*To be pedantic they'll actually slowly spiral in to each other, eventually merging and exploding in what's called a Type I supernova. But that's not likely to happen for something like a trillion trillion trillion years (honestly, I don't know how many trillions, but it's a lot of them), so I figure it's safe to not worry about that for now.