A dying star, dancing in the night

Behold, I bring you great tidings of joy this holiday season, for unto us comes awesome:

Planetary Nebula NGC 5189. Image credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA)
Planetary Nebula NGC 5189. Image credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA) Click here to get the full-resolution version!

Do you see what I see? A star, a star, dying in the night, and it’s bringing us goodness and light! This is NGC 5189, which lies about 1,800 light-years away in the southern constellation Musca. Once upon a time it was a star very much like our own Sun, but is now in its death throes. When stars like our Sun die, they cast off their outer atmosphere in a spectacular fashion, forming what is known as a planetary nebula (because way back in the day, their fuzzy blob-like appearance reminded astronomers of planets; since astronomers are terrible at naming things, the term planetary nebula stuck – hey, don’t blame me, I’m just the messenger).

Although imaged in great detail by the Gemini South Telescope, it wasn’t until astronomers used the Hubble Space Telescope to create the most detailed image of NGC 5189 yet. And what an image it is!

There’s a lot going on here, so you may want to grab the full-resolution version of this image and play along at home because it’s a gold mine of stellar wreckage.

The first thing I noticed is the twisted, reverse “S”-shaped structure. The “S” is fragmented into comet-shaped structures like this one, taken from the upper left-hand corner of the image:

Fragments of debris, blown out from the central star in NGC 5189. Image Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA)
Fragments of debris, blown out from the central star in NGC 5189. Image Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA)

Each of those knots is a clump of what used to be the central star’s outer atmosphere. We’ve seen these spoke-like clumps before; they are the result of slower-moving material blown out by the star in an earlier wind that have since been blasted again by a later, faster wind from the same central star. The clumps are a powerful reminder of just how vast NGC 5189 is, because each of those clumps are about the size of our entire Solar System!

The second major feature of NGC 5189 are the bipolar (perhaps quadrupolar) lobes blowing out from the central star. The lobes are arranged in an hourglass shape with one lobe coming toward us (moving toward the upper-right) and the other moving away from us (toward the lower left). These lobes are being driven by the star’s howling winds, which are reaching 2,700 kilometers (about 1,700 miles) per second. And it’s these same winds that sculpted the knotty clumps as they slammed into the slower-moving material in the mebula’s “arms.”

HD 117622 (center), the white dwarf at the heart of NGC 5189. Image credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA)

Finally, at the center of it all, is the now-exposed core of the star itself, known as a white dwarf. Designated HD 117622, this white dwarf is a hot (10,000K), dense ball of degenerate helium, no larger than the Earth. (Note: the light from the star saturates Hubble’s detectors and “spills” into adjacent pixels. The white dwarf itself simply too small to be seen – all we can see is its light.) Even so, it’s hot enough to illuminate the surrounding nebula, which by now is more than 2 light-years across!

Demonstration of orbital precession, using the Earth's orbit around the Sun Demonstration of orbital precession, using the Earth’s orbit around the Sun

So what is responsible for the strange shape of this nebula? The most likely explanation is that HD 117622 has an as yet undetected companion. That would allow for HD 117622 to wobble, or precess in its rotation as it lost mass. Furthermore, its orbit with its companion would also precess, creating a “wobble within a wobble.”

All the while, HD 117622 is loosing mass, first in a slow, gradual wind, perhaps creating the reverse “S” shape over a long period of time, much like a stellar garden sprinkler. Later, the second wind emerges, creating the lobes but also slamming into the slower-moving material in the “S” creating the comet-shaped fragments.

That said, I’m making an educated (to be generous) speculation here  because no such companion to HD 117622 has been detected as of yet. There is also a lot about how planetary nebulae form that astronomers still do not yet fully understand. There may be some other mechanism at work here, waiting to be discovered.

In the meantime, we can sit back and gaze in amazement at its full beauty. It’s a sobering reminder of our own Sun’s demise to come, billions of years from now. But for the moment, we are here to admire the universe that created us.

Happy Holidays.

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