A Dying Star’s Flickering Ghost

NGC 2452 a planetary nebula located in the southern constellation of Puppis. Image Credit:  ESA/Hubble & NASA,  Acknowledgements: Luca Limatola, Budeanu Cosmin Mirel
NGC 2452 a planetary nebula located in the southern constellation of Puppis. Click to Ennebulate. Image Credit: ESA/Hubble & NASA, Acknowledgements: Luca Limatola, Budeanu Cosmin Mirel

As stars like our own Sun die, they do so with beautiful complexity. First, they exhaust the hydrogen in their cores, having fused it into helium. As the star contracts, the core gets hotter until helium begins fusing into carbon, nitrogen, and oxygen. The resulting heat and pressure causes the outer envelope to expand, and eventually dissipate out into the cosmos. The result, is a planetary nebula like the one you see here.

At the center of the nebula lies the relic of what used to be the star’s core, known as a White Dwarf. What’s particularly interesting about this white dwarf is that it pulsates over time.[1. This pulsation shouldn’t be confused with a pulsar, which is a far denser object and a very different animal.] In NGC 2452’s case, the white dwarf seems to undergo occasional gravitational wave disturbances that propagate across the surface, like a ripple in a pond traversing the entire surface of the star.

NGC 2542 appears almost ghost-like in this Hubble Space Telescope image, which is perfect since we’re nearing Halloween.  It’s blue-like appearance is the result of the optical (colored cyan) and infrared (colored red) filters used to make this image. It’s eerie and awesome at the same time.

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.

Hubble catches a stellar garden sprinkler

As stars die, they form the most beautiful objects in the cosmos. As the star ejects its outer atmosphere into space, it forms a beautiful planetary nebula. Planetary nebulae are like snowflakes: no two are exactly the same. This is for several reasons – the stars that create them can have different masses, sizes, temperatures, and chemical compositions. Moreover, we see different planetary nebulae at different moments in their evolution. Sometimes, they are later stage and we see them fully formed; other times, we see them just as they are beginning to form, much like Hen 3-1475:

Hubble Space Telescope image of Hen 3-1475. Credit: NASA/ESA

Hen 3-1475 is actually a proto-planetary nebula. That is, a planetary nebula in the making! The central star is surrounded by a thick disk of gas and dust, which almost blocks it from our view. Fortunately, it’s tilted just enough such that we can look “down” the disk and catch a glimpse of the star within.

The star is blowing out a fast stellar wind, which is funneled by the disk into a bi-polar outflow at several hundred kilometers per second.  If you look closely at the outflow funnels, you can even see “shock diamonds“, which are compression waves caused by the gas blasting out between 150-200 kilometers per second!

The central star is very bright – more than 12,000 times brighter than our Sun but not yet hot enough to cause the outflowing gas to ionize. Instead, the gas glows by scattering and reflecting starlight inside the funnel. Cool!

Finally, the shape of the outer lobes are curved into an extended “s” shape (or for you math geeks, an integral symbol). That’s because the star is precessing, like a spinning top as it starts to wobble. The star’s precession is only about 1,000 years, causing the outflowing material to “sprinkle” out into space in a gentle s-shaped pattern.

I love this image not just because it’s beautiful, but because it also reflects the same kinds of everyday physics we witness here on earth (ok, maybe not everybody gets to see shock diamonds from jets everyday but you get the picture). It’s a reminder of how some relatively simple laws of physics can give us such intricate and lovely cosmic sculptures.

The Helix Nebula in a multi-wavelength mashup

One of the great things about having space telescopes sensitive to different wavelengths is that you can you can combine them to make really cool “mashup” images of celestial objects. And the folks at Caltech have done just that to produce this amazing image of the Helix Nebula:

The blue eye of Sauron, ok the Helix Nebula, is the remnant of a star that was once not unlike our own Sun. The exposed core, called a White Dwarf, shines bright in the ultraviolet and illuminates the shroud of its former self. Get the full 6019×6019 version here! (Image credit: NASA/JPL-Caltech)

It looks like a blue Eye of Sauron, doesn’t it? It’s really the remnants of a dying star that once upon a time was very much like our own Sun. About 10,600 years ago, it ran out of the hydrogen fuel in its core and literally blew its outer atmosphere out into space in a series of powerful stellar winds. At the very center lies the exposed core, now an earth-sized sphere of degenerate helium known as a white dwarf.

The remnant, known as a planetary nebula* has expanded to a whopping 2.5 light-years across. Of course, that’s much smaller compared to star-forming regions, galaxies and the like, but way, waaay larger than its original solar system.

The image’s eerie, purple-ish complexion is the result of a combination of ultraviolet images taken by the Galaxy Evolution Explorer satellite (colored blue), and infrared images from the Spitzer Space Telescope and the Wide-Field Infrared Survey Explorer (WISE). The result is a “skeleton view” of what we more commonly see in visible light:

A Hubble Space Telescope image of the Helix Nebula, taken at visual wavelengths. Image credit: NASA

As you can see, the Hubble image of the Helix is gorgeous, but the GALEX/Spitzer/WISE image reveals many fine details of the inner structure of the expanding cloud. Most notable are the spindly comet-shaped “spokes” pointing toward the central white dwarf. These spokes were formed as slow-moving material that was ejected in an earlier outflow was overtaken by a wave of fast-moving material blown out in a later wind. The fast wind slammed into the densest “chunks” of the slower-moving material, forming the comet tail-like spokes in the process.

Take a look at both images and compare the brightness of the central star in visible light in the Hubble image and in ultraviolet light in the GALEX image at top. That white dwarf might be dim in visible light but it’s absolutely blinding at ultraviolet! That’s because it is still incredibly hot – 100,000 K – causing it to shine much more brightly in the ultraviolet.

The outer envelope will continue to expand away, reaching a distance far enough away from the white dwarf that it will no longer glow. The white dwarf itself will eventually cool to become a dead black dwarf – a charred cosmic cinder alone in the night.

Fortunately, we get to enjoy these beautiful objects now in their full expanding splendor.

* Planetary nebulae got their name because when they were first discovered in the 1800’s their round shape reminded astronomers of the way gas giant planets like Jupiter appeared in their telescopes. Thus ensued the confusion among non-experts as to the nature of a planet vs. a nebula for centuries to come. Astronomers are horrible at naming stuff.