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.

A galaxy in perspective

One of the things that’s very difficult to appreciate is the relative sizes and distances of celestial objects. After all, their sizes and distances aren’t remotely small enough for us to appreciate. As a result, everything we see looks in some way compressed – one object in an image may just as easily be twice as distant, or ten times as near, as another with little way of knowing the difference.

But if we apply a little knowledge to these images, the relative distances seem to jump out at us, like this:

ESO 318-13 as imaged by the Hubble Space Telescope using its Advanced Camera for Surveys. Image credit: NASA/ESA & Hubble

This is a Hubble Space Telescope image of ESO 318-13 taken with Hubble’s Advanced Camera for Surveys. ESO 318-13 is an irregular-shaped galaxy. That is, a galaxy that has no formal structure such as spiral arms or even a well-defined central nuclear region.

It’s a beautiful image, but what I really love about it is that in one shot we can discern a tremendous amount of perspective and depth. Let’s start with the nearby stuff and work our way outward.

The brightest objects are foreground stars in our own Milky Galaxy. Coincidentally the foreground star in the middle right of the image happens to line up with the middle of ESO 318-13, but keep in mind that the galaxy is itself a collection of stars millions of light years beyond.

If you look at the full-resolution version of the image, you’ll see individual stars within ESO 318-13. And if you look toward the right-edge of the galaxy you’ll spot a beautiful face-on spiral tens of millions of light-years in the distance behind the galaxy. For all of their grandeur, galaxies are mostly empty space, and can be transparent enough to allow background galaxies to easily be seen through them.

Pan around the image and you’ll see a lot more galaxies in the background, including a giant elliptical galaxy in the upper right-hand corner of the image. ESO 318-13 is a dwarf galaxy and may contain a few tens of millions of stars. But the elliptical galaxy in the background is a cosmic titan containing several hundred billion suns!

The distances to objects in this image are all relative – there isn’t anything that tells us exactly how far away they are just by looking at them. To get those measurements we need to use indirect methods such as spectroscopy. But in one image we can begin to get an idea of the sheer scale of the cosmos – keeping in mind that this is still a fairly local region of the universe!

NGC3344 – a mini-me Galaxy in a spin

I can’t say I ever tire of looking at galaxies – the great islands of the universe home to billions of Suns. but the Hubble Space Telescope’s image of NGC 3344 shows a galaxy that in some ways is a lot like our own. Behold:

Nearby spiral galaxy NGC 3344. Tightly wound spiral arms surround a subtle bar of older stars at the center. In this regard, NG 3344 is a smaller analogue to our Milky Way. Image Credit: ESA/Hubble & NASA. Want the giant 3845×3049 version? Git sum!

The bright stars with the diffraction spikes are nearby stars in our own Milky Way, so let’s get them (figuratively) out of the way and admire the beautiful galaxy beyond. NGC 3344 is 25 million light-years away, making it a cosmic neighbor to our own Milky Way, though it contains about half as many stars.

NGC 3344 is a spiral, like our Milky Way. It even has a subtle central bar, oriented vertically in the image. Our own galaxy has a similar bar at the center, though it is believed to be better defined NGC 3344’s. Because it’s relatively nearby, the galaxy covers a wider region of the sky than Hubble’s camera sees, so we only see about 1/3 of it here.

However, a wider field of view shows that the NGC 3344 has an extended, faint ring of stars surrounding it. It turns out this outer ring of stars is orbiting galactic central point in an opposite direction than the inner spiral arms. It’s not clear why this is the case, but it could be due to the cannibalization of a smaller passing galaxy some time ago.

If you haven’t already, check out the giant 3845×3049 version and admire this galaxy in all of Hubble’s glorious detail!

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.

NGC 1672, a galaxy a-bursting with stars

Galaxies are the great islands in the universe where stars live out their lives. But over the course of a galaxy’s evolution, there are times when it is active with more star formation than others, like this gorgeous example of NGC 1672 courtesy of the Hubble Space Telescope:

NGC 1672
NGC 1672 is a barred spiral galaxy with populations of stars forming in the arms and in the nucleus. Click for the 1280×919 version or get the 5302×3805 version

You really want to click that image to see the large version or, if you’re keen to experience the finer details, get the amazing 5302×3805 version.

You’re welcome 🙂

As I was saying, galaxies evolve over time and go through periods where there is a lot more star formation going on than others. NGC 1672 is one of those active galaxies, with star formation taking place not only in the spiral arms, but also in its nucleus as well:

Dude! Star formation in the nucleus of NGC 1672!

NGC 1672 is a type of galaxy known as a Seyfert galaxy. Galaxies typically have quiet nuclear regions – that is, they are dominated by older stars and have very little activity going on in those parts. Seyfert galaxies are quite different from typical spiral galaxies in that their nuclei are are very bright and are typically active with star formation, which you can easily see in the close-up.

So what’s going on here? The answer may lie in the fact that NGC 1672 is also a barred spiral galaxy. The Hubble image shows the central region of the galaxy but this ground-based image shows NGC 1672 in all of its barred-spiral glory:

Portion of a wide field ground-based image of NGC 1672’s taken by the Digitized Sky Survey 2 (DSS2). The image was rotated to match the Hubble Space Telescope image. Note the bar!

As you can see, the bar is chocka-block of stars, gas, and dust that orbit the core in a highly inclined orbit. In other words, the gas is largely “aimed” toward the supermassive black hole at the very center. This in turn creates an accretion disk around the black hole which makes for a very bright nucleus.

But it also means that there is a lot of moving material in the outer region of the nucleus as well, and that means star formation around the nucleus!

There’s still a lot about barred spirals that we don’t yet know. Our own home galaxy contains a bar as well and barred spirals are not uncommon. Astronomers believe that bars are temporary but many questions remain. How do bars form? How long do they last? When do they form – do the form early in the galaxy’s evolution or late? And perhaps most interesting, why do they form in the first place?

There are lots of other little amazing details in the image, and I invite you to grab the 5302×3805 version and start digging around. The brightest stars are foreground stars that live right here in the Milky Way, but there are lots galaxies deep in the background, some of which can be seen through NGC 1672, like this one:

Dude! A galaxy in the background of of NGC 1672!

Notice the color of this galaxy – that’s not because the background galaxy is really that color, but because it’s blue light is scattered by the dust in NGC 1672 itself, letting the yellow, orange, and red light through, giving the background galaxy a caramel color. Pretty cool!

There’s lots of beautiful gems in this image so dig away!