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 a ring

Now here’s something you don’t see every day:

NGC 660, captured with the Gemini Multi-Object Spectrograph on the Fredrick C. Gillett Gemini North telescope on Mauna Kea in Hawai‘i in August of 2012. The optical image, made using g, r, I, and hydrogen alpha filters, were assigned the colors of blue, green, orange and red respectively. The field of view is 9.3×5.6 arcminutes and is oriented 8 degrees clockwise from north at right and east up. The total exposure (integration) time was 1,620 seconds cumulative for all filters.
Color composite produced by Travis Rector, University of Alaska Anchorage.
Credit: Gemini Observatory/AURA Get the 3844×2444 version!

This is NGC 660 as seen by the Gemini North observatory at Mauna Kea, Hawaii and before we go any further, you definitely want to grab the full-resolution version. NGC 660 is an example of a rare polar-ring galaxy – that is, a galaxy surrounded by a ring of stars that rotates over the poles of the galaxy. First we have the main galaxy itself, seen edge-on to us in the middle. The host galaxy has a very thick central bulge, which classifies it as a lenticular galaxy.

Surrounding the central galaxy is a ring of stars and gas. And the ring itself is bursting with star formation! Take a look in that full-resolution version and you’ll see nebulae illuminated by hot young stars, and bubbles blown out by massive stars that went supernova.

Of course, neither the galaxy nor the ring are solid, and both are warped by their mutual tidal forces on each other. Just wow!

So how did this whole thing come to be? Polar-ring galaxies are thought to form in one of two ways: either in a head-on collision or when one galaxy rips apart a passer by, strewing the former galaxy into a ring.

In the merger scenario, one galaxy pierces the heart of another at a right angle, and the “pierced” galaxy ends up as a ring around the intruder galaxy. In such a scenario, you end up with a collapsed core and a burst of star formation, and NGC 660 certainly has both.

But there are some compelling reasons why the piercing scenario may not be the case here. For one, the galaxy and its ring aren’t at right angles to one another, but rather at roughly 45-degree angles. For comparison, here’s NGC 4650, another polar ring galaxy that was probably formed as a result of such a collision:

NGC 4650 – a polar-ring galaxy most likely formed in a collision scenario. Note that the galaxy and its ring are at ~90-degrees to one another. Image credit: Credit:Hubble Space Telescope/NASA/ESA.

Another problem is that in a piercing scenario, the gas gets concentrated into the host galaxy’s central region, while the ring is left largely stripped of this gas. In NGC 660, not only is there a lot of gas and star formation taking place in the ring, but the host galaxy itself is “thick” with gas, and there’s quite a bit of star formation going on in there as well. In addition, ring itself is tilted about 45-degrees. Simulations of piercing mergers don’t get us gas-rich, 45-degree ring that we see.

That leaves us with with another possibility, called tidal accretion. In other words, the host galaxy “shredded” a lower-mass passer-by galaxy into the ring. There’s some compelling evidence for that here. In tidal accretion, you can have a wider distribution of gas both in the ring and in the host galaxy itself, allowing for the star formation we see in both. It is also possible for the ring to be oriented at any angle with respect to the host, since you don’t have to start out with a right-angle collision. Finally, in a right-angle collision, you’d expect the pierced galaxy’s nucleus to fall inward toward the host, giving you a “double nucleus” at the center. But NGC 660 only has its original nucleus.

That means that we’re likely looking at the aftermath of a lower-mass galaxy that passed close to NGC 660 long ago, and lost a lot of its stars and gas to the larger galaxy as it passed by. NGC pulled this “chunk” of the passer-by into a ring and both the ring and the galaxy tidally distorted themselves into the shapes we see today.

There’s one more very cool thing going on here, hidden from our view but bright at radio wavelengths is a compact , 32 light-year diameter source near the center of NGC 660. This source is most likely a super cluster of stars in a dense cloud of dust and gas, hidden from view but transparent at radio. This source contains perhaps a few thousand hot, blue youthful stars.

Whatever caused the ring, it led to a lot of star formation in NGC 660, making this already rare polar-ring galaxy even rarer – a polar-ring starburst galaxy! Very cool!

It’s truly amazing how much we can figure out about the origins and evolution of galaxies “just” by looking at them and critically examining their features. The answers don’t jump out at us, but we can take what we see and propose models to explain them. Those models that don’t explain the observations are discarded, while those that do are refined to come up with a picture that better matches what we see. This is how science – astronomy in particular – works!

A cosmic bubble fit for a Viking god of thunder

Amateur astronomer Brigitte Bailleul won herself a chance to observe a target of her choice at Very Large Telescope (VLT) in Chile – an observatory that most professional astronomers only wish they could use. Brigitte was given the chance to observe any target of her choosing. And man, did she chose well:

Thor's Helmet Nebula
Thor’s Helmet as imaged by the European Southern Observatory’s (ESO) Very Large Telescope (VLT) as part of ESO’s 50th anniversary. Credit: ESO/B. Bailleul

Holy Helm of the Gods! This is the Thor’s Helmet Nebula. It’s a giant bubble of glowing gas being blown by the hot, massive star at the center, a special type of star called a Wolf-Rayet star. Wolf-Rayet stars are giant stars that are in a brief period of their lives where they have evolved from the day-to-day hydrogen fusing stage of their lives but not yet ready to explode in a supernova.

The star is losing mass somewhere between 10-6 and 10-5 of a solar mass per year. By comparison, our Sun loses about 10-14 solar masses per year in its solar wind, so this star is losing quite a lot of mass at a fairly high rate. This forms a powerful wind that “inflates” the bubble as it expands outward from the star. The bubble is huge, about 30 light-years across. But the star is so hot and energetic it still ionizes the gas, causing it to glow.

It gets better – the giant full resolution (424×3437) image reveals a lot of detail. You’ll definitely want to click that link, even if you have to go grab a cup of tea while it downloads. The swirls and arcs in the expanding bubble shows wave after wave of powerful winds being blown out by the central star, slamming into cooler, darker gas and dust in the surrounding nebula. Near the edges of the image are the “wings” of the nebula that extent well outside of VLT’s view. This wide-field view from the Digital Sky Survey shows its resemblance to Thor’s helmet:

This wide-field view shows the rich region of sky around the Thor’s Helmet Nebula (NGC 2359) in the constellation of Canis Major (The Great Dog). Image credit: ESO/Digitized Sky Survey 2. Acknowledgement: Davide De Martin

Thor blows a mighty wind, indeed!

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!

The deepest view of the universe: the Hubble eXtreme Deep Field

How deep into the universe have we looked? As of today, this deep:

The Hubble eXtreme Deep Field – Credit: NASAESA, G. Illingworth, D. Magee, and P. Oesch (University of California, Santa Cruz), R. Bouwens (Leiden University), and the HUDF09 Team

This is the Hubble eXtreme Deep Field, (XDF), and it’s a masterwork ten years in the making*. What you’re seeing is what you get when you take a very long exposure with two of Hubble’s best cameras of a region of the sky that contains no known stars – an ocean of 5,000 galaxies! And it’s a very deep ocean, indeed. More than 5,500 galaxies are crammed into a field of view just a fraction of the size of the full moon.

The galaxies are arranged at varying distances from us. Some are relatively bright and even have spiral arms as seen in nearby spiral and elliptical galaxies:

Nearby galaxies in the XDF resemble modern-day spiral and elliptical-shaped galaxies.

But others, way, way, waaaay in the background, don’t appear to have any structure at all. Instead, they just look like little blobs of stars and gas:

A portion of the HUDF. The tiny points of light are primordial clumps of newly formed stars, gas, and dust that would combine to form modern-day galaxies.

So what’s going on here? It turns out that these fainter galaxies are so far away, their light took billions of years to reach us. In other words, we’re seeing these galaxies as they were several billion years ago when the universe was only a few hundred million years old!

To put that into perspective, it helps to think of the XDF as a kind of “core sample” of the cosmos; the deeper into the field we look, the farther back into the universe’s past we can probe:

The XDF, separated by the distances of objects within it. The most distant objects within the XDF are more than 95% of the way back to the Big Bang.

Our universe is 13.7 billion years old. Thanks to Hubble, we can see what galaxies looked like in the current era, what they looked like in its earlier years, and what they looked like a relatively short time after the Big Bang.

And so, in just one image, we can trace the evolution of galaxies over time – from small embryonic building blocks of fluff to beautiful spirals, to giant ellipticals that are the relic of collisions of multiple galaxies. It’s the story of the universe, writ in a single image.

I’ll never tire of looking at this image, and marveling at just how far we’ve come in our understanding of the universe in so short a time.

But what really gives me goosebumps is what’s left to discover.

* I realize in retrospect I didn’t explain this elsewhere in the post. XDF is actually part of the Hubble Ultra Deep Field, which was made with Hubble’s Advanced Camera for Surveys (ACS) from September 2003 through January 2004. But this new image was made with additional ACS images taken since then, as well as Hubble’s new Wide Field Camera 3 (WFC3) which was installed in 2009. WFC3 is sensitive to near-infrared, allowing even fainter, more distant proto galaxies to be imaged. Hence my comments about this image being ten years in the making, as well as the deepest view ever!