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.

A High Energy X-Ray Superbubble

A long, long time ago in a galaxy not that far away at all as a matter of fact, a cluster of stars formed inside a great cloud of gas and dust. Among them were giant stars whose masses may have been between 10-30 times the mass of our Sun. These behemoth stars lived fast, fusing their once hydrogen cores into heavier and heavier elements until one day they could no longer sustain themselves and they explodeed as supernovae.

Like TNT exploding in a mine, the supernovae sent out powerful shock waves that blew a giant bubble in the cloud 1,200 light-years across. Behold:

Massive stars exploded as supernovae, creating superbubbles in the surrounding gas. X-Ray (blue), Optical (yellow/green), Infrared (red) composite of N44 in the Large Magellanic Cloud. Click for the 864×690 version, or get the ridiculously  cool 3600×2874 version Credit: X-ray: NASA/CXC/U.Mich./S.Oey, IR: NASA/JPL, Optical: ESO/WFI/2.2-m

Holy Cosmic Cauldrons! This region is known as N44 and resides in the Large Magellanic Cloud, a satellite galaxy to our own Milky Way. It’s actually a composite of an image of N44 taken by the Chandra X-Ray Observatory, an optical image taken by the 2.2-meter Max-Planck-ESO telescope in Chile, and the Spitzer Space Telescope.

The colors correspond to different wavelengths imaged by the different observatories. Red stands in for infrared emission captured by Spitzer. This is cooler gas and dust and is relatively “intact” surrounding the bubble.

Yellow corresponds to imagery from the Max-Planck-ESO telescope and shows hot ionized gas. This gas is glowing from the ultraviolet radiation from hot, young stars, in much the same way that gas glows in a florescent light bulb.

Finally, we have the really high-energy stuff, the x-ray emission (shown here in blue), taken by the Chandra X-Ray Observatory. In fact, let’s take a look at just the Chandra X-Ray image:

Chandra X-Ray image of star cluster NGC 1929 inside N44.

Here’s where the story gets really interesting. For a long time, it was assumed that the bubbles were created by winds from the hot stars that eventually went supernova. That makes sense. After all, the more massive and hotter they are, the more mass they lose as stellar winds.

Well, the winds from these stars certainly did “blow” out the bubbles. However, it turns out there is a lot more x-ray radiation coming from inside the bubbles than was expected from just the winds alone.

So where is this extra x-ray radiation coming from? Dr. Anne  Jaskot from the University of Michigan and her team used Chandra to find out. It turns out that there are two extra sources of x-ray radiation: the supernova shock waves striking the walls of the cavities, and hot material evaporating from the cavity walls. In fact, if you compare the two images above, you can actually see the X-ray radiation coming off the walls of the bubbles. Cool!

In other words, the supernova explosions themselves generated a lot of high-energy x-ray radiation by slamming into the walls of the bubble making the walls so hot they give off x-rays as the shock waves passed through.

Moreover, these shockwaves, created in the supernovae of these massive stars, compressed the surrounding gas. This in turn triggered the formation of even more stars.

And there it is, the cycle of stellar life: the ashes of stars are blown along the expanding wave of a cosmic bubble, and seed the next generation of stars.