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.

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