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March 31, 2013

The Egg Nebula, a protoplanetary nebula in Cygnus

The Egg Nebula, a protoplanetary nebula in Cygnus

Image Credit: W. Sparks (STScI) and R. Sahai (JPL), NASA and The Hubble Heritage Team (STScI/AURA)


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The Egg Nebula (designated CRL 2688) is a bipolar protoplanetary nebula with an estimated diameter of about 0.4 light-year that lies approximately 3,000 light-years away from Earth in the constellation of Cygnus, the Swan. It was first discovered on the Palomar Observatory Sky Survey as a small-egg-shaped nebulosity, hence its name.

Despite their name, protoplanetary nebulae have nothing to do with planets: they are clouds of dust and gas formed from material shed by an aging central star similar in mass to our Sun. For such a star death is a long process. After billions of years, the hydrogen fuel that powers the star begins to run out. The star balloons to great size and becomes a red giant. Eventually, however, the star collapses back on itself. This increases the temperature at its core and most of the stars material is catapulted into space, enveloping itself in clouds of gas, but the core is not yet hot enough to make the gas itself glow on its own. Instead, the gas is merely reflecting the light from the star.

But as the star continues to evolve, it becomes hot enough to emit strong ultraviolet light. At that point it will have the power needed to make the gas glow, and will become a real full-fledged planetary nebula. But before the nebula begins to shine, fierce winds of material ejected from the star will continue to shape the surrounding gas into intricate patterns. Then the star cools down and all that is left after this process is the exposed, hot and dead core, known as a white dwarf.

A protoplanetary nebula is a relatively short-lived phenomenon, what means there are relatively few of them in existence at any one time. Moreover, they are very dim, requiring powerful telescopes to be seen. This combination of rarity and faintness means they were only discovered comparatively recently. The Egg Nebula, the first to be discovered, was first spotted less than 40 years ago, and this image of it shows one of the best views to date of this brief but dramatic phase in a star’s life.

Resembling a rippling pool illuminated by underwater lights, the Egg Nebula offers astronomers a special look at the normally invisible dust shells hiding an aging star. These dust layers, extending almost two-tenth of a light-year from the central star, have an onionskin structure that forms concentric rings around the star. A thicker dust belt, running almost vertically through the image, blocks off light from the central star.

While we can’t see the star directly, four searchlight beams of light coming from it shine out through the nebula. It is thought that ring-shaped holes in the thick cocoon of dust, carved by jets coming from the star, let the beams of light emerge through the otherwise opaque cloud. The precise mechanism by which stellar jets produce these holes is not known for certain, but one possible explanation is that a binary star system, rather than a single star, exists at the center of the nebula.

The onion-like layered structure of the more diffuse cloud surrounding the central cocoon is caused by periodic bursts of material being ejected from the dying star. The bursts typically occur every few hundred years.

Dust in our atmosphere reflects sunlight such that only light waves vibrating in a certain orientation get reflected toward us. Polarizing sunglasses take advantage of this effect to block out all reflections, except those that align to the polarizing filter material. The artificial “Easter-Egg” colors in this image are used to dissect how the light reflects off the smoke-sized dust particles and then heads toward Earth.

Hubble’s Advanced Camera for Surveys has polarizing filters that accept light that vibrates at select angles. In this composite image, the light from one of the polarizing filters has been colored red and only admits light from about one-third of the nebula. Another polarizing filter accepts light reflected from a different swath of the nebula. This light is colored blue. Light from the final third of the nebula is from a third polarizing filter and is colored green. Some of the inner regions of the nebula appear whitish because the dust is thicker and the light is scattered many times in random directions before reaching us. (Likewise, polarizing sunglasses are less effective if the sky is very dusty).

By studying polarized light from the Egg Nebula, scientists can tell a lot about the physical properties of the material responsible for the scattering, as well as the precise location of the central (hidden) star. The fine dust is largely carbon, manufactured by nuclear fusion in the heart of the star and then ejected into space as the star sheds material. Such dust grains are essential ingredients for building dusty disks around future generations of young stars, and possibly in the formation of planets around those stars.

This image was taken with Hubble’s Advanced Camera for Surveys in September and October 2002.

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