mrt 292013

March 29, 2013

The Retina Nebula, a planetary nebula in Lupus

IC 4406

Image Credit: C. R. O’Dell (Vanderbilt Univ.) et al., Hubble Heritage Team, NASA

The Retina Nebula (designated IC 4406) is a bipolar planetary nebula that measures about 0.13 × 0.45 light-year. It is located some 2,000 light-years away, near the western border of the southern constellation of Lupus (the Wolf) while it is approaching us at approximately 22 kilometers per second.

Despite their name, planetary nebulae have nothing to do with planets. The name of planetary nebulae arose because of the visual similarity between some round planetary nebulae and the planets Uranus and Neptune when viewed through early telescopes.

When a star with a mass up to eight times that of the Sun runs out of fuel at the end of its life, it blows off its outer shells and begins to lose mass. This allows the hot, inner core of the star (collapsing from a red giant to a white dwarf) to radiate strongly, causing this outward-moving cocoon of gas to glow brightly.

Over the next several thousand years, the nebula will gradually disperse into space, and then the white dwarf will cool and fade away for billions of years. Our own Sun is expected to undergo a similar fate, but fortunately this will not occur until some 5 billion years from now.

Like many other planetary nebulae, IC 4406 is highly symmetric; the left and right halves of the nebula are nearly mirror images of the other. And, although the Retina Nebula looks like a square, evidence indicates that IC 4406 is actually donut-shaped. From Earth, we are viewing the donut from the side, but were IC 4406 viewed from the top, it would likely look similar to the Ring Nebula (Messier 57 or NGC 6720).

This side view allows us to see the intricate tendrils of dust that have been compared to the eye’s retina. If we could fly around IC 4406, we would see that the gas and dust that form the vast donut of material are streaming outward from the central dying star. The donut of material confines the intense radiation coming from the remnant of the dying star. Gas on the inside of the donut is ionized by light from the central star and glows.

One of the most interesting features of IC 4406 is the irregular lattice of dark lanes that criss-cross the center of the nebula. These lanes are about 160 astronomical units wide (1 astronomical unit is the distance between the Earth and Sun). They are located right at the boundary between the hot glowing gas that produces the visual light imaged here and the neutral gas seen with radio telescopes.

We see the lanes in silhouette because they have a density of dust and gas that is a thousand times higher than the rest of the nebula. The dust lanes are like a rather open mesh veil that has been wrapped around the bright donut. The fate of these dense knots of material with a “lacy” appearance is unknown. Will they survive the nebula’s expansion and become dark denizens of the space between the stars or simply dissipate in a few million years? Will the only thing left visible of the Retina Nebula be a fading white dwarf star?

This color picture is a composite made by combining images taken by the Wide Field Planetary Camera 2 onboard the Hubble Space Telescope in 2001 and 2002. Light from oxygen atoms is rendered blue; hydrogen is shown as green, and nitrogen as red. The range of color in the final image shows the differences in concentration of these three gases in the nebula. Unseen in the image is a larger zone of neutral gas that is not emitting visible light, but which can be seen by radio telescopes.

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