Hubble's eye in the sky

Hubble's latest picture in its series of images of dying stars is one of its most remarkable.

Like many other so-called planetary nebulae, IC 4406 exhibits a marked degree of symmetry; the left and right halves of the Hubble image are very nearly mirror images of each other.

Astronomers say that if they could fly around IC4406 in a starship, they would see that the gas and dust form a vast donut of material streaming outward from the dying star.

They have dubbed it "the retina" after the similarity of the dust lanes to blood vessels in the eye.

Hot, glowing gas

The view from planet Earth of this nebula is from the side, allowing the intricate tendrils of dust to be studied. In other planetary nebulae, like the famous Ring Nebula, we view the donut from above.

The gas cloud confines the intense radiation coming from the hot remnant of the dying star.

Gas on the inside of the donut is ionized by light from the central star - light from oxygen atoms is coloured blue in the Hubble image; hydrogen is shown as green; and nitrogen as red.

Unseen in the Hubble image is a larger zone of neutral gas that does not radiate visible light, but which can be seen by radio telescopes.

Researchers say that one of the most interesting features of IC 4406 is the irregular lattice of dark lanes that criss-cross the nebula. These lanes are about 160 astronomical units (AU) wide (1 AU is the distance between the Earth and Sun).

The dark lanes are located at the boundary between the hot glowing gas that produces visual light and the neutral gas seen with radio telescopes. We see the lanes in silhouette because they have a density of dust and gas much higher than the rest of the nebula.

The fate of these dense knots of material is unknown. If they survive the nebula's expansion they may condense into small dark objects or simply dissipate.