Like many other so-called planetary nebulas, IC 4406 – a dying star dubbed “Retina Nebula” – exhibits a high degree of symmetry; the left and right halves of the Hubble image are nearly mirror images of the other. If we could fly around IC 4406 in a starship, we would see that the gas and dust form a vast doughnut of material streaming outward from the dying star. From Earth, we are viewing the doughnut from the side. This side view allows us to see the intricate tendrils of dust that have been compared to the eye’s retina. In other planetary nebulas, like the Ring Nebula (NGC 6720), we view the doughnut from the top.
The doughnut of material confines the intense radiation coming from the remnant of the dying star. Gas on the inside of the doughnut is ionised by light from the central star and glows. Light from oxygen atoms is rendered blue in this image; hydrogen is shown as green, and nitrogen as red. The range of colour in the final image shows the differences in concentration of these three gases in the nebula. Unseen in this Hubble image is a larger zone of neutral gas that is not emitting visible light, but which can be seen by radio telescopes.
One of the most interesting features of IC 4406 is the irregular lattice of dark lanes that criss-cross the centre 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 doughnut.
The fate of these dense knots of material is unknown. Will they survive the nebula’s expansion and become dark denizens of the space between the stars or simply dissipate?
Image credit: Nasa, The Hubble Heritage Team (STScI/AURA), and CR O’Dell (Vanderbilt University)