Also known as IC 4406, it is a planetary nebula near the western border of the constellation Lupus, the Wolf, and about 2,000 to 5,000 light years away from us. It looks somewhat rectangular because it is seen from its side as viewed from Earth, almost in the plane of its equator. The Retina Nebula as a planetary nebula, represents the final stage of a star’s life. When stars like our sun reach the end of their nuclear fuel burning lifetimes they make one final grab for glory. The outer layers of these stars, which have swollen to something approaching the size of Earth’s orbit, get blown into space forming what has been dubbed a “planetary nebula.” The expelled gas from the star glows brightly, illuminated by the ultraviolet light from the surviving stellar core, known as a “white dwarf.”
The Retina Nebula is bipolar and appears to be a prolate spheroid with strong concentrations of material in its equator. IC 4406 exhibits a high degree of symmetry. The nebula’s left and right halves are nearly mirror images of the other. If we could fly around IC 4406 in a spaceship, we would see that the gas and dust form a vast donut of material streaming outward from the dying star. We don’t see the donut shape in this photograph because we are viewing IC 4406 from the Earth-orbiting Hubble telescope. From this vantage point, we are seeing the side of the donut. This side view allows us to see the intricate tendrils of material that have been compared to the eye’s retina. The donut of material confines the intense radiation coming from the remnant dying star. Gas on the inside of the donut is ionized by light from the central star and glows, where oxygen atoms is rendered blue in this image, 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 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 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.
Credits: NASA, Wikipedia.