ABOUT THIS IMAGE:

The Fine Ring Nebula (Shapley 1 or PK 329+02.1) is an unusual planetary nebula, between 1000-4700 light-years away in Norma. The perfect ring is caused by the central object, thought to be a binary star system of white dwarfs

SP 1, the hazy and aptly named Fine Ring Nebula discovered in 1936 by Harlow Shapley, shown here, is an unusual planetary nebula. Planetary nebulae form when some dying stars, having expanded into a red giant phase, expel a shell of gas as they evolve into white dwarfs. Most planetary nebulae are either spherical or elliptical in shape, or bipolar (featuring two symmetric lobes of material).

But the Fine Ring Nebula - captured here by the ESO Faint Object Spectrograph and Camera mounted on the New Technology Telescope at the La Silla Observatory in Chile - looks like an almost perfect circular ring. Astronomers believe that some of these more unusually shaped planetary nebulae are formed when the progenitor star is actually a binary system. The interaction between the primary star and its orbiting companion shapes the ejected material.

The stellar object at the center of the Fine Ring Nebula is indeed thought to be a binary system of white dwarf stars (mag. 14.03), orbiting with a period of 2.9 days. Observations suggest that the binary pair is almost perfectly face-on from our vantage point, implying that the planetary nebula's structure is aligned in the same way. We are looking down on a torus (doughnut shape) of ejected material, leading to the strikingly circular ring shape in the image.

Planetary nebulae are shaped by the complex interplay of many physical processes. Not only can these celestial objects be admired for their beauty, but the study of precisely how they form their striking shapes is a fascinating topic in astronomical research.

This image was made using multiple filters: light observed through B and O-III filters is shown in blue, V is shown in green, R is shown in orange, and H-alpha in red. The image is approximately 200 arcseconds across.

[1] It is very hard to accurately measure the distance to planetary nebulae. Different techniques tend to produce wildly varying estimates. Some nearby nebulae such as the Helix nebula and the Dumbell nebula have crude parallaxes available for them, but otherwise only indirect methods can be used which are usually very inaccurate and errors are often ±50% or more.