Why Do Dying Stars Glow in Rings? A Name That Means Nothing
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
These glowing shells have nothing to do with planets and are the discarded outer layers of a dying star, lit up by the exposed core. The name is an eighteenth century mistake that stuck.
Where the misleading name came from
William Herschel coined the term in the 1780s because these objects appeared through his telescopes as small rounded greenish discs, resembling the disc of a planet rather than a point of light or a diffuse patch. The resemblance was entirely superficial and was an artefact of the resolution available, but the description was useful for cataloguing and it survived long after the objects were understood. By the time spectroscopy in the nineteenth century showed them to be glowing gas rather than anything solid, the name was established in the literature and changing it would have caused more confusion than keeping it. Astronomers have used it apologetically ever since, and it appears in essentially every account of these objects with an immediate explanation attached.
How one forms
The sequence runs over a few tens of thousands of years:
- •A star of modest mass exhausts the hydrogen and then the helium in its core
- •It swells enormously and becomes unstable, pulsing over periods of a year or more
- •Those pulsations and a strong stellar wind eject the outer layers over millennia
- •The exposed core is extremely hot, radiating strongly in the ultraviolet
- •That radiation ionises the ejected gas, which glows as the electrons recombine
- •The gas disperses within tens of thousands of years and the core cools into a white dwarf
Why so few are round
A star is roughly spherical and loses mass more or less evenly, so the shells ought to be spherical, and the great majority are not. Surveys find a strong preference for bipolar, elliptical and elaborately structured shapes, with jets, rings, knots and multiple nested shells being common, and simple spheres being distinctly unusual. The leading explanation is that most of these stars are not alone, with a companion star or possibly a large planet shaping the outflow through its gravity and through a disc of material around the pair, channelling the ejected gas into the observed structures. Magnetic fields and rotation contribute. The question of how much of the shaping requires a companion remains genuinely open and is one of the active arguments in the field.
The best known examples
A handful of these objects have been observed so often that they anchor the subject. The Ring in Lyra is the standard illustration, a bright annulus that is almost certainly a barrel or cylinder seen end on rather than an actual ring. The Helix in Aquarius is among the nearest and largest in apparent size, close enough that the knots of denser gas within it are resolved individually and studied as objects in their own right. The Cat Eye in Draco shows nested shells and jets in a structure elaborate enough that no single mechanism explains it. The Dumbbell in Vulpecula was the first of the class ever found, recorded by Charles Messier in 1764. Several are bright enough to be seen in a modest amateur telescope, appearing grey rather than coloured because the eye detects no colour at low light levels.
Why they matter
These objects do real work in astronomy beyond being photogenic. They return processed material to interstellar space, including carbon and nitrogen manufactured inside the star, and a substantial share of the carbon in the universe including the carbon in living things arrived this way rather than from supernovae. They are bright and recognisable at great distances, which makes them useful for measuring distances to other galaxies, since the brightest ones in any galaxy reach a consistent limit. Their spectra are dominated by a few strong emission lines from ionised gas, which makes them straightforward to study and made them important historically in working out how such spectra arise. And the expansion of the shell can be measured directly, giving the age.
The takeaway
The name records an eighteenth century resemblance to a planetary disc and nothing else, since these are ejected stellar envelopes lit by the exposed core. Most are bipolar or elaborately structured rather than spherical, which probably requires a companion shaping the outflow. They return carbon and nitrogen to interstellar space, supplying a large share of the carbon that ends up in living things.