The Bug Nebula* and the (no) One Ring

How many times in astronomy (and, in general, in the world of science) did we discover something that wasn’t the goal that moved us in the beginning? These “collateral” discoveries are sometimes more relevant than originally sought. Other times, they complement certain fields of study with their contributions.

Miguel Santander (researcher at the Molecular Astrophysics Group of the Institute of Materials Science of Madrid (CSIC)) knows something about this, as he sometimes finds, along with his team, surprises that have led him to publish his results in the journal Nature (you can see one of these stories in his Talk of Naukas “How to be a Star, Die Twice and Do It in Style”). [1]

However, on this occasion, this is not about stars, but about rings.

To start where it’s right, we need to explain what a planetary nebula is. Well, a planetary nebula is the corpse of a low-mass or intermediate star (usually stars that have up to eight solar masses). They draw our attention because it is difficult to explain how a spherical object (we assume that stars are essentially spherical) can give rise, when dying, to such diverse and fantastic forms.

The star, by depleting the hydrogen from its nucleus, goes through several phases that will cause it to swell, multiply its size hundreds of times, and end up releasing its matter into the medium, leaving its remains in the center in the form of a dense white dwarf star. Around it, the gas that was once part of it desegregates, partially condensing into dust grains and forming different molecules. Its final destiny will be the total disappearance of the planetary nebula as we see it now. It will eventually fade into the interstellar environment and, most likely, the life cycle of the stars will begin again when gas and dust gather elsewhere and condense enough to generate nuclear reactions. But that’s another story. Let’s get on with the planetary nebula.

A while ago, the result of the work of a team (led by Valentín Bujarrabal of the OAN-IGN) that studied the presence of material discs around evolved stars was unveiled. These are discs very similar to those created when the stars are born, although we do not know many of their characteristics and we also do not know if planets could be born in these discs of dying stars. With the intention of further researching these interesting discs, observation time was obtained with the ALMA interferometer, a radio telescope formed by 66 antennas and located in the Atacama Desert in Chile.

And when they received the data, there was a surprise.

A nebula not only asymmetrical

We’re still asking the question. How is it possible for spherical objects such as stars to give rise to such different symmetries and, in some cases, so extreme? This is what happens to our protagonist, the Bug Nebula (NGC 6302), a relatively young planetary nebula whose central star has a very high temperature (it is not really yet known if there are one or more stars in the center, but that is the subject of another study).

The Ring Nebula.

The shape is impressive (see image 2). A fiery center from which stellar winds come out, caused by the white dwarf, that ionize the whole medium and shape bipolar jets, also known as lobes (those that make the nebula appear to be a bug with wings or have the shape of a diabolo). But, wait a moment… We don’t see the center.

And we don’t see it in this image because there’s a dust and gas ring that prevents it. First, let’s make a clear distinction between ring and disc. We were looking for a much smaller type of disk than this ring and we haven’t detected it. But there’s that ring and, if you look at it, in the visible range of light we see an arc-shaped filament wrapped in the main lobes. Although we do not know very well what it is… unless we observe in other light ranges, such as millimeter and submillimeter, the ranges in which ALMA observes and which manage to provide us with surprising information.

The One Ring?

Some nebulae have, around the nucleus, a very dense and thick gas and dust ring that is usually associated with its extreme symmetry and that we believe is related to the star’s winds, the presence of a companion or magnetic fields.

In the case of the Bug Nebula, the ring creation process began about 5,000 years ago and lasted approximately 2,000 years. Later, in a space of time that would go between 3,600 and 4,700 years ago, the lobes were created. But the planetary nebula does not have a single axis of symmetry or a single bipolar jet. About 2,200 years ago, another jet emerged from the nucleus, this one with a different symmetry. That is, there is a third lobe, younger and with a different axis than the main lobes.

But that’s not all.

In parallel, at a similar time, another structure was formed whose existence was not known until now: a second ring, younger than the first, that is oriented in another direction (see image 3) and which also expands faster.

Although it is not the first planetary nebula discovered with several rings with different degrees of inclination, it is the first time that it is estimated that there is quite a difference in age and mass between the rings. The secondary rings of other planetary nebulae are almost as massive as the primary ones and, in this case, if the primary ring has 0.1 solar masses, the secondary ring has only 2.8 Jupiter masses.

Whom are you from?

Both the origin and orientation of this second ring of the Bug Nebula are a mystery to researchers, but there are several theories that speculate on its possible formation. One of them sets the stage for a triple system in which one of the stars would have gone through the red giant phase, destabilizing the entire system. The other two stars could have originated the new ring.

There’s another hypothesis that’s much riskier, but equally interesting. May the ring be the result of the destruction of a gas giant planet that would have been in an orbit too close to the star during its process of evolution to red giant.

In both cases it is speculation and reaching some plausible conclusion would require more accurate data from the particular area.

The case is that no, the discs originally sought and for which observation time was requested have not been found, but a new type of ring has been discovered casually. Science gives us surprises.


[1] This talk is inspired by the results of this scientific paper: “The double-degenerate, super-Chandrasekhar nucleus of the planetary nebula Henize 2–428”.

Paper: ALMA high spatial resolution observations of the dense molecular region of NGC 6302


Press Release (in Spanish): Descubierto un segundo anillo en la Nebulosa del Insecto


Image 1. Planetary nebulae. Credit:  Montage by Judy Schmidt.

Image 2. Bug Nebula. Credit: NASA, ESA and the Hubble SM4 ERO team.

Image 3. Rings of the dense molecular region of the Bug Nebula seen by ALMA. Credit: M. Santander-García et al./ALMA/HST


ALMA observations in 12CO and 13CO (carbon monoxide isotopologues) overlapped on a background image of the Hubble Space Telescope. The number shown at the bottom corresponds to the velocity relative to the Local Standard of Rest (LSR) in km/s (the systemic velocity -the center of mass of the system is -30.4 km/s). The emission traces the structure and speed pattern of both rings. The left (west) region of the inner ring is associated with the arc-shaped filament visible in the Hubble image. Credit: M. Santander-García et al./ALMA/HST

Originally published in Spanish on the CulturaCientífica website: “La nebulosa del Bicho* y el anillo no único” (2016/09/30).