The Calabash Nebula, caught mid-transformation

Most astronomical objects are photographed in a state they will hold for
millions of years. The Calabash Nebula — formally OH 231.8+04.2 — is not. It is
a star caught partway through a transition that takes roughly a thousand years
from start to finish, which is close to instantaneous on any stellar clock.

The transition

A star like the Sun ends in a sequence. First it swells into a red giant,
cool and enormous, shedding its outer layers as a slow dense wind. Eventually
enough of the envelope has gone that the hot core is exposed, its ultraviolet
light ionises the surrounding gas, and the whole thing lights up as a
planetary nebula.

Between those two states is a gap: the star has stopped being a red giant but
the shell is not yet lit. This is the protoplanetary or pre-planetary
phase, and objects in it are rare to catch — not because they are unusual, but
because the phase is so short that few stars are in it at any moment.

The Calabash is in it.

Why it is shaped like that

The nebula is strikingly bipolar — two lobes of material streaming in opposite
directions from the centre, rather than an expanding sphere.

The gas is moving fast. Measurements put parts of the outflow at around
a million kilometres per hour, which is far quicker than a red giant's
ordinary wind. Something has accelerated it, and the collision between that
fast outflow and the slower material shed earlier is what is producing the
shock fronts visible in the image.

The bipolarity itself points to a companion. A single star losing mass should
lose it more or less evenly; making it come out along one axis requires a disc
or a magnetic field to collimate it, and a binary system supplies a natural
mechanism for both.

The unflattering name

The nickname is the Rotten Egg Nebula, and it is chemical rather than
aesthetic. Spectra show an unusual abundance of sulphur compounds — the
family of molecules responsible for the smell of rotten eggs.

That is more than a curiosity. The chemistry of these outflows is how heavier
elements manufactured inside the star get returned to interstellar space. The
sulphur here was made in the star and is now being scattered into the galaxy,
to end up in the next generation of stars, planets and — eventually —
everything built from them.

Where and when

The Calabash is about 5,000 light years away in Puppis. Within roughly a
thousand years it is expected to be ionised and shining as a fully fledged
planetary nebula. For an object that has existed for billions of years, we are
watching a moment.