A Wolf-Rayet Ring in Carina

This star is throwing away its own outer layers, and what you see is the part that has already left.

WR 40, and the gas it has shed

RCW 58 is a ring nebula in Carina built from material lost by the star at its centre: WR 40, also catalogued HD 96548, of spectral type WN8. The bright blue star in the middle of the field is that star. Wolf-Rayet stars are massive — WR 40 is thought to hold around twenty times the mass of the Sun at this stage — and they are in the process of losing what remains of their envelopes to stellar winds far stronger than anything the Sun produces.

Hot enough to light what it loses

Surface temperatures in this class run from around 30,000 kelvin to well over 100,000, and the ultraviolet output that goes with that is what makes the nebula visible: the star ionises the gas it has just expelled, and the gas radiates. Several hundred Wolf-Rayet stars are catalogued in the Milky Way. Most are expected to end as supernovae, which puts a limit on how long any of this lasts.

Where the oxygen runs ahead of the hydrogen

There is a detail in these objects worth knowing before looking at one. Across eight of the best-studied Wolf-Rayet ring nebulae in the galaxy, the outer edge of the OIII emission sits further out than the outer edge of the H-alpha. The two are not tracing the same boundary: the offset is read as the shock front where the expanding bubble meets the material the star shed earlier, with the oxygen marking the shock and the hydrogen marking what has been swept up behind it.

Thirty hours, split evenly

The CDK17 was working at f/6.8 with adaptive optics ahead of the sensor, from Rio Hurtado in May 2020. Thirty hours in all, and the narrowband is split exactly in half: twelve hours of H-alpha, twelve of OIII, with six across red, green and blue for the star colours. That even split is the point. If the OIII edge sits outside the H-alpha edge, then under-exposing either one moves an apparent boundary that is a real measurement — half the time on OIII would have brought the shock front in without changing anything about the object. What thirty hours does not reach is the outermost material, where both lines fade together. Martin Pugh gathered the data at El Sauce.

HOO palette: H-alpha in the red channel, OIII in green and blue. The nebula's colours are not its true colours; the two lines are mapped to make the structure readable, not to reproduce what an eye would see.